// Copyright 2014 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package runtime

import (
	"internal/abi"
	"internal/cpu"
	"internal/goarch"
	"internal/goexperiment"
	"internal/goos"
	"internal/runtime/atomic"
	"internal/runtime/exithook"
	"internal/runtime/maps"
	"internal/runtime/sys"
	"internal/strconv"
	"internal/stringslite"
	"unsafe"
)

// set using cmd/go/internal/modload.ModInfoProg
var modinfo string

// Goroutine scheduler
// The scheduler's job is to distribute ready-to-run goroutines over worker threads.
//
// The main concepts are:
// G - goroutine.
// M - worker thread, or machine.
// P - processor, a resource that is required to execute Go code.
//     M must have an associated P to execute Go code, however it can be
//     blocked or in a syscall w/o an associated P.
//
// Design doc at https://golang.org/s/go11sched.

// Worker thread parking/unparking.
// We need to balance between keeping enough running worker threads to utilize
// available hardware parallelism and parking excessive running worker threads
// to conserve CPU resources and power. This is not simple for two reasons:
// (1) scheduler state is intentionally distributed (in particular, per-P work
// queues), so it is not possible to compute global predicates on fast paths;
// (2) for optimal thread management we would need to know the future (don't park
// a worker thread when a new goroutine will be readied in near future).
//
// Three rejected approaches that would work badly:
// 1. Centralize all scheduler state (would inhibit scalability).
// 2. Direct goroutine handoff. That is, when we ready a new goroutine and there
//    is a spare P, unpark a thread and handoff it the thread and the goroutine.
//    This would lead to thread state thrashing, as the thread that readied the
//    goroutine can be out of work the very next moment, we will need to park it.
//    Also, it would destroy locality of computation as we want to preserve
//    dependent goroutines on the same thread; and introduce additional latency.
// 3. Unpark an additional thread whenever we ready a goroutine and there is an
//    idle P, but don't do handoff. This would lead to excessive thread parking/
//    unparking as the additional threads will instantly park without discovering
//    any work to do.
//
// The current approach:
//
// This approach applies to three primary sources of potential work: readying a
// goroutine, new/modified-earlier timers, and idle-priority GC. See below for
// additional details.
//
// We unpark an additional thread when we submit work if (this is wakep()):
// 1. There is an idle P, and
// 2. There are no "spinning" worker threads.
//
// A worker thread is considered spinning if it is out of local work and did
// not find work in the global run queue or netpoller; the spinning state is
// denoted in m.spinning and in sched.nmspinning. Threads unparked this way are
// also considered spinning; we don't do goroutine handoff so such threads are
// out of work initially. Spinning threads spin on looking for work in per-P
// run queues and timer heaps or from the GC before parking. If a spinning
// thread finds work it takes itself out of the spinning state and proceeds to
// execution. If it does not find work it takes itself out of the spinning
// state and then parks.
//
// If there is at least one spinning thread (sched.nmspinning>1), we don't
// unpark new threads when submitting work. To compensate for that, if the last
// spinning thread finds work and stops spinning, it must unpark a new spinning
// thread. This approach smooths out unjustified spikes of thread unparking,
// but at the same time guarantees eventual maximal CPU parallelism
// utilization.
//
// The main implementation complication is that we need to be very careful
// during spinning->non-spinning thread transition. This transition can race
// with submission of new work, and either one part or another needs to unpark
// another worker thread. If they both fail to do that, we can end up with
// semi-persistent CPU underutilization.
//
// The general pattern for submission is:
// 1. Submit work to the local or global run queue, timer heap, or GC state.
// 2. #StoreLoad-style memory barrier.
// 3. Check sched.nmspinning.
//
// The general pattern for spinning->non-spinning transition is:
// 1. Decrement nmspinning.
// 2. #StoreLoad-style memory barrier.
// 3. Check all per-P work queues and GC for new work.
//
// Note that all this complexity does not apply to global run queue as we are
// not sloppy about thread unparking when submitting to global queue. Also see
// comments for nmspinning manipulation.
//
// How these different sources of work behave varies, though it doesn't affect
// the synchronization approach:
// * Ready goroutine: this is an obvious source of work; the goroutine is
//   immediately ready and must run on some thread eventually.
// * New/modified-earlier timer: The current timer implementation (see time.go)
//   uses netpoll in a thread with no work available to wait for the soonest
//   timer. If there is no thread waiting, we want a new spinning thread to go
//   wait.
// * Idle-priority GC: The GC wakes a stopped idle thread to contribute to
//   background GC work (note: currently disabled per golang.org/issue/19112).
//   Also see golang.org/issue/44313, as this should be extended to all GC
//   workers.

var (
	m0           m
	g0           g
	mcache0      *mcache
	raceprocctx0 uintptr
	raceFiniLock mutex
)

// This slice records the initializing tasks that need to be
// done to start up the runtime. It is built by the linker.
var runtime_inittasks []*initTask

// mainInitDone is a signal used by cgocallbackg that initialization
// has been completed. If this is false, wait on mainInitDoneChan.
var mainInitDone atomic.Bool

// mainInitDoneChan is closed after initialization has been completed.
// It is made before _cgo_notify_runtime_init_done, so all cgo
// calls can rely on it existing.
var mainInitDoneChan chan bool

//go:linkname main_main main.main
func main_main()

// mainStarted indicates that the main M has started.
var mainStarted bool

// runtimeInitTime is the nanotime() at which the runtime started.
var runtimeInitTime int64

// Value to use for signal mask for newly created M's.
var initSigmask sigset

// The main goroutine.
func main() {
	mp := getg().m

	// Racectx of m0->g0 is used only as the parent of the main goroutine.
	// It must not be used for anything else.
	mp.g0.racectx = 0

	// Max stack size is 1 GB on 64-bit, 250 MB on 32-bit.
	// Using decimal instead of binary GB and MB because
	// they look nicer in the stack overflow failure message.
	if goarch.PtrSize == 8 {
		maxstacksize = 1000000000
	} else {
		maxstacksize = 250000000
	}

	// An upper limit for max stack size. Used to avoid random crashes
	// after calling SetMaxStack and trying to allocate a stack that is too big,
	// since stackalloc works with 32-bit sizes.
	maxstackceiling = 2 * maxstacksize

	// Allow newproc to start new Ms.
	mainStarted = true

	if haveSysmon {
		systemstack(func() {
			newm(sysmon, nil, -1)
		})
	}

	// Lock the main goroutine onto this, the main OS thread,
	// during initialization. Most programs won't care, but a few
	// do require certain calls to be made by the main thread.
	// Those can arrange for main.main to run in the main thread
	// by calling runtime.LockOSThread during initialization
	// to preserve the lock.
	lockOSThread()

	if mp != &m0 {
		throw("runtime.main not on m0")
	}

	// Record when the world started.
	// Must be before doInit for tracing init.
	runtimeInitTime = nanotime()
	if runtimeInitTime == 0 {
		throw("nanotime returning zero")
	}

	if debug.inittrace != 0 {
		inittrace.id = getg().goid
		inittrace.active = true
	}

	doInit(runtime_inittasks) // Must be before defer.

	// Defer unlock so that runtime.Goexit during init does the unlock too.
	needUnlock := true
	defer func() {
		if needUnlock {
			unlockOSThread()
		}
	}()

	gcenable()
	defaultGOMAXPROCSUpdateEnable() // don't STW before runtime initialized.

	// If we encountered a removed GODEBUG during startup we can panic now.
	if k := invalidGODEBUG.key; k != "" {
		v := invalidGODEBUG.value
		r := strconv.Itoa(invalidGODEBUG.removed)
		fatal(`removed GODEBUG "` + k + `" set to old value "` + v + `" in environment (https://go.dev/doc/godebug#go-1` + r + `)`)
	}

	mainInitDoneChan = make(chan bool)
	if iscgo {
		if _cgo_pthread_key_created == nil {
			throw("_cgo_pthread_key_created missing")
		}

		if GOOS != "windows" {
			if _cgo_thread_start == nil {
				throw("_cgo_thread_start missing")
			}
			if _cgo_setenv == nil {
				throw("_cgo_setenv missing")
			}
			if _cgo_unsetenv == nil {
				throw("_cgo_unsetenv missing")
			}
		}
		if _cgo_notify_runtime_init_done == nil {
			throw("_cgo_notify_runtime_init_done missing")
		}

		// Set the x_crosscall2_ptr C function pointer variable point to crosscall2.
		if set_crosscall2 == nil {
			throw("set_crosscall2 missing")
		}
		set_crosscall2()

		// Start the template thread in case we enter Go from
		// a C-created thread and need to create a new thread.
		startTemplateThread()
		cgocall(_cgo_notify_runtime_init_done, nil)
	}

	// Run the initializing tasks. Depending on build mode this
	// list can arrive a few different ways, but it will always
	// contain the init tasks computed by the linker for all the
	// packages in the program (excluding those added at runtime
	// by package plugin). Run through the modules in dependency
	// order (the order they are initialized by the dynamic
	// loader, i.e. they are added to the moduledata linked list).
	last := lastmoduledatap // grab before loop starts. Any added modules after this point will do their own doInit calls.
	for m := &firstmoduledata; true; m = m.next {
		doInit(m.inittasks)
		if m == last {
			break
		}
	}

	// Disable init tracing after main init done to avoid overhead
	// of collecting statistics in malloc and newproc
	inittrace.active = false

	mainInitDone.Store(true)
	close(mainInitDoneChan)

	needUnlock = false
	unlockOSThread()

	if isarchive || islibrary {
		// A program compiled with -buildmode=c-archive or c-shared
		// has a main, but it is not executed.
		if GOARCH == "wasm" {
			// On Wasm, pause makes it return to the host.
			// Unlike cgo callbacks where Ms are created on demand,
			// on Wasm we have only one M. So we keep this M (and this
			// G) for callbacks.
			// Using the caller's SP unwinds this frame and backs to
			// goexit. The -16 is: 8 for goexit's (fake) return PC,
			// and pause's epilogue pops 8.
			pause(sys.GetCallerSP() - 16) // should not return
			panic("unreachable")
		}
		return
	}
	fn := main_main // make an indirect call, as the linker doesn't know the address of the main package when laying down the runtime
	fn()

	// Check for C memory leaks if using ASAN and we've made cgo calls,
	// or if we are running as a library in a C program.
	// We always make one cgo call, above, to notify_runtime_init_done,
	// so we ignore that one.
	// No point in leak checking if no cgo calls, since leak checking
	// just looks for objects allocated using malloc and friends.
	// Just checking iscgo doesn't help because asan implies iscgo.
	exitHooksRun := false
	if asanenabled && (isarchive || islibrary || NumCgoCall() > 1) {
		runExitHooks(0) // lsandoleakcheck may not return
		exitHooksRun = true
		lsandoleakcheck()
	}

	// Make racy client program work: if panicking on
	// another goroutine at the same time as main returns,
	// let the other goroutine finish printing the panic trace.
	// Once it does, it will exit. See issues 3934 and 20018.
	if runningPanicDefers.Load() != 0 {
		// Running deferred functions should not take long.
		for c := 0; c < 1000; c++ {
			if runningPanicDefers.Load() == 0 {
				break
			}
			Gosched()
		}
	}
	if panicking.Load() != 0 {
		gopark(nil, nil, waitReasonPanicWait, traceBlockForever, 1)
	}
	if !exitHooksRun {
		runExitHooks(0)
	}
	if raceenabled {
		racefini() // does not return
	}

	exit(0)
	for {
		var x *int32
		*x = 0
	}
}

// os_beforeExit is called from os.Exit(0).
//
//go:linkname os_beforeExit os.runtime_beforeExit
func os_beforeExit(exitCode int) {
	runExitHooks(exitCode)
	if exitCode == 0 && raceenabled {
		racefini()
	}

	// See comment in main, above.
	if exitCode == 0 && asanenabled && (isarchive || islibrary || NumCgoCall() > 1) {
		lsandoleakcheck()
	}
}

func init() {
	exithook.Gosched = Gosched
	exithook.Goid = func() uint64 { return getg().goid }
	exithook.Throw = throw
}

func runExitHooks(code int) {
	exithook.Run(code)
}

// start forcegc helper goroutine
func init() {
	go forcegchelper()
}

func forcegchelper() {
	forcegc.g = getg()
	lockInit(&forcegc.lock, lockRankForcegc)
	for {
		lock(&forcegc.lock)
		if forcegc.idle.Load() {
			throw("forcegc: phase error")
		}
		forcegc.idle.Store(true)
		goparkunlock(&forcegc.lock, waitReasonForceGCIdle, traceBlockSystemGoroutine, 1)
		// this goroutine is explicitly resumed by sysmon
		if debug.gctrace > 0 {
			println("GC forced")
		}
		// Time-triggered, fully concurrent.
		gcStart(gcTrigger{kind: gcTriggerTime, now: nanotime()})
	}
}

// Gosched yields the processor, allowing other goroutines to run. It does not
// suspend the current goroutine, so execution resumes automatically.
//
//go:nosplit
func Gosched() {
	checkTimeouts()
	mcall(gosched_m)
}

// goschedguarded yields the processor like gosched, but also checks
// for forbidden states and opts out of the yield in those cases.
//
//go:nosplit
func goschedguarded() {
	mcall(goschedguarded_m)
}

// goschedIfBusy yields the processor like gosched, but only does so if
// there are no idle Ps or if we're on the only P and there's nothing in
// the run queue. In both cases, there is freely available idle time.
//
//go:nosplit
func goschedIfBusy() {
	gp := getg()
	// Call gosched if gp.preempt is set; we may be in a tight loop that
	// doesn't otherwise yield.
	if !gp.preempt && sched.npidle.Load() > 0 {
		return
	}
	mcall(gosched_m)
}

// Puts the current goroutine into a waiting state and calls unlockf on the
// system stack.
//
// If unlockf returns false, the goroutine is resumed.
//
// unlockf must not access this G's stack, as it may be moved between
// the call to gopark and the call to unlockf.
//
// Note that because unlockf is called after putting the G into a waiting
// state, the G may have already been readied by the time unlockf is called
// unless there is external synchronization preventing the G from being
// readied. If unlockf returns false, it must guarantee that the G cannot be
// externally readied.
//
// Reason explains why the goroutine has been parked. It is displayed in stack
// traces and heap dumps. Reasons should be unique and descriptive. Do not
// re-use reasons, add new ones.
//
// gopark should be an internal detail,
// but widely used packages access it using linkname.
// Notable members of the hall of shame include:
//   - gvisor.dev/gvisor
//   - github.com/sagernet/gvisor
//
// Do not remove or change the type signature.
// See go.dev/issue/67401.
//
//go:linkname gopark
func gopark(unlockf func(*g, unsafe.Pointer) bool, lock unsafe.Pointer, reason waitReason, traceReason traceBlockReason, traceskip int) {
	if reason != waitReasonSleep {
		checkTimeouts() // timeouts may expire while two goroutines keep the scheduler busy
	}
	mp := acquirem()
	gp := mp.curg
	status := readgstatus(gp)
	if status != _Grunning && status != _Gscanrunning {
		throw("gopark: bad g status")
	}
	mp.waitlock = lock
	mp.waitunlockf = unlockf
	gp.waitreason = reason
	mp.waitTraceBlockReason = traceReason
	mp.waitTraceSkip = traceskip
	releasem(mp)
	// can't do anything that might move the G between Ms here.
	mcall(park_m)
}

// Puts the current goroutine into a waiting state and unlocks the lock.
// The goroutine can be made runnable again by calling goready(gp).
func goparkunlock(lock *mutex, reason waitReason, traceReason traceBlockReason, traceskip int) {
	gopark(parkunlock_c, unsafe.Pointer(lock), reason, traceReason, traceskip)
}

// goready should be an internal detail,
// but widely used packages access it using linkname.
// Notable members of the hall of shame include:
//   - gvisor.dev/gvisor
//   - github.com/sagernet/gvisor
//
// Do not remove or change the type signature.
// See go.dev/issue/67401.
//
//go:linkname goready
func goready(gp *g, traceskip int) {
	systemstack(func() {
		ready(gp, traceskip, true)
	})
}

//go:nosplit
func acquireSudog() *sudog {
	// Delicate dance: the semaphore implementation calls
	// acquireSudog, acquireSudog calls new(sudog),
	// new calls malloc, malloc can call the garbage collector,
	// and the garbage collector calls the semaphore implementation
	// in stopTheWorld.
	// Break the cycle by doing acquirem/releasem around new(sudog).
	// The acquirem/releasem increments m.locks during new(sudog),
	// which keeps the garbage collector from being invoked.
	mp := acquirem()
	pp := mp.p.ptr()
	if len(pp.sudogcache) == 0 {
		lock(&sched.sudoglock)
		// First, try to grab a batch from central cache.
		for len(pp.sudogcache) < cap(pp.sudogcache)/2 && sched.sudogcache != nil {
			s := sched.sudogcache
			sched.sudogcache = s.next
			s.next = nil
			pp.sudogcache = append(pp.sudogcache, s)
		}
		unlock(&sched.sudoglock)
		// If the central cache is empty, allocate a new one.
		if len(pp.sudogcache) == 0 {
			pp.sudogcache = append(pp.sudogcache, new(sudog))
		}
	}
	n := len(pp.sudogcache)
	s := pp.sudogcache[n-1]
	pp.sudogcache[n-1] = nil
	pp.sudogcache = pp.sudogcache[:n-1]
	if s.elem.get() != nil {
		throw("acquireSudog: found s.elem != nil in cache")
	}
	releasem(mp)
	return s
}

//go:nosplit
func releaseSudog(s *sudog) {
	if s.elem.get() != nil {
		throw("runtime: sudog with non-nil elem")
	}
	if s.isSelect {
		throw("runtime: sudog with non-false isSelect")
	}
	if s.next != nil {
		throw("runtime: sudog with non-nil next")
	}
	if s.prev != nil {
		throw("runtime: sudog with non-nil prev")
	}
	if s.waitlink != nil {
		throw("runtime: sudog with non-nil waitlink")
	}
	if s.c.get() != nil {
		throw("runtime: sudog with non-nil c")
	}
	gp := getg()
	if gp.param != nil {
		throw("runtime: releaseSudog with non-nil gp.param")
	}
	mp := acquirem() // avoid rescheduling to another P
	pp := mp.p.ptr()
	if len(pp.sudogcache) == cap(pp.sudogcache) {
		// Transfer half of local cache to the central cache.
		var first, last *sudog
		for len(pp.sudogcache) > cap(pp.sudogcache)/2 {
			n := len(pp.sudogcache)
			p := pp.sudogcache[n-1]
			pp.sudogcache[n-1] = nil
			pp.sudogcache = pp.sudogcache[:n-1]
			if first == nil {
				first = p
			} else {
				last.next = p
			}
			last = p
		}
		lock(&sched.sudoglock)
		last.next = sched.sudogcache
		sched.sudogcache = first
		unlock(&sched.sudoglock)
	}
	pp.sudogcache = append(pp.sudogcache, s)
	releasem(mp)
}

// called from assembly.
func badmcall(fn func(*g)) {
	throw("runtime: mcall called on m->g0 stack")
}

func badmcall2(fn func(*g)) {
	throw("runtime: mcall function returned")
}

func badreflectcall() {
	panic(plainError("arg size to reflect.call more than 1GB"))
}

//go:nosplit
//go:nowritebarrierrec
func badmorestackg0() {
	if !crashStackImplemented {
		writeErrStr("fatal: morestack on g0\n")
		return
	}

	g := getg()
	switchToCrashStack(func() {
		print("runtime: morestack on g0, stack [", hex(g.stack.lo), " ", hex(g.stack.hi), "], sp=", hex(g.sched.sp), ", called from\n")
		g.m.traceback = 2 // include pc and sp in stack trace
		traceback1(g.sched.pc, g.sched.sp, g.sched.lr, g, 0)
		print("\n")

		throw("morestack on g0")
	})
}

//go:nosplit
//go:nowritebarrierrec
func badmorestackgsignal() {
	writeErrStr("fatal: morestack on gsignal\n")
}

//go:nosplit
func badctxt() {
	throw("ctxt != 0")
}

// gcrash is a fake g that can be used when crashing due to bad
// stack conditions.
var gcrash g

var crashingG atomic.Pointer[g]

// Switch to crashstack and call fn, with special handling of
// concurrent and recursive cases.
//
// Nosplit as it is called in a bad stack condition (we know
// morestack would fail).
//
//go:nosplit
//go:nowritebarrierrec
func switchToCrashStack(fn func()) {
	me := getg()
	if crashingG.CompareAndSwapNoWB(nil, me) {
		switchToCrashStack0(fn) // should never return
		abort()
	}
	if crashingG.Load() == me {
		// recursive crashing. too bad.
		writeErrStr("fatal: recursive switchToCrashStack\n")
		abort()
	}
	// Another g is crashing. Give it some time, hopefully it will finish traceback.
	usleep_no_g(100)
	writeErrStr("fatal: concurrent switchToCrashStack\n")
	abort()
}

// Disable crash stack on Windows for now. Apparently, throwing an exception
// on a non-system-allocated crash stack causes EXCEPTION_STACK_OVERFLOW and
// hangs the process (see issue 63938).
const crashStackImplemented = GOOS != "windows"

//go:noescape
func switchToCrashStack0(fn func()) // in assembly

func lockedOSThread() bool {
	gp := getg()
	return gp.lockedm != 0 && gp.m.lockedg != 0
}

var (
	// allgs contains all Gs ever created (including dead Gs), and thus
	// never shrinks.
	//
	// Access via the slice is protected by allglock or stop-the-world.
	// Readers that cannot take the lock may (carefully!) use the atomic
	// variables below.
	allglock mutex
	allgs    []*g

	// allglen and allgptr are atomic variables that contain len(allgs) and
	// &allgs[0] respectively. Proper ordering depends on totally-ordered
	// loads and stores. Writes are protected by allglock.
	//
	// allgptr is updated before allglen. Readers should read allglen
	// before allgptr to ensure that allglen is always <= len(allgptr). New
	// Gs appended during the race can be missed. For a consistent view of
	// all Gs, allglock must be held.
	//
	// allgptr copies should always be stored as a concrete type or
	// unsafe.Pointer, not uintptr, to ensure that GC can still reach it
	// even if it points to a stale array.
	allglen uintptr
	allgptr **g
)

func allgadd(gp *g) {
	if readgstatus(gp) == _Gidle {
		throw("allgadd: bad status Gidle")
	}

	lock(&allglock)
	allgs = append(allgs, gp)
	if &allgs[0] != allgptr {
		atomicstorep(unsafe.Pointer(&allgptr), unsafe.Pointer(&allgs[0]))
	}
	atomic.Storeuintptr(&allglen, uintptr(len(allgs)))
	unlock(&allglock)
}

// allGsSnapshot returns a snapshot of the slice of all Gs.
//
// The world must be stopped or allglock must be held.
func allGsSnapshot() []*g {
	assertWorldStoppedOrLockHeld(&allglock)

	// Because the world is stopped or allglock is held, allgadd
	// cannot happen concurrently with this. allgs grows
	// monotonically and existing entries never change, so we can
	// simply return a copy of the slice header. For added safety,
	// we trim everything past len because that can still change.
	return allgs[:len(allgs):len(allgs)]
}

// atomicAllG returns &allgs[0] and len(allgs) for use with atomicAllGIndex.
func atomicAllG() (**g, uintptr) {
	length := atomic.Loaduintptr(&allglen)
	ptr := (**g)(atomic.Loadp(unsafe.Pointer(&allgptr)))
	return ptr, length
}

// atomicAllGIndex returns ptr[i] with the allgptr returned from atomicAllG.
func atomicAllGIndex(ptr **g, i uintptr) *g {
	return *(**g)(add(unsafe.Pointer(ptr), i*goarch.PtrSize))
}

// forEachG calls fn on every G from allgs.
//
// forEachG takes a lock to exclude concurrent addition of new Gs.
func forEachG(fn func(gp *g)) {
	lock(&allglock)
	for _, gp := range allgs {
		fn(gp)
	}
	unlock(&allglock)
}

// forEachGRace calls fn on every G from allgs.
//
// forEachGRace avoids locking, but does not exclude addition of new Gs during
// execution, which may be missed.
func forEachGRace(fn func(gp *g)) {
	ptr, length := atomicAllG()
	for i := uintptr(0); i < length; i++ {
		gp := atomicAllGIndex(ptr, i)
		fn(gp)
	}
	return
}

const (
	// Number of goroutine ids to grab from sched.goidgen to local per-P cache at once.
	// 16 seems to provide enough amortization, but other than that it's mostly arbitrary number.
	_GoidCacheBatch = 16
)

// cpuinit sets up CPU feature flags and calls internal/cpu.Initialize. env should be the complete
// value of the GODEBUG environment variable.
func cpuinit(env string) {
	cpu.Initialize(env)

	// Support cpu feature variables are used in code generated by the compiler
	// to guard execution of instructions that can not be assumed to be always supported.
	switch GOARCH {
	case "386", "amd64":
		x86HasAVX = cpu.X86.HasAVX
		x86HasFMA = cpu.X86.HasFMA
		x86HasPOPCNT = cpu.X86.HasPOPCNT
		x86HasSSE41 = cpu.X86.HasSSE41

	case "arm":
		armHasVFPv4 = cpu.ARM.HasVFPv4

	case "arm64":
		arm64HasATOMICS = cpu.ARM64.HasATOMICS

	case "loong64":
		loong64HasLAMCAS = cpu.Loong64.HasLAMCAS
		loong64HasLAM_BH = cpu.Loong64.HasLAM_BH
		loong64HasDBAR_HINTS = cpu.Loong64.HasDBAR_HINTS
		loong64HasLSX = cpu.Loong64.HasLSX

	case "riscv64":
		riscv64HasZbb = cpu.RISCV64.HasZbb
	}
}

// getGodebugEarly extracts the environment variable GODEBUG from the environment on
// Unix-like operating systems and returns it. This function exists to extract GODEBUG
// early before much of the runtime is initialized.
//
// Returns nil, false if OS doesn't provide env vars early in the init sequence.
func getGodebugEarly() (string, bool) {
	const prefix = "GODEBUG="
	var env string
	switch GOOS {
	case "aix", "darwin", "ios", "dragonfly", "freebsd", "netbsd", "openbsd", "illumos", "solaris", "linux":
		// Similar to goenv_unix but extracts the environment value for
		// GODEBUG directly.
		// TODO(moehrmann): remove when general goenvs() can be called before cpuinit()
		n := int32(0)
		for argv_index(argv, argc+1+n) != nil {
			n++
		}

		for i := int32(0); i < n; i++ {
			p := argv_index(argv, argc+1+i)
			s := unsafe.String(p, findnull(p))

			if stringslite.HasPrefix(s, prefix) {
				env = gostringnocopy(p)[len(prefix):]
				break
			}
		}
		break

	default:
		return "", false
	}
	return env, true
}

// The bootstrap sequence is:
//
//	call osinit
//	call schedinit
//	make & queue new G
//	call runtime·mstart
//
// The new G calls runtime·main.
func schedinit() {
	lockInit(&sched.lock, lockRankSched)
	lockInit(&sched.sysmonlock, lockRankSysmon)
	lockInit(&sched.deferlock, lockRankDefer)
	lockInit(&sched.sudoglock, lockRankSudog)
	lockInit(&deadlock, lockRankDeadlock)
	lockInit(&paniclk, lockRankPanic)
	lockInit(&allglock, lockRankAllg)
	lockInit(&allpLock, lockRankAllp)
	lockInit(&reflectOffs.lock, lockRankReflectOffs)
	lockInit(&finlock, lockRankFin)
	lockInit(&cpuprof.lock, lockRankCpuprof)
	lockInit(&computeMaxProcsLock, lockRankComputeMaxProcs)
	allocmLock.init(lockRankAllocmR, lockRankAllocmRInternal, lockRankAllocmW)
	execLock.init(lockRankExecR, lockRankExecRInternal, lockRankExecW)
	traceLockInit()
	// Enforce that this lock is always a leaf lock.
	// All of this lock's critical sections should be
	// extremely short.
	lockInit(&memstats.heapStats.noPLock, lockRankLeafRank)

	lockVerifyMSize()

	sched.midle.init(unsafe.Offsetof(m{}.idleNode))

	// raceinit must be the first call to race detector.
	// In particular, it must be done before mallocinit below calls racemapshadow.
	gp := getg()
	if raceenabled {
		gp.racectx, raceprocctx0 = raceinit()
	}

	sched.maxmcount = 10000
	crashFD.Store(^uintptr(0))

	// The world starts stopped.
	worldStopped()

	godebug, parsedGodebug := getGodebugEarly()
	if parsedGodebug {
		parseRuntimeDebugVars(godebug)
	}
	ticks.init() // run as early as possible
	moduledataverify()
	stackinit()
	randinit() // must run before mallocinit, AlgInit, mcommoninit
	mallocinit()
	cpuinit(godebug) // must run before AlgInit
	maps.AlgInit()   // maps, hash, rand must not be used before this call
	mcommoninit(gp.m, -1)
	modulesinit()   // provides activeModules
	typelinksinit() // uses maps, activeModules
	itabsinit()     // uses activeModules
	stkobjinit()    // must run before GC starts

	sigsave(&gp.m.sigmask)
	initSigmask = gp.m.sigmask

	goargs()
	goenvs()
	secure()
	checkfds()
	if !parsedGodebug {
		// Some platforms, e.g., Windows, didn't make env vars available "early",
		// so try again now.
		parseRuntimeDebugVars(gogetenv("GODEBUG"))
	}
	finishDebugVarsSetup()
	gcinit()

	// Allocate stack space that can be used when crashing due to bad stack
	// conditions, e.g. morestack on g0.
	gcrash.stack = stackalloc(16384)
	gcrash.stackguard0 = gcrash.stack.lo + 1000
	gcrash.stackguard1 = gcrash.stack.lo + 1000

	// if disableMemoryProfiling is set, update MemProfileRate to 0 to turn off memprofile.
	// Note: parsedebugvars may update MemProfileRate, but when disableMemoryProfiling is
	// set to true by the linker, it means that nothing is consuming the profile, it is
	// safe to set MemProfileRate to 0.
	if disableMemoryProfiling {
		MemProfileRate = 0
	}

	// mcommoninit runs before parsedebugvars, so init profstacks again.
	mProfStackInit(gp.m)
	defaultGOMAXPROCSInit()

	lock(&sched.lock)
	sched.lastpoll.Store(nanotime())
	var procs int32
	if n, err := strconv.ParseInt(gogetenv("GOMAXPROCS"), 10, 32); err == nil && n > 0 {
		procs = int32(n)
		sched.customGOMAXPROCS = true
	} else {
		// Use numCPUStartup for initial GOMAXPROCS for two reasons:
		//
		// 1. We just computed it in osinit, recomputing is (minorly) wasteful.
		//
		// 2. More importantly, if debug.containermaxprocs == 0 &&
		//    debug.updatemaxprocs == 0, we want to guarantee that
		//    runtime.GOMAXPROCS(0) always equals runtime.NumCPU (which is
		//    just numCPUStartup).
		procs = defaultGOMAXPROCS(numCPUStartup)
	}
	if procresize(procs) != nil {
		throw("unknown runnable goroutine during bootstrap")
	}
	unlock(&sched.lock)

	// World is effectively started now, as P's can run.
	worldStarted()

	if buildVersion == "" {
		// Condition should never trigger. This code just serves
		// to ensure runtime·buildVersion is kept in the resulting binary.
		buildVersion = "unknown"
	}
	if len(modinfo) == 1 {
		// Condition should never trigger. This code just serves
		// to ensure runtime·modinfo is kept in the resulting binary.
		modinfo = ""
	}
}

func dumpgstatus(gp *g) {
	thisg := getg()
	print("runtime:   gp: gp=", gp, ", goid=", gp.goid, ", gp->atomicstatus=", readgstatus(gp), "\n")
	print("runtime: getg:  g=", thisg, ", goid=", thisg.goid, ",  g->atomicstatus=", readgstatus(thisg), "\n")
}

// sched.lock must be held.
func checkmcount() {
	assertLockHeld(&sched.lock)

	// Exclude extra M's, which are used for cgocallback from threads
	// created in C.
	//
	// The purpose of the SetMaxThreads limit is to avoid accidental fork
	// bomb from something like millions of goroutines blocking on system
	// calls, causing the runtime to create millions of threads. By
	// definition, this isn't a problem for threads created in C, so we
	// exclude them from the limit. See https://go.dev/issue/60004.
	count := mcount() - int32(extraMInUse.Load()) - int32(extraMLength.Load())
	if count > sched.maxmcount {
		print("runtime: program exceeds ", sched.maxmcount, "-thread limit\n")
		throw("thread exhaustion")
	}
}

// mReserveID returns the next ID to use for a new m. This new m is immediately
// considered 'running' by checkdead.
//
// sched.lock must be held.
func mReserveID() int64 {
	assertLockHeld(&sched.lock)

	if sched.mnext+1 < sched.mnext {
		throw("runtime: thread ID overflow")
	}
	id := sched.mnext
	sched.mnext++
	checkmcount()
	return id
}

// Pre-allocated ID may be passed as 'id', or omitted by passing -1.
func mcommoninit(mp *m, id int64) {
	gp := getg()

	// g0 stack won't make sense for user (and is not necessary unwindable).
	if gp != gp.m.g0 {
		callers(1, mp.createstack[:])
	}

	lock(&sched.lock)

	if id >= 0 {
		mp.id = id
	} else {
		mp.id = mReserveID()
	}

	mp.self = newMWeakPointer(mp)

	mrandinit(mp)

	mpreinit(mp)
	if mp.gsignal != nil {
		mp.gsignal.stackguard1 = mp.gsignal.stack.lo + stackGuard
	}

	// Add to allm so garbage collector doesn't free g->m
	// when it is just in a register or thread-local storage.
	mp.alllink = allm

	// NumCgoCall and others iterate over allm w/o schedlock,
	// so we need to publish it safely.
	atomicstorep(unsafe.Pointer(&allm), unsafe.Pointer(mp))
	unlock(&sched.lock)

	// Allocate memory to hold a cgo traceback if the cgo call crashes.
	if iscgo || GOOS == "solaris" || GOOS == "illumos" || GOOS == "windows" {
		mp.cgoCallers = new(cgoCallers)
	}
	mProfStackInit(mp)
}

// mProfStackInit is used to eagerly initialize stack trace buffers for
// profiling. Lazy allocation would have to deal with reentrancy issues in
// malloc and runtime locks for mLockProfile.
// TODO(mknyszek): Implement lazy allocation if this becomes a problem.
func mProfStackInit(mp *m) {
	if debug.profstackdepth == 0 {
		// debug.profstack is set to 0 by the user, or we're being called from
		// schedinit before parsedebugvars.
		return
	}
	mp.profStack = makeProfStackFP()
	mp.mLockProfile.stack = makeProfStackFP()
}

// makeProfStackFP creates a buffer large enough to hold a maximum-sized stack
// trace as well as any additional frames needed for frame pointer unwinding
// with delayed inline expansion.
func makeProfStackFP() []uintptr {
	// The "1" term is to account for the first stack entry being
	// taken up by a "skip" sentinel value for profilers which
	// defer inline frame expansion until the profile is reported.
	// The "maxSkip" term is for frame pointer unwinding, where we
	// want to end up with debug.profstackdebth frames but will discard
	// some "physical" frames to account for skipping.
	return make([]uintptr, 1+maxSkip+debug.profstackdepth)
}

// makeProfStack returns a buffer large enough to hold a maximum-sized stack
// trace.
func makeProfStack() []uintptr { return make([]uintptr, debug.profstackdepth) }

//go:linkname pprof_makeProfStack
func pprof_makeProfStack() []uintptr { return makeProfStack() }

func (mp *m) becomeSpinning() {
	mp.spinning = true
	sched.nmspinning.Add(1)
	sched.needspinning.Store(0)
}

// Take a snapshot of allp, for use after dropping the P.
//
// Must be called with a P, but the returned slice may be used after dropping
// the P. The M holds a reference on the snapshot to keep the backing array
// alive.
//
//go:yeswritebarrierrec
func (mp *m) snapshotAllp() []*p {
	mp.allpSnapshot = allp
	return mp.allpSnapshot
}

// Clear the saved allp snapshot. Should be called as soon as the snapshot is
// no longer required.
//
// Must be called after reacquiring a P, as it requires a write barrier.
//
//go:yeswritebarrierrec
func (mp *m) clearAllpSnapshot() {
	mp.allpSnapshot = nil
}

func (mp *m) hasCgoOnStack() bool {
	return mp.ncgo > 0 || mp.isextra
}

const (
	// osHasLowResTimer indicates that the platform's internal timer system has a low resolution,
	// typically on the order of 1 ms or more.
	osHasLowResTimer = GOOS == "windows" || GOOS == "openbsd" || GOOS == "netbsd"

	// osHasLowResClockInt is osHasLowResClock but in integer form, so it can be used to create
	// constants conditionally.
	osHasLowResClockInt = goos.IsWindows

	// osHasLowResClock indicates that timestamps produced by nanotime on the platform have a
	// low resolution, typically on the order of 1 ms or more.
	osHasLowResClock = osHasLowResClockInt > 0
)

// Mark gp ready to run.
func ready(gp *g, traceskip int, next bool) {
	status := readgstatus(gp)

	// Mark runnable.
	mp := acquirem() // disable preemption because it can be holding p in a local var
	if status&^_Gscan != _Gwaiting {
		dumpgstatus(gp)
		throw("bad g->status in ready")
	}

	// status is Gwaiting or Gscanwaiting, make Grunnable and put on runq
	trace := traceAcquire()
	casgstatus(gp, _Gwaiting, _Grunnable)
	if trace.ok() {
		trace.GoUnpark(gp, traceskip)
		traceRelease(trace)
	}
	runqput(mp.p.ptr(), gp, next)
	wakep()
	releasem(mp)
}

// freezeStopWait is a large value that freezetheworld sets
// sched.stopwait to in order to request that all Gs permanently stop.
const freezeStopWait = 0x7fffffff

// freezing is set to non-zero if the runtime is trying to freeze the
// world.
var freezing atomic.Bool

// Similar to stopTheWorld but best-effort and can be called several times.
// There is no reverse operation, used during crashing.
// This function must not lock any mutexes.
func freezetheworld() {
	freezing.Store(true)
	if debug.dontfreezetheworld > 0 {
		// Don't prempt Ps to stop goroutines. That will perturb
		// scheduler state, making debugging more difficult. Instead,
		// allow goroutines to continue execution.
		//
		// fatalpanic will tracebackothers to trace all goroutines. It
		// is unsafe to trace a running goroutine, so tracebackothers
		// will skip running goroutines. That is OK and expected, we
		// expect users of dontfreezetheworld to use core files anyway.
		//
		// However, allowing the scheduler to continue running free
		// introduces a race: a goroutine may be stopped when
		// tracebackothers checks its status, and then start running
		// later when we are in the middle of traceback, potentially
		// causing a crash.
		//
		// To mitigate this, when an M naturally enters the scheduler,
		// schedule checks if freezing is set and if so stops
		// execution. This guarantees that while Gs can transition from
		// running to stopped, they can never transition from stopped
		// to running.
		//
		// The sleep here allows racing Ms that missed freezing and are
		// about to run a G to complete the transition to running
		// before we start traceback.
		usleep(1000)
		return
	}

	// stopwait and preemption requests can be lost
	// due to races with concurrently executing threads,
	// so try several times
	for i := 0; i < 5; i++ {
		// this should tell the scheduler to not start any new goroutines
		sched.stopwait = freezeStopWait
		sched.gcwaiting.Store(true)
		// this should stop running goroutines
		if !preemptall() {
			break // no running goroutines
		}
		usleep(1000)
	}
	// to be sure
	usleep(1000)
	preemptall()
	usleep(1000)
}

// All reads and writes of g's status go through readgstatus, casgstatus
// castogscanstatus, casfrom_Gscanstatus.
//
//go:nosplit
func readgstatus(gp *g) uint32 {
	return gp.atomicstatus.Load()
}

// The Gscanstatuses are acting like locks and this releases them.
// If it proves to be a performance hit we should be able to make these
// simple atomic stores but for now we are going to throw if
// we see an inconsistent state.
func casfrom_Gscanstatus(gp *g, oldval, newval uint32) {
	success := false

	// Check that transition is valid.
	switch oldval {
	default:
		print("runtime: casfrom_Gscanstatus bad oldval gp=", gp, ", oldval=", hex(oldval), ", newval=", hex(newval), "\n")
		dumpgstatus(gp)
		throw("casfrom_Gscanstatus:top gp->status is not in scan state")
	case _Gscanrunnable,
		_Gscanwaiting,
		_Gscanrunning,
		_Gscansyscall,
		_Gscanleaked,
		_Gscanpreempted,
		_Gscandeadextra:
		if newval == oldval&^_Gscan {
			success = gp.atomicstatus.CompareAndSwap(oldval, newval)
		}
	}
	if !success {
		print("runtime: casfrom_Gscanstatus failed gp=", gp, ", oldval=", hex(oldval), ", newval=", hex(newval), "\n")
		dumpgstatus(gp)
		throw("casfrom_Gscanstatus: gp->status is not in scan state")
	}
	releaseLockRankAndM(lockRankGscan)
}

// This will return false if the gp is not in the expected status and the cas fails.
// This acts like a lock acquire while the casfromgstatus acts like a lock release.
func castogscanstatus(gp *g, oldval, newval uint32) bool {
	switch oldval {
	case _Grunnable,
		_Grunning,
		_Gwaiting,
		_Gleaked,
		_Gsyscall,
		_Gdeadextra:
		if newval == oldval|_Gscan {
			r := gp.atomicstatus.CompareAndSwap(oldval, newval)
			if r {
				acquireLockRankAndM(lockRankGscan)
			}
			return r

		}
	}
	print("runtime: castogscanstatus oldval=", hex(oldval), " newval=", hex(newval), "\n")
	throw("bad oldval passed to castogscanstatus")
	return false
}

// casgstatusAlwaysTrack is a debug flag that causes casgstatus to always track
// various latencies on every transition instead of sampling them.
var casgstatusAlwaysTrack = false

// If asked to move to or from a Gscanstatus this will throw. Use the castogscanstatus
// and casfrom_Gscanstatus instead.
// casgstatus will loop if the g->atomicstatus is in a Gscan status until the routine that
// put it in the Gscan state is finished.
//
//go:nosplit
func casgstatus(gp *g, oldval, newval uint32) {
	if (oldval&_Gscan != 0) || (newval&_Gscan != 0) || oldval == newval {
		systemstack(func() {
			// Call on the systemstack to prevent print and throw from counting
			// against the nosplit stack reservation.
			print("runtime: casgstatus: oldval=", hex(oldval), " newval=", hex(newval), "\n")
			throw("casgstatus: bad incoming values")
		})
	}

	lockWithRankMayAcquire(nil, lockRankGscan)

	// See https://golang.org/cl/21503 for justification of the yield delay.
	const yieldDelay = 5 * 1000
	var nextYield int64

	// loop if gp->atomicstatus is in a scan state giving
	// GC time to finish and change the state to oldval.
	for i := 0; !gp.atomicstatus.CompareAndSwap(oldval, newval); i++ {
		if oldval == _Gwaiting && gp.atomicstatus.Load() == _Grunnable {
			systemstack(func() {
				// Call on the systemstack to prevent throw from counting
				// against the nosplit stack reservation.
				throw("casgstatus: waiting for Gwaiting but is Grunnable")
			})
		}
		if i == 0 {
			nextYield = nanotime() + yieldDelay
		}
		if nanotime() < nextYield {
			for x := 0; x < 10 && gp.atomicstatus.Load() != oldval; x++ {
				procyield(1)
			}
		} else {
			osyield()
			nextYield = nanotime() + yieldDelay/2
		}
	}

	if gp.bubble != nil {
		systemstack(func() {
			gp.bubble.changegstatus(gp, oldval, newval)
		})
	}

	if (oldval == _Grunning || oldval == _Gsyscall) && (newval != _Grunning && newval != _Gsyscall) {
		// Track every gTrackingPeriod time a goroutine transitions out of _Grunning or _Gsyscall.
		// Do not track _Grunning <-> _Gsyscall transitions, since they're two very similar states.
		if casgstatusAlwaysTrack || gp.trackingSeq%gTrackingPeriod == 0 {
			gp.tracking = true
		}
		gp.trackingSeq++
	}
	if !gp.tracking {
		return
	}

	// Handle various kinds of tracking.
	//
	// Currently:
	// - Time spent in runnable.
	// - Time spent blocked on a sync.Mutex or sync.RWMutex.
	switch oldval {
	case _Grunnable:
		// We transitioned out of runnable, so measure how much
		// time we spent in this state and add it to
		// runnableTime.
		now := nanotime()
		gp.runnableTime += now - gp.trackingStamp
		gp.trackingStamp = 0
	case _Gwaiting:
		if !gp.waitreason.isMutexWait() {
			// Not blocking on a lock.
			break
		}
		// Blocking on a lock, measure it. Note that because we're
		// sampling, we have to multiply by our sampling period to get
		// a more representative estimate of the absolute value.
		// gTrackingPeriod also represents an accurate sampling period
		// because we can only enter this state from _Grunning.
		now := nanotime()
		sched.totalMutexWaitTime.Add((now - gp.trackingStamp) * gTrackingPeriod)
		gp.trackingStamp = 0
	}
	switch newval {
	case _Gwaiting:
		if !gp.waitreason.isMutexWait() {
			// Not blocking on a lock.
			break
		}
		// Blocking on a lock. Write down the timestamp.
		now := nanotime()
		gp.trackingStamp = now
	case _Grunnable:
		// We just transitioned into runnable, so record what
		// time that happened.
		now := nanotime()
		gp.trackingStamp = now
	case _Grunning:
		// We're transitioning into running, so turn off
		// tracking and record how much time we spent in
		// runnable.
		gp.tracking = false
		sched.timeToRun.record(gp.runnableTime)
		gp.runnableTime = 0
	}
}

// casGToWaiting transitions gp from old to _Gwaiting, and sets the wait reason.
//
// Use this over casgstatus when possible to ensure that a waitreason is set.
func casGToWaiting(gp *g, old uint32, reason waitReason) {
	// Set the wait reason before calling casgstatus, because casgstatus will use it.
	gp.waitreason = reason
	casgstatus(gp, old, _Gwaiting)
}

// casGToWaitingForSuspendG transitions gp from old to _Gwaiting, and sets the wait reason.
// The wait reason must be a valid isWaitingForSuspendG wait reason.
//
// While a goroutine is in this state, it's stack is effectively pinned.
// The garbage collector must not shrink or otherwise mutate the goroutine's stack.
//
// Use this over casgstatus when possible to ensure that a waitreason is set.
func casGToWaitingForSuspendG(gp *g, old uint32, reason waitReason) {
	if !reason.isWaitingForSuspendG() {
		throw("casGToWaitingForSuspendG with non-isWaitingForSuspendG wait reason")
	}
	casGToWaiting(gp, old, reason)
}

// casGToPreemptScan transitions gp from _Grunning to _Gscan|_Gpreempted.
//
// TODO(austin): This is the only status operation that both changes
// the status and locks the _Gscan bit. Rethink this.
func casGToPreemptScan(gp *g, old, new uint32) {
	if old != _Grunning || new != _Gscan|_Gpreempted {
		throw("bad g transition")
	}
	acquireLockRankAndM(lockRankGscan)
	for !gp.atomicstatus.CompareAndSwap(_Grunning, _Gscan|_Gpreempted) {
	}
	// We never notify gp.bubble that the goroutine state has moved
	// from _Grunning to _Gpreempted. We call bubble.changegstatus
	// after status changes happen, but doing so here would violate the
	// ordering between the gscan and synctest locks. The bubble doesn't
	// distinguish between _Grunning and _Gpreempted anyway, so not
	// notifying it is fine.
}

// casGFromPreempted attempts to transition gp from _Gpreempted to
// _Gwaiting. If successful, the caller is responsible for
// re-scheduling gp.
func casGFromPreempted(gp *g, old, new uint32) bool {
	if old != _Gpreempted || new != _Gwaiting {
		throw("bad g transition")
	}
	gp.waitreason = waitReasonPreempted
	if !gp.atomicstatus.CompareAndSwap(_Gpreempted, _Gwaiting) {
		return false
	}
	if bubble := gp.bubble; bubble != nil {
		bubble.changegstatus(gp, _Gpreempted, _Gwaiting)
	}
	return true
}

// stwReason is an enumeration of reasons the world is stopping.
type stwReason uint8

// Reasons to stop-the-world.
//
// Avoid reusing reasons and add new ones instead.
const (
	stwUnknown                     stwReason = iota // "unknown"
	stwGCMarkTerm                                   // "GC mark termination"
	stwGCSweepTerm                                  // "GC sweep termination"
	stwWriteHeapDump                                // "write heap dump"
	stwGoroutineProfile                             // "goroutine profile"
	stwGoroutineProfileCleanup                      // "goroutine profile cleanup"
	stwAllGoroutinesStack                           // "all goroutines stack trace"
	stwReadMemStats                                 // "read mem stats"
	stwAllThreadsSyscall                            // "AllThreadsSyscall"
	stwGOMAXPROCS                                   // "GOMAXPROCS"
	stwStartTrace                                   // "start trace"
	stwStopTrace                                    // "stop trace"
	stwForTestCountPagesInUse                       // "CountPagesInUse (test)"
	stwForTestReadMetricsSlow                       // "ReadMetricsSlow (test)"
	stwForTestReadMemStatsSlow                      // "ReadMemStatsSlow (test)"
	stwForTestPageCachePagesLeaked                  // "PageCachePagesLeaked (test)"
	stwForTestResetDebugLog                         // "ResetDebugLog (test)"
)

func (r stwReason) String() string {
	return stwReasonStrings[r]
}

func (r stwReason) isGC() bool {
	return r == stwGCMarkTerm || r == stwGCSweepTerm
}

// If you add to this list, also add it to src/internal/trace/parser.go.
// If you change the values of any of the stw* constants, bump the trace
// version number and make a copy of this.
var stwReasonStrings = [...]string{
	stwUnknown:                     "unknown",
	stwGCMarkTerm:                  "GC mark termination",
	stwGCSweepTerm:                 "GC sweep termination",
	stwWriteHeapDump:               "write heap dump",
	stwGoroutineProfile:            "goroutine profile",
	stwGoroutineProfileCleanup:     "goroutine profile cleanup",
	stwAllGoroutinesStack:          "all goroutines stack trace",
	stwReadMemStats:                "read mem stats",
	stwAllThreadsSyscall:           "AllThreadsSyscall",
	stwGOMAXPROCS:                  "GOMAXPROCS",
	stwStartTrace:                  "start trace",
	stwStopTrace:                   "stop trace",
	stwForTestCountPagesInUse:      "CountPagesInUse (test)",
	stwForTestReadMetricsSlow:      "ReadMetricsSlow (test)",
	stwForTestReadMemStatsSlow:     "ReadMemStatsSlow (test)",
	stwForTestPageCachePagesLeaked: "PageCachePagesLeaked (test)",
	stwForTestResetDebugLog:        "ResetDebugLog (test)",
}

// worldStop provides context from the stop-the-world required by the
// start-the-world.
type worldStop struct {
	reason           stwReason
	startedStopping  int64
	finishedStopping int64
	stoppingCPUTime  int64
}

// Temporary variable for stopTheWorld, when it can't write to the stack.
//
// Protected by worldsema.
var stopTheWorldContext worldStop

// stopTheWorld stops all P's from executing goroutines, interrupting
// all goroutines at GC safe points and records reason as the reason
// for the stop. On return, only the current goroutine's P is running.
// stopTheWorld must not be called from a system stack and the caller
// must not hold worldsema. The caller must call startTheWorld when
// other P's should resume execution.
//
// stopTheWorld is safe for multiple goroutines to call at the
// same time. Each will execute its own stop, and the stops will
// be serialized.
//
// This is also used by routines that do stack dumps. If the system is
// in panic or being exited, this may not reliably stop all
// goroutines.
//
// Returns the STW context. When starting the world, this context must be
// passed to startTheWorld.
func stopTheWorld(reason stwReason) worldStop {
	semacquire(&worldsema)
	gp := getg()
	gp.m.preemptoff = reason.String()
	systemstack(func() {
		stopTheWorldContext = stopTheWorldWithSema(reason) // avoid write to stack
	})
	return stopTheWorldContext
}

// startTheWorld undoes the effects of stopTheWorld.
//
// w must be the worldStop returned by stopTheWorld.
func startTheWorld(w worldStop) {
	systemstack(func() { startTheWorldWithSema(0, w) })

	// worldsema must be held over startTheWorldWithSema to ensure
	// gomaxprocs cannot change while worldsema is held.
	//
	// Release worldsema with direct handoff to the next waiter, but
	// acquirem so that semrelease1 doesn't try to yield our time.
	//
	// Otherwise if e.g. ReadMemStats is being called in a loop,
	// it might stomp on other attempts to stop the world, such as
	// for starting or ending GC. The operation this blocks is
	// so heavy-weight that we should just try to be as fair as
	// possible here.
	//
	// We don't want to just allow us to get preempted between now
	// and releasing the semaphore because then we keep everyone
	// (including, for example, GCs) waiting longer.
	mp := acquirem()
	mp.preemptoff = ""
	semrelease1(&worldsema, true, 0)
	releasem(mp)
}

// stopTheWorldGC has the same effect as stopTheWorld, but blocks
// until the GC is not running. It also blocks a GC from starting
// until startTheWorldGC is called.
func stopTheWorldGC(reason stwReason) worldStop {
	semacquire(&gcsema)
	return stopTheWorld(reason)
}

// startTheWorldGC undoes the effects of stopTheWorldGC.
//
// w must be the worldStop returned by stopTheWorld.
func startTheWorldGC(w worldStop) {
	startTheWorld(w)
	semrelease(&gcsema)
}

// Holding worldsema grants an M the right to try to stop the world.
var worldsema uint32 = 1

// Holding gcsema grants the M the right to block a GC, and blocks
// until the current GC is done. In particular, it prevents gomaxprocs
// from changing concurrently.
//
// TODO(mknyszek): Once gomaxprocs and the execution tracer can handle
// being changed/enabled during a GC, remove this.
var gcsema uint32 = 1

// stopTheWorldWithSema is the core implementation of stopTheWorld.
// The caller is responsible for acquiring worldsema and disabling
// preemption first and then should stopTheWorldWithSema on the system
// stack:
//
//	semacquire(&worldsema, 0)
//	m.preemptoff = "reason"
//	var stw worldStop
//	systemstack(func() {
//		stw = stopTheWorldWithSema(reason)
//	})
//
// When finished, the caller must either call startTheWorld or undo
// these three operations separately:
//
//	m.preemptoff = ""
//	systemstack(func() {
//		now = startTheWorldWithSema(stw)
//	})
//	semrelease(&worldsema)
//
// It is allowed to acquire worldsema once and then execute multiple
// startTheWorldWithSema/stopTheWorldWithSema pairs.
// Other P's are able to execute between successive calls to
// startTheWorldWithSema and stopTheWorldWithSema.
// Holding worldsema causes any other goroutines invoking
// stopTheWorld to block.
//
// Returns the STW context. When starting the world, this context must be
// passed to startTheWorldWithSema.
//
//go:systemstack
func stopTheWorldWithSema(reason stwReason) worldStop {
	// Mark the goroutine which called stopTheWorld preemptible so its
	// stack may be scanned by the GC or observed by the execution tracer.
	//
	// This lets a mark worker scan us or the execution tracer take our
	// stack while we try to stop the world since otherwise we could get
	// in a mutual preemption deadlock.
	//
	// casGToWaitingForSuspendG marks the goroutine as ineligible for a
	// stack shrink, effectively pinning the stack in memory for the duration.
	//
	// N.B. The execution tracer is not aware of this status transition and
	// handles it specially based on the wait reason.
	casGToWaitingForSuspendG(getg().m.curg, _Grunning, waitReasonStoppingTheWorld)

	trace := traceAcquire()
	if trace.ok() {
		trace.STWStart(reason)
		traceRelease(trace)
	}
	gp := getg()

	// If we hold a lock, then we won't be able to stop another M
	// that is blocked trying to acquire the lock.
	if gp.m.locks > 0 {
		throw("stopTheWorld: holding locks")
	}

	lock(&sched.lock)
	start := nanotime() // exclude time waiting for sched.lock from start and total time metrics.
	sched.stopwait = gomaxprocs
	sched.gcwaiting.Store(true)
	preemptall()

	// Stop current P.
	gp.m.p.ptr().status = _Pgcstop // Pgcstop is only diagnostic.
	gp.m.p.ptr().gcStopTime = start
	sched.stopwait--

	// Try to retake all P's in syscalls.
	for _, pp := range allp {
		if thread, ok := setBlockOnExitSyscall(pp); ok {
			thread.gcstopP()
			thread.resume()
		}
	}

	// Stop idle Ps.
	now := nanotime()
	for {
		pp, _ := pidleget(now)
		if pp == nil {
			break
		}
		pp.status = _Pgcstop
		pp.gcStopTime = nanotime()
		sched.stopwait--
	}
	wait := sched.stopwait > 0
	unlock(&sched.lock)

	// Wait for remaining Ps to stop voluntarily.
	if wait {
		for {
			// wait for 100us, then try to re-preempt in case of any races
			if notetsleep(&sched.stopnote, 100*1000) {
				noteclear(&sched.stopnote)
				break
			}
			preemptall()
		}
	}

	finish := nanotime()
	startTime := finish - start
	if reason.isGC() {
		sched.stwStoppingTimeGC.record(startTime)
	} else {
		sched.stwStoppingTimeOther.record(startTime)
	}

	// Double-check we actually stopped everything, and all the invariants hold.
	// Also accumulate all the time spent by each P in _Pgcstop up to the point
	// where everything was stopped. This will be accumulated into the total pause
	// CPU time by the caller.
	stoppingCPUTime := int64(0)
	bad := ""
	if sched.stopwait != 0 {
		bad = "stopTheWorld: not stopped (stopwait != 0)"
	} else {
		for _, pp := range allp {
			if pp.status != _Pgcstop {
				bad = "stopTheWorld: not stopped (status != _Pgcstop)"
			}
			if pp.gcStopTime == 0 && bad == "" {
				bad = "stopTheWorld: broken CPU time accounting"
			}
			stoppingCPUTime += finish - pp.gcStopTime
			pp.gcStopTime = 0
		}
	}
	if freezing.Load() {
		// Some other thread is panicking. This can cause the
		// sanity checks above to fail if the panic happens in
		// the signal handler on a stopped thread. Either way,
		// we should halt this thread.
		lock(&deadlock)
		lock(&deadlock)
	}
	if bad != "" {
		throw(bad)
	}

	worldStopped()

	// Switch back to _Grunning, now that the world is stopped.
	casgstatus(getg().m.curg, _Gwaiting, _Grunning)

	return worldStop{
		reason:           reason,
		startedStopping:  start,
		finishedStopping: finish,
		stoppingCPUTime:  stoppingCPUTime,
	}
}

// reason is the same STW reason passed to stopTheWorld. start is the start
// time returned by stopTheWorld.
//
// now is the current time; prefer to pass 0 to capture a fresh timestamp.
//
// stattTheWorldWithSema returns now.
func startTheWorldWithSema(now int64, w worldStop) int64 {
	assertWorldStopped()

	mp := acquirem() // disable preemption because it can be holding p in a local var
	if netpollinited() {
		list, delta := netpoll(0) // non-blocking
		injectglist(&list)
		netpollAdjustWaiters(delta)
	}
	lock(&sched.lock)

	procs := gomaxprocs
	if newprocs != 0 {
		procs = newprocs
		newprocs = 0
	}
	p1 := procresize(procs)
	sched.gcwaiting.Store(false)
	if sched.sysmonwait.Load() {
		sched.sysmonwait.Store(false)
		notewakeup(&sched.sysmonnote)
	}
	unlock(&sched.lock)

	worldStarted()

	for p1 != nil {
		p := p1
		p1 = p1.link.ptr()
		if p.m != 0 {
			mp := p.m.ptr()
			p.m = 0
			if mp.nextp != 0 {
				throw("startTheWorld: inconsistent mp->nextp")
			}
			mp.nextp.set(p)
			notewakeup(&mp.park)
		} else {
			// Start M to run P.  Do not start another M below.
			newm(nil, p, -1)
		}
	}

	// Capture start-the-world time before doing clean-up tasks.
	if now == 0 {
		now = nanotime()
	}
	totalTime := now - w.startedStopping
	if w.reason.isGC() {
		sched.stwTotalTimeGC.record(totalTime)
	} else {
		sched.stwTotalTimeOther.record(totalTime)
	}
	trace := traceAcquire()
	if trace.ok() {
		trace.STWDone()
		traceRelease(trace)
	}

	// Wakeup an additional proc in case we have excessive runnable goroutines
	// in local queues or in the global queue. If we don't, the proc will park itself.
	// If we have lots of excessive work, resetspinning will unpark additional procs as necessary.
	wakep()

	releasem(mp)

	return now
}

// usesLibcall indicates whether this runtime performs system calls
// via libcall.
func usesLibcall() bool {
	switch GOOS {
	case "aix", "darwin", "illumos", "ios", "openbsd", "solaris", "windows":
		return true
	}
	return false
}

// mStackIsSystemAllocated indicates whether this runtime starts on a
// system-allocated stack.
func mStackIsSystemAllocated() bool {
	switch GOOS {
	case "aix", "darwin", "plan9", "illumos", "ios", "openbsd", "solaris", "windows":
		return true
	}
	return false
}

// mstart is the entry-point for new Ms.
// It is written in assembly, uses ABI0, is marked TOPFRAME, and calls mstart0.
func mstart()

// mstart0 is the Go entry-point for new Ms.
// This must not split the stack because we may not even have stack
// bounds set up yet.
//
// May run during STW (because it doesn't have a P yet), so write
// barriers are not allowed.
//
//go:nosplit
//go:nowritebarrierrec
func mstart0() {
	gp := getg()

	osStack := gp.stack.lo == 0
	if osStack {
		// Initialize stack bounds from system stack.
		// Cgo may have left stack size in stack.hi.
		// minit may update the stack bounds.
		//
		// Note: these bounds may not be very accurate.
		// We set hi to &size, but there are things above
		// it. The 1024 is supposed to compensate this,
		// but is somewhat arbitrary.
		size := gp.stack.hi
		if size == 0 {
			size = 16384 * sys.StackGuardMultiplier
		}
		gp.stack.hi = uintptr(noescape(unsafe.Pointer(&size)))
		gp.stack.lo = gp.stack.hi - size + 1024
	}
	// Initialize stack guard so that we can start calling regular
	// Go code.
	gp.stackguard0 = gp.stack.lo + stackGuard
	// This is the g0, so we can also call go:systemstack
	// functions, which check stackguard1.
	gp.stackguard1 = gp.stackguard0
	mstart1()

	// Exit this thread.
	if mStackIsSystemAllocated() {
		// Windows, Solaris, illumos, Darwin, AIX and Plan 9 always system-allocate
		// the stack, but put it in gp.stack before mstart,
		// so the logic above hasn't set osStack yet.
		osStack = true
	}
	mexit(osStack)
}

// The go:noinline is to guarantee the sys.GetCallerPC/sys.GetCallerSP below are safe,
// so that we can set up g0.sched to return to the call of mstart1 above.
//
//go:noinline
func mstart1() {
	gp := getg()

	if gp != gp.m.g0 {
		throw("bad runtime·mstart")
	}

	// Set up m.g0.sched as a label returning to just
	// after the mstart1 call in mstart0 above, for use by goexit0 and mcall.
	// We're never coming back to mstart1 after we call schedule,
	// so other calls can reuse the current frame.
	// And goexit0 does a gogo that needs to return from mstart1
	// and let mstart0 exit the thread.
	gp.sched.g = guintptr(unsafe.Pointer(gp))
	gp.sched.pc = sys.GetCallerPC()
	gp.sched.sp = sys.GetCallerSP()

	asminit()
	minit()

	// Install signal handlers; after minit so that minit can
	// prepare the thread to be able to handle the signals.
	if gp.m == &m0 {
		mstartm0()
	}

	if debug.dataindependenttiming == 1 {
		sys.EnableDIT()
	}

	if fn := gp.m.mstartfn; fn != nil {
		fn()
	}

	if gp.m != &m0 {
		acquirep(gp.m.nextp.ptr())
		gp.m.nextp = 0
	}
	schedule()
}

// mstartm0 implements part of mstart1 that only runs on the m0.
//
// Write barriers are allowed here because we know the GC can't be
// running yet, so they'll be no-ops.
//
//go:yeswritebarrierrec
func mstartm0() {
	// Create an extra M for callbacks on threads not created by Go.
	// An extra M is also needed on Windows for callbacks created by
	// syscall.NewCallback. See issue #6751 for details.
	if (iscgo || GOOS == "windows") && !cgoHasExtraM {
		cgoHasExtraM = true
		newextram()
	}
	initsig(false)
}

// mPark causes a thread to park itself, returning once woken.
//
//go:nosplit
func mPark() {
	gp := getg()
	// This M might stay parked through an entire GC cycle.
	// Erase any leftovers on the signal stack.
	if goexperiment.RuntimeSecret {
		eraseSecretsSignalStk()
	}
	notesleep(&gp.m.park)
	noteclear(&gp.m.park)
}

// mexit tears down and exits the current thread.
//
// Don't call this directly to exit the thread, since it must run at
// the top of the thread stack. Instead, use gogo(&gp.m.g0.sched) to
// unwind the stack to the point that exits the thread.
//
// It is entered with m.p != nil, so write barriers are allowed. It
// will release the P before exiting.
//
//go:yeswritebarrierrec
func mexit(osStack bool) {
	mp := getg().m

	if mp == &m0 {
		// This is the main thread. Just wedge it.
		//
		// On Linux, exiting the main thread puts the process
		// into a non-waitable zombie state. On Plan 9,
		// exiting the main thread unblocks wait even though
		// other threads are still running. On Solaris we can
		// neither exitThread nor return from mstart. Other
		// bad things probably happen on other platforms.
		//
		// We could try to clean up this M more before wedging
		// it, but that complicates signal handling.
		handoffp(releasep())
		lock(&sched.lock)
		sched.nmfreed++
		checkdead()
		unlock(&sched.lock)
		mPark()
		throw("locked m0 woke up")
	}

	sigblock(true)
	unminit()

	// Free the gsignal stack.
	if mp.gsignal != nil {
		stackfree(mp.gsignal.stack)
		if valgrindenabled {
			valgrindDeregisterStack(mp.gsignal.valgrindStackID)
			mp.gsignal.valgrindStackID = 0
		}
		// On some platforms, when calling into VDSO (e.g. nanotime)
		// we store our g on the gsignal stack, if there is one.
		// Now the stack is freed, unlink it from the m, so we
		// won't write to it when calling VDSO code.
		mp.gsignal = nil
	}

	// Free vgetrandom state.
	vgetrandomDestroy(mp)

	// Clear the self pointer so Ps don't access this M after it is freed,
	// or keep it alive.
	mp.self.clear()

	// Remove m from allm.
	lock(&sched.lock)
	for pprev := &allm; *pprev != nil; pprev = &(*pprev).alllink {
		if *pprev == mp {
			*pprev = mp.alllink
			goto found
		}
	}
	throw("m not found in allm")
found:
	// Events must not be traced after this point.

	// Delay reaping m until it's done with the stack.
	//
	// Put mp on the free list, though it will not be reaped while freeWait
	// is freeMWait. mp is no longer reachable via allm, so even if it is
	// on an OS stack, we must keep a reference to mp alive so that the GC
	// doesn't free mp while we are still using it.
	//
	// Note that the free list must not be linked through alllink because
	// some functions walk allm without locking, so may be using alllink.
	//
	// N.B. It's important that the M appears on the free list simultaneously
	// with it being removed so that the tracer can find it.
	mp.freeWait.Store(freeMWait)
	mp.freelink = sched.freem
	sched.freem = mp
	unlock(&sched.lock)

	atomic.Xadd64(&ncgocall, int64(mp.ncgocall))
	sched.totalRuntimeLockWaitTime.Add(mp.mLockProfile.waitTime.Load())

	// Release the P.
	handoffp(releasep())
	// After this point we must not have write barriers.

	// Invoke the deadlock detector. This must happen after
	// handoffp because it may have started a new M to take our
	// P's work.
	lock(&sched.lock)
	sched.nmfreed++
	checkdead()
	unlock(&sched.lock)

	if GOOS == "darwin" || GOOS == "ios" {
		// Make sure pendingPreemptSignals is correct when an M exits.
		// For #41702.
		if mp.signalPending.Load() != 0 {
			pendingPreemptSignals.Add(-1)
		}
	}

	// Destroy all allocated resources. After this is called, we may no
	// longer take any locks.
	mdestroy(mp)

	if osStack {
		// No more uses of mp, so it is safe to drop the reference.
		mp.freeWait.Store(freeMRef)

		// Return from mstart and let the system thread
		// library free the g0 stack and terminate the thread.
		return
	}

	// mstart is the thread's entry point, so there's nothing to
	// return to. Exit the thread directly. exitThread will clear
	// m.freeWait when it's done with the stack and the m can be
	// reaped.
	exitThread(&mp.freeWait)
}

// forEachP calls fn(p) for every P p when p reaches a GC safe point.
// If a P is currently executing code, this will bring the P to a GC
// safe point and execute fn on that P. If the P is not executing code
// (it is idle or in a syscall), this will call fn(p) directly while
// preventing the P from exiting its state. This does not ensure that
// fn will run on every CPU executing Go code, but it acts as a global
// memory barrier. GC uses this as a "ragged barrier."
//
// The caller must hold worldsema. fn must not refer to any
// part of the current goroutine's stack, since the GC may move it.
func forEachP(reason waitReason, fn func(*p)) {
	systemstack(func() {
		gp := getg().m.curg
		// Mark the user stack as preemptible so that it may be scanned
		// by the GC or observed by the execution tracer. Otherwise, our
		// attempt to force all P's to a safepoint could result in a
		// deadlock as we attempt to preempt a goroutine that's trying
		// to preempt us (e.g. for a stack scan).
		//
		// casGToWaitingForSuspendG marks the goroutine as ineligible for a
		// stack shrink, effectively pinning the stack in memory for the duration.
		//
		// N.B. The execution tracer is not aware of this status transition and
		// handles it specially based on the wait reason.
		casGToWaitingForSuspendG(gp, _Grunning, reason)
		forEachPInternal(fn)
		casgstatus(gp, _Gwaiting, _Grunning)
	})
}

// forEachPInternal calls fn(p) for every P p when p reaches a GC safe point.
// It is the internal implementation of forEachP.
//
// The caller must hold worldsema and either must ensure that a GC is not
// running (otherwise this may deadlock with the GC trying to preempt this P)
// or it must leave its goroutine in a preemptible state before it switches
// to the systemstack. Due to these restrictions, prefer forEachP when possible.
//
//go:systemstack
func forEachPInternal(fn func(*p)) {
	mp := acquirem()
	pp := getg().m.p.ptr()

	lock(&sched.lock)
	if sched.safePointWait != 0 {
		throw("forEachP: sched.safePointWait != 0")
	}
	sched.safePointWait = gomaxprocs - 1
	sched.safePointFn = fn

	// Ask all Ps to run the safe point function.
	for _, p2 := range allp {
		if p2 != pp {
			atomic.Store(&p2.runSafePointFn, 1)
		}
	}
	preemptall()

	// Any P entering _Pidle or a system call from now on will observe
	// p.runSafePointFn == 1 and will call runSafePointFn when
	// changing its status to _Pidle.

	// Run safe point function for all idle Ps. sched.pidle will
	// not change because we hold sched.lock.
	for p := sched.pidle.ptr(); p != nil; p = p.link.ptr() {
		if atomic.Cas(&p.runSafePointFn, 1, 0) {
			fn(p)
			sched.safePointWait--
		}
	}

	wait := sched.safePointWait > 0
	unlock(&sched.lock)

	// Run fn for the current P.
	fn(pp)

	// Force Ps currently in a system call into _Pidle and hand them
	// off to induce safe point function execution.
	for _, p2 := range allp {
		if atomic.Load(&p2.runSafePointFn) != 1 {
			// Already ran it.
			continue
		}
		if thread, ok := setBlockOnExitSyscall(p2); ok {
			thread.takeP()
			thread.resume()
			handoffp(p2)
		}
	}

	// Wait for remaining Ps to run fn.
	if wait {
		for {
			// Wait for 100us, then try to re-preempt in
			// case of any races.
			//
			// Requires system stack.
			if notetsleep(&sched.safePointNote, 100*1000) {
				noteclear(&sched.safePointNote)
				break
			}
			preemptall()
		}
	}
	if sched.safePointWait != 0 {
		throw("forEachP: not done")
	}
	for _, p2 := range allp {
		if p2.runSafePointFn != 0 {
			throw("forEachP: P did not run fn")
		}
	}

	lock(&sched.lock)
	sched.safePointFn = nil
	unlock(&sched.lock)
	releasem(mp)
}

// runSafePointFn runs the safe point function, if any, for this P.
// This should be called like
//
//	if getg().m.p.runSafePointFn != 0 {
//	    runSafePointFn()
//	}
//
// runSafePointFn must be checked on any transition in to _Pidle or
// when entering a system call to avoid a race where forEachP sees
// that the P is running just before the P goes into _Pidle/system call
// and neither forEachP nor the P run the safe-point function.
func runSafePointFn() {
	p := getg().m.p.ptr()
	// Resolve the race between forEachP running the safe-point
	// function on this P's behalf and this P running the
	// safe-point function directly.
	if !atomic.Cas(&p.runSafePointFn, 1, 0) {
		return
	}
	sched.safePointFn(p)
	lock(&sched.lock)
	sched.safePointWait--
	if sched.safePointWait == 0 {
		notewakeup(&sched.safePointNote)
	}
	unlock(&sched.lock)
}

// When running with cgo, we call _cgo_thread_start
// to start threads for us so that we can play nicely with
// foreign code.
var cgoThreadStart unsafe.Pointer

type cgothreadstart struct {
	g   guintptr
	tls *uint64
	fn  unsafe.Pointer
}

// Allocate a new m unassociated with any thread.
// Can use p for allocation context if needed.
// fn is recorded as the new m's m.mstartfn.
// id is optional pre-allocated m ID. Omit by passing -1.
//
// This function is allowed to have write barriers even if the caller
// isn't because it borrows pp.
//
//go:yeswritebarrierrec
func allocm(pp *p, fn func(), id int64) *m {
	allocmLock.rlock()

	// The caller owns pp, but we may borrow (i.e., acquirep) it. We must
	// disable preemption to ensure it is not stolen, which would make the
	// caller lose ownership.
	acquirem()

	gp := getg()
	if gp.m.p == 0 {
		acquirep(pp) // temporarily borrow p for mallocs in this function
	}

	// Release the free M list. We need to do this somewhere and
	// this may free up a stack we can use.
	if sched.freem != nil {
		lock(&sched.lock)
		var newList *m
		for freem := sched.freem; freem != nil; {
			// Wait for freeWait to indicate that freem's stack is unused.
			wait := freem.freeWait.Load()
			if wait == freeMWait {
				next := freem.freelink
				freem.freelink = newList
				newList = freem
				freem = next
				continue
			}
			// Drop any remaining trace resources.
			// Ms can continue to emit events all the way until wait != freeMWait,
			// so it's only safe to call traceThreadDestroy at this point.
			if traceEnabled() || traceShuttingDown() {
				traceThreadDestroy(freem)
			}
			// Free the stack if needed. For freeMRef, there is
			// nothing to do except drop freem from the sched.freem
			// list.
			if wait == freeMStack {
				// stackfree must be on the system stack, but allocm is
				// reachable off the system stack transitively from
				// startm.
				systemstack(func() {
					stackfree(freem.g0.stack)
					if valgrindenabled {
						valgrindDeregisterStack(freem.g0.valgrindStackID)
						freem.g0.valgrindStackID = 0
					}
				})
			}
			freem = freem.freelink
		}
		sched.freem = newList
		unlock(&sched.lock)
	}

	mp := &new(mPadded).m
	mp.mstartfn = fn
	mcommoninit(mp, id)

	// In case of cgo or Solaris or illumos or Darwin, pthread_create will make us a stack.
	// Windows and Plan 9 will layout sched stack on OS stack.
	if iscgo || mStackIsSystemAllocated() {
		mp.g0 = malg(-1)
	} else {
		mp.g0 = malg(16384 * sys.StackGuardMultiplier)
	}
	mp.g0.m = mp

	if pp == gp.m.p.ptr() {
		releasep()
	}

	releasem(gp.m)
	allocmLock.runlock()
	return mp
}

// needm is called when a cgo callback happens on a
// thread without an m (a thread not created by Go).
// In this case, needm is expected to find an m to use
// and return with m, g initialized correctly.
// Since m and g are not set now (likely nil, but see below)
// needm is limited in what routines it can call. In particular
// it can only call nosplit functions (textflag 7) and cannot
// do any scheduling that requires an m.
//
// In order to avoid needing heavy lifting here, we adopt
// the following strategy: there is a stack of available m's
// that can be stolen. Using compare-and-swap
// to pop from the stack has ABA races, so we simulate
// a lock by doing an exchange (via Casuintptr) to steal the stack
// head and replace the top pointer with MLOCKED (1).
// This serves as a simple spin lock that we can use even
// without an m. The thread that locks the stack in this way
// unlocks the stack by storing a valid stack head pointer.
//
// In order to make sure that there is always an m structure
// available to be stolen, we maintain the invariant that there
// is always one more than needed. At the beginning of the
// program (if cgo is in use) the list is seeded with a single m.
// If needm finds that it has taken the last m off the list, its job
// is - once it has installed its own m so that it can do things like
// allocate memory - to create a spare m and put it on the list.
//
// Each of these extra m's also has a g0 and a curg that are
// pressed into service as the scheduling stack and current
// goroutine for the duration of the cgo callback.
//
// It calls dropm to put the m back on the list,
// 1. when the callback is done with the m in non-pthread platforms,
// 2. or when the C thread exiting on pthread platforms.
//
// The signal argument indicates whether we're called from a signal
// handler.
//
//go:nosplit
func needm(signal bool) {
	if (iscgo || GOOS == "windows") && !cgoHasExtraM {
		// Can happen if C/C++ code calls Go from a global ctor.
		// Can also happen on Windows if a global ctor uses a
		// callback created by syscall.NewCallback. See issue #6751
		// for details.
		//
		// Can not throw, because scheduler is not initialized yet.
		writeErrStr("fatal error: cgo callback before cgo call\n")
		exit(1)
	}

	// Save and block signals before getting an M.
	// The signal handler may call needm itself,
	// and we must avoid a deadlock. Also, once g is installed,
	// any incoming signals will try to execute,
	// but we won't have the sigaltstack settings and other data
	// set up appropriately until the end of minit, which will
	// unblock the signals. This is the same dance as when
	// starting a new m to run Go code via newosproc.
	var sigmask sigset
	sigsave(&sigmask)
	sigblock(false)

	// getExtraM is safe here because of the invariant above,
	// that the extra list always contains or will soon contain
	// at least one m.
	mp, last := getExtraM()

	// Set needextram when we've just emptied the list,
	// so that the eventual call into cgocallbackg will
	// allocate a new m for the extra list. We delay the
	// allocation until then so that it can be done
	// after exitsyscall makes sure it is okay to be
	// running at all (that is, there's no garbage collection
	// running right now).
	mp.needextram = last

	// Store the original signal mask for use by minit.
	mp.sigmask = sigmask

	// Install TLS on some platforms (previously setg
	// would do this if necessary).
	osSetupTLS(mp)

	// Install g (= m->g0) and set the stack bounds
	// to match the current stack.
	setg(mp.g0)
	sp := sys.GetCallerSP()
	callbackUpdateSystemStack(mp, sp, signal)

	// We must mark that we are already in Go now.
	// Otherwise, we may call needm again when we get a signal, before cgocallbackg1,
	// which means the extram list may be empty, that will cause a deadlock.
	mp.isExtraInC = false

	// Initialize this thread to use the m.
	asminit()
	minit()

	// Emit a trace event for this dead -> syscall transition,
	// but only if we're not in a signal handler.
	//
	// N.B. the tracer can run on a bare M just fine, we just have
	// to make sure to do this before setg(nil) and unminit.
	var trace traceLocker
	if !signal {
		trace = traceAcquire()
	}

	// mp.curg is now a real goroutine.
	casgstatus(mp.curg, _Gdeadextra, _Gsyscall)
	sched.ngsys.Add(-1)

	// This is technically inaccurate, but we set isExtraInC to false above,
	// and so we need to update addGSyscallNoP to keep the two pieces of state
	// consistent (it's only updated when isExtraInC is false). More specifically,
	// When we get to cgocallbackg and exitsyscall, we'll be looking for a P, and
	// since isExtraInC is false, we will decrement this metric.
	//
	// The inaccuracy is thankfully transient: only until this thread can get a P.
	// We're going into Go anyway, so it's okay to pretend we're a real goroutine now.
	addGSyscallNoP(mp)

	if !signal {
		if trace.ok() {
			trace.GoCreateSyscall(mp.curg)
			traceRelease(trace)
		}
	}
	mp.isExtraInSig = signal
}

// Acquire an extra m and bind it to the C thread when a pthread key has been created.
//
//go:nosplit
func needAndBindM() {
	needm(false)

	if _cgo_pthread_key_created != nil && *(*uintptr)(_cgo_pthread_key_created) != 0 {
		cgoBindM()
	}
}

// newextram allocates m's and puts them on the extra list.
// It is called with a working local m, so that it can do things
// like call schedlock and allocate.
func newextram() {
	c := extraMWaiters.Swap(0)
	if c > 0 {
		for i := uint32(0); i < c; i++ {
			oneNewExtraM()
		}
	} else if extraMLength.Load() == 0 {
		// Make sure there is at least one extra M.
		oneNewExtraM()
	}
}

// oneNewExtraM allocates an m and puts it on the extra list.
func oneNewExtraM() {
	// Create extra goroutine locked to extra m.
	// The goroutine is the context in which the cgo callback will run.
	// The sched.pc will never be returned to, but setting it to
	// goexit makes clear to the traceback routines where
	// the goroutine stack ends.
	mp := allocm(nil, nil, -1)
	gp := malg(4096)
	gp.sched.pc = abi.FuncPCABI0(goexit) + sys.PCQuantum
	gp.sched.sp = gp.stack.hi
	gp.sched.sp -= 4 * goarch.PtrSize // extra space in case of reads slightly beyond frame
	gp.sched.lr = 0
	gp.sched.g = guintptr(unsafe.Pointer(gp))
	gp.syscallpc = gp.sched.pc
	gp.syscallsp = gp.sched.sp
	gp.stktopsp = gp.sched.sp
	// malg returns status as _Gidle. Change to _Gdeadextra before
	// adding to allg where GC can see it. _Gdeadextra hides this
	// from traceback and stack scans.
	casgstatus(gp, _Gidle, _Gdeadextra)
	gp.m = mp
	mp.curg = gp
	mp.isextra = true
	// mark we are in C by default.
	mp.isExtraInC = true
	mp.lockedInt++
	mp.lockedg.set(gp)
	gp.lockedm.set(mp)
	gp.goid = sched.goidgen.Add(1)
	if raceenabled {
		gp.racectx = racegostart(abi.FuncPCABIInternal(newextram) + sys.PCQuantum)
	}
	// put on allg for garbage collector
	allgadd(gp)

	// gp is now on the allg list, but we don't want it to be
	// counted by gcount. It would be more "proper" to increment
	// sched.ngfree, but that requires locking. Incrementing ngsys
	// has the same effect.
	sched.ngsys.Add(1)

	// Add m to the extra list.
	addExtraM(mp)
}

// dropm puts the current m back onto the extra list.
//
// 1. On systems without pthreads, like Windows
// dropm is called when a cgo callback has called needm but is now
// done with the callback and returning back into the non-Go thread.
//
// The main expense here is the call to signalstack to release the
// m's signal stack, and then the call to needm on the next callback
// from this thread. It is tempting to try to save the m for next time,
// which would eliminate both these costs, but there might not be
// a next time: the current thread (which Go does not control) might exit.
// If we saved the m for that thread, there would be an m leak each time
// such a thread exited. Instead, we acquire and release an m on each
// call. These should typically not be scheduling operations, just a few
// atomics, so the cost should be small.
//
// 2. On systems with pthreads
// dropm is called while a non-Go thread is exiting.
// We allocate a pthread per-thread variable using pthread_key_create,
// to register a thread-exit-time destructor.
// And store the g into a thread-specific value associated with the pthread key,
// when first return back to C.
// So that the destructor would invoke dropm while the non-Go thread is exiting.
// This is much faster since it avoids expensive signal-related syscalls.
//
// This may run without a P, so //go:nowritebarrierrec is required.
//
// This may run with a different stack than was recorded in g0 (there is no
// call to callbackUpdateSystemStack prior to dropm), so this must be
// //go:nosplit to avoid the stack bounds check.
//
//go:nowritebarrierrec
//go:nosplit
func dropm() {
	// Clear m and g, and return m to the extra list.
	// After the call to setg we can only call nosplit functions
	// with no pointer manipulation.
	mp := getg().m

	// Emit a trace event for this syscall -> dead transition.
	//
	// N.B. the tracer can run on a bare M just fine, we just have
	// to make sure to do this before setg(nil) and unminit.
	var trace traceLocker
	if !mp.isExtraInSig {
		trace = traceAcquire()
	}

	// Return mp.curg to _Gdeadextra state.
	casgstatus(mp.curg, _Gsyscall, _Gdeadextra)
	mp.curg.preemptStop = false
	sched.ngsys.Add(1)
	decGSyscallNoP(mp)

	if !mp.isExtraInSig {
		if trace.ok() {
			trace.GoDestroySyscall()
			traceRelease(trace)
		}
	}

	// Trash syscalltick so that it doesn't line up with mp.old.syscalltick anymore.
	//
	// In the new tracer, we model needm and dropm and a goroutine being created and
	// destroyed respectively. The m then might get reused with a different procid but
	// still with a reference to oldp, and still with the same syscalltick. The next
	// time a G is "created" in needm, it'll return and quietly reacquire its P from a
	// different m with a different procid, which will confuse the trace parser. By
	// trashing syscalltick, we ensure that it'll appear as if we lost the P to the
	// tracer parser and that we just reacquired it.
	//
	// Trash the value by decrementing because that gets us as far away from the value
	// the syscall exit code expects as possible. Setting to zero is risky because
	// syscalltick could already be zero (and in fact, is initialized to zero).
	mp.syscalltick--

	// Reset trace state unconditionally. This goroutine is being 'destroyed'
	// from the perspective of the tracer.
	mp.curg.trace.reset()

	// Flush all the M's buffers. This is necessary because the M might
	// be used on a different thread with a different procid, so we have
	// to make sure we don't write into the same buffer.
	if traceEnabled() || traceShuttingDown() {
		// Acquire sched.lock across thread destruction. One of the invariants of the tracer
		// is that a thread cannot disappear from the tracer's view (allm or freem) without
		// it noticing, so it requires that sched.lock be held over traceThreadDestroy.
		//
		// This isn't strictly necessary in this case, because this thread never leaves allm,
		// but the critical section is short and dropm is rare on pthread platforms, so just
		// take the lock and play it safe. traceThreadDestroy also asserts that the lock is held.
		lock(&sched.lock)
		traceThreadDestroy(mp)
		unlock(&sched.lock)
	}
	mp.isExtraInSig = false

	// Block signals before unminit.
	// Unminit unregisters the signal handling stack (but needs g on some systems).
	// Setg(nil) clears g, which is the signal handler's cue not to run Go handlers.
	// It's important not to try to handle a signal between those two steps.
	sigmask := mp.sigmask
	sigblock(false)
	unminit()

	setg(nil)

	// Clear g0 stack bounds to ensure that needm always refreshes the
	// bounds when reusing this M.
	g0 := mp.g0
	g0.stack.hi = 0
	g0.stack.lo = 0
	g0.stackguard0 = 0
	g0.stackguard1 = 0
	mp.g0StackAccurate = false

	putExtraM(mp)

	msigrestore(sigmask)
}

// bindm store the g0 of the current m into a thread-specific value.
//
// We allocate a pthread per-thread variable using pthread_key_create,
// to register a thread-exit-time destructor.
// We are here setting the thread-specific value of the pthread key, to enable the destructor.
// So that the pthread_key_destructor would dropm while the C thread is exiting.
//
// And the saved g will be used in pthread_key_destructor,
// since the g stored in the TLS by Go might be cleared in some platforms,
// before the destructor invoked, so, we restore g by the stored g, before dropm.
//
// We store g0 instead of m, to make the assembly code simpler,
// since we need to restore g0 in runtime.cgocallback.
//
// On systems without pthreads, like Windows, bindm shouldn't be used.
//
// NOTE: this always runs without a P, so, nowritebarrierrec required.
//
//go:nosplit
//go:nowritebarrierrec
func cgoBindM() {
	if GOOS == "windows" || GOOS == "plan9" {
		fatal("bindm in unexpected GOOS")
	}
	g := getg()
	if g.m.g0 != g {
		fatal("the current g is not g0")
	}
	if _cgo_bindm != nil {
		asmcgocall(_cgo_bindm, unsafe.Pointer(g))
	}
}

// A helper function for EnsureDropM.
//
// getm should be an internal detail,
// but widely used packages access it using linkname.
// Notable members of the hall of shame include:
//   - fortio.org/log
//
// Do not remove or change the type signature.
// See go.dev/issue/67401.
//
//go:linkname getm
func getm() uintptr {
	return uintptr(unsafe.Pointer(getg().m))
}

var (
	// Locking linked list of extra M's, via mp.schedlink. Must be accessed
	// only via lockextra/unlockextra.
	//
	// Can't be atomic.Pointer[m] because we use an invalid pointer as a
	// "locked" sentinel value. M's on this list remain visible to the GC
	// because their mp.curg is on allgs.
	extraM atomic.Uintptr
	// Number of M's in the extraM list.
	extraMLength atomic.Uint32
	// Number of waiters in lockextra.
	extraMWaiters atomic.Uint32

	// Number of extra M's in use by threads.
	extraMInUse atomic.Uint32
)

// lockextra locks the extra list and returns the list head.
// The caller must unlock the list by storing a new list head
// to extram. If nilokay is true, then lockextra will
// return a nil list head if that's what it finds. If nilokay is false,
// lockextra will keep waiting until the list head is no longer nil.
//
//go:nosplit
func lockextra(nilokay bool) *m {
	const locked = 1

	incr := false
	for {
		old := extraM.Load()
		if old == locked {
			osyield_no_g()
			continue
		}
		if old == 0 && !nilokay {
			if !incr {
				// Add 1 to the number of threads
				// waiting for an M.
				// This is cleared by newextram.
				extraMWaiters.Add(1)
				incr = true
			}
			usleep_no_g(1)
			continue
		}
		if extraM.CompareAndSwap(old, locked) {
			return (*m)(unsafe.Pointer(old))
		}
		osyield_no_g()
		continue
	}
}

//go:nosplit
func unlockextra(mp *m, delta int32) {
	extraMLength.Add(delta)
	extraM.Store(uintptr(unsafe.Pointer(mp)))
}

// Return an M from the extra M list. Returns last == true if the list becomes
// empty because of this call.
//
// Spins waiting for an extra M, so caller must ensure that the list always
// contains or will soon contain at least one M.
//
//go:nosplit
func getExtraM() (mp *m, last bool) {
	mp = lockextra(false)
	extraMInUse.Add(1)
	unlockextra(mp.schedlink.ptr(), -1)
	return mp, mp.schedlink.ptr() == nil
}

// Returns an extra M back to the list. mp must be from getExtraM. Newly
// allocated M's should use addExtraM.
//
//go:nosplit
func putExtraM(mp *m) {
	extraMInUse.Add(-1)
	addExtraM(mp)
}

// Adds a newly allocated M to the extra M list.
//
//go:nosplit
func addExtraM(mp *m) {
	mnext := lockextra(true)
	mp.schedlink.set(mnext)
	unlockextra(mp, 1)
}

var (
	// allocmLock is locked for read when creating new Ms in allocm and their
	// addition to allm. Thus acquiring this lock for write blocks the
	// creation of new Ms.
	allocmLock rwmutex

	// execLock serializes exec and clone to avoid bugs or unspecified
	// behaviour around exec'ing while creating/destroying threads. See
	// issue #19546.
	execLock rwmutex
)

// These errors are reported (via writeErrStr) by some OS-specific
// versions of newosproc and newosproc0.
const (
	failthreadcreate  = "runtime: failed to create new OS thread\n"
	failallocatestack = "runtime: failed to allocate stack for the new OS thread\n"
)

// newmHandoff contains a list of m structures that need new OS threads.
// This is used by newm in situations where newm itself can't safely
// start an OS thread.
var newmHandoff struct {
	lock mutex

	// newm points to a list of M structures that need new OS
	// threads. The list is linked through m.schedlink.
	newm muintptr

	// waiting indicates that wake needs to be notified when an m
	// is put on the list.
	waiting bool
	wake    note

	// haveTemplateThread indicates that the templateThread has
	// been started. This is not protected by lock. Use cas to set
	// to 1.
	haveTemplateThread uint32
}

// Create a new m. It will start off with a call to fn, or else the scheduler.
// fn needs to be static and not a heap allocated closure.
// May run with m.p==nil, so write barriers are not allowed.
//
// id is optional pre-allocated m ID. Omit by passing -1.
//
//go:nowritebarrierrec
func newm(fn func(), pp *p, id int64) {
	// allocm adds a new M to allm, but they do not start until created by
	// the OS in newm1 or the template thread.
	//
	// doAllThreadsSyscall requires that every M in allm will eventually
	// start and be signal-able, even with a STW.
	//
	// Disable preemption here until we start the thread to ensure that
	// newm is not preempted between allocm and starting the new thread,
	// ensuring that anything added to allm is guaranteed to eventually
	// start.
	acquirem()

	mp := allocm(pp, fn, id)
	mp.nextp.set(pp)
	mp.sigmask = initSigmask
	if gp := getg(); gp != nil && gp.m != nil && (gp.m.lockedExt != 0 || gp.m.incgo) && GOOS != "plan9" {
		// We're on a locked M or a thread that may have been
		// started by C. The kernel state of this thread may
		// be strange (the user may have locked it for that
		// purpose). We don't want to clone that into another
		// thread. Instead, ask a known-good thread to create
		// the thread for us.
		//
		// This is disabled on Plan 9. See golang.org/issue/22227.
		//
		// TODO: This may be unnecessary on Windows, which
		// doesn't model thread creation off fork.
		lock(&newmHandoff.lock)
		if newmHandoff.haveTemplateThread == 0 {
			throw("on a locked thread with no template thread")
		}
		mp.schedlink = newmHandoff.newm
		newmHandoff.newm.set(mp)
		if newmHandoff.waiting {
			newmHandoff.waiting = false
			notewakeup(&newmHandoff.wake)
		}
		unlock(&newmHandoff.lock)
		// The M has not started yet, but the template thread does not
		// participate in STW, so it will always process queued Ms and
		// it is safe to releasem.
		releasem(getg().m)
		return
	}
	newm1(mp)
	releasem(getg().m)
}

func newm1(mp *m) {
	if iscgo && _cgo_thread_start != nil {
		var ts cgothreadstart
		ts.g.set(mp.g0)
		ts.tls = (*uint64)(unsafe.Pointer(&mp.tls[0]))
		ts.fn = unsafe.Pointer(abi.FuncPCABI0(mstart))
		if msanenabled {
			msanwrite(unsafe.Pointer(&ts), unsafe.Sizeof(ts))
		}
		if asanenabled {
			asanwrite(unsafe.Pointer(&ts), unsafe.Sizeof(ts))
		}
		execLock.rlock() // Prevent process clone.
		asmcgocall(_cgo_thread_start, unsafe.Pointer(&ts))
		execLock.runlock()
		return
	}
	execLock.rlock() // Prevent process clone.
	newosproc(mp)
	execLock.runlock()
}

// startTemplateThread starts the template thread if it is not already
// running.
//
// The calling thread must itself be in a known-good state.
func startTemplateThread() {
	if GOARCH == "wasm" { // no threads on wasm yet
		return
	}

	// Disable preemption to guarantee that the template thread will be
	// created before a park once haveTemplateThread is set.
	mp := acquirem()
	if !atomic.Cas(&newmHandoff.haveTemplateThread, 0, 1) {
		releasem(mp)
		return
	}
	newm(templateThread, nil, -1)
	releasem(mp)
}

// templateThread is a thread in a known-good state that exists solely
// to start new threads in known-good states when the calling thread
// may not be in a good state.
//
// Many programs never need this, so templateThread is started lazily
// when we first enter a state that might lead to running on a thread
// in an unknown state.
//
// templateThread runs on an M without a P, so it must not have write
// barriers.
//
//go:nowritebarrierrec
func templateThread() {
	lock(&sched.lock)
	sched.nmsys++
	checkdead()
	unlock(&sched.lock)

	for {
		lock(&newmHandoff.lock)
		for newmHandoff.newm != 0 {
			newm := newmHandoff.newm.ptr()
			newmHandoff.newm = 0
			unlock(&newmHandoff.lock)
			for newm != nil {
				next := newm.schedlink.ptr()
				newm.schedlink = 0
				newm1(newm)
				newm = next
			}
			lock(&newmHandoff.lock)
		}
		newmHandoff.waiting = true
		noteclear(&newmHandoff.wake)
		unlock(&newmHandoff.lock)
		notesleep(&newmHandoff.wake)
	}
}

// Stops execution of the current m until new work is available.
// Returns with acquired P.
func stopm() {
	gp := getg()

	if gp.m.locks != 0 {
		throw("stopm holding locks")
	}
	if gp.m.p != 0 {
		throw("stopm holding p")
	}
	if gp.m.spinning {
		throw("stopm spinning")
	}

	lock(&sched.lock)
	mput(gp.m)
	unlock(&sched.lock)
	mPark()
	acquirep(gp.m.nextp.ptr())
	gp.m.nextp = 0
}

func mspinning() {
	// startm's caller incremented nmspinning. Set the new M's spinning.
	getg().m.spinning = true
}

// Schedules some M to run the p (creates an M if necessary).
// If p==nil, tries to get an idle P, if no idle P's does nothing.
// May run with m.p==nil, so write barriers are not allowed.
// If spinning is set, the caller has incremented nmspinning and must provide a
// P. startm will set m.spinning in the newly started M.
//
// Callers passing a non-nil P must call from a non-preemptible context. See
// comment on acquirem below.
//
// Argument lockheld indicates whether the caller already acquired the
// scheduler lock. Callers holding the lock when making the call must pass
// true. The lock might be temporarily dropped, but will be reacquired before
// returning.
//
// Must not have write barriers because this may be called without a P.
//
//go:nowritebarrierrec
func startm(pp *p, spinning, lockheld bool) {
	// Disable preemption.
	//
	// Every owned P must have an owner that will eventually stop it in the
	// event of a GC stop request. startm takes transient ownership of a P
	// (either from argument or pidleget below) and transfers ownership to
	// a started M, which will be responsible for performing the stop.
	//
	// Preemption must be disabled during this transient ownership,
	// otherwise the P this is running on may enter GC stop while still
	// holding the transient P, leaving that P in limbo and deadlocking the
	// STW.
	//
	// Callers passing a non-nil P must already be in non-preemptible
	// context, otherwise such preemption could occur on function entry to
	// startm. Callers passing a nil P may be preemptible, so we must
	// disable preemption before acquiring a P from pidleget below.
	mp := acquirem()
	if !lockheld {
		lock(&sched.lock)
	}
	if pp == nil {
		if spinning {
			// TODO(prattmic): All remaining calls to this function
			// with _p_ == nil could be cleaned up to find a P
			// before calling startm.
			throw("startm: P required for spinning=true")
		}
		pp, _ = pidleget(0)
		if pp == nil {
			if !lockheld {
				unlock(&sched.lock)
			}
			releasem(mp)
			return
		}
	}
	nmp := mget()
	if nmp == nil {
		// No M is available, we must drop sched.lock and call newm.
		// However, we already own a P to assign to the M.
		//
		// Once sched.lock is released, another G (e.g., in a syscall),
		// could find no idle P while checkdead finds a runnable G but
		// no running M's because this new M hasn't started yet, thus
		// throwing in an apparent deadlock.
		// This apparent deadlock is possible when startm is called
		// from sysmon, which doesn't count as a running M.
		//
		// Avoid this situation by pre-allocating the ID for the new M,
		// thus marking it as 'running' before we drop sched.lock. This
		// new M will eventually run the scheduler to execute any
		// queued G's.
		id := mReserveID()
		unlock(&sched.lock)

		var fn func()
		if spinning {
			// The caller incremented nmspinning, so set m.spinning in the new M.
			fn = mspinning
		}
		newm(fn, pp, id)

		if lockheld {
			lock(&sched.lock)
		}
		// Ownership transfer of pp committed by start in newm.
		// Preemption is now safe.
		releasem(mp)
		return
	}
	if !lockheld {
		unlock(&sched.lock)
	}
	if nmp.spinning {
		throw("startm: m is spinning")
	}
	if nmp.nextp != 0 {
		throw("startm: m has p")
	}
	if spinning && !runqempty(pp) {
		throw("startm: p has runnable gs")
	}
	// The caller incremented nmspinning, so set m.spinning in the new M.
	nmp.spinning = spinning
	nmp.nextp.set(pp)
	notewakeup(&nmp.park)
	// Ownership transfer of pp committed by wakeup. Preemption is now
	// safe.
	releasem(mp)
}

// Hands off P from syscall or locked M.
// Always runs without a P, so write barriers are not allowed.
//
//go:nowritebarrierrec
func handoffp(pp *p) {
	// handoffp must start an M in any situation where
	// findRunnable would return a G to run on pp.

	// if it has local work, start it straight away
	if !runqempty(pp) || !sched.runq.empty() {
		startm(pp, false, false)
		return
	}
	// if there's trace work to do, start it straight away
	if (traceEnabled() || traceShuttingDown()) && traceReaderAvailable() != nil {
		startm(pp, false, false)
		return
	}
	// if it has GC work, start it straight away
	if gcBlackenEnabled != 0 && gcShouldScheduleWorker(pp) {
		startm(pp, false, false)
		return
	}
	// no local work, check that there are no spinning/idle M's,
	// otherwise our help is not required
	if sched.nmspinning.Load()+sched.npidle.Load() == 0 && sched.nmspinning.CompareAndSwap(0, 1) { // TODO: fast atomic
		sched.needspinning.Store(0)
		startm(pp, true, false)
		return
	}
	lock(&sched.lock)
	if sched.gcwaiting.Load() {
		pp.status = _Pgcstop
		pp.gcStopTime = nanotime()
		sched.stopwait--
		if sched.stopwait == 0 {
			notewakeup(&sched.stopnote)
		}
		unlock(&sched.lock)
		return
	}
	if pp.runSafePointFn != 0 && atomic.Cas(&pp.runSafePointFn, 1, 0) {
		sched.safePointFn(pp)
		sched.safePointWait--
		if sched.safePointWait == 0 {
			notewakeup(&sched.safePointNote)
		}
	}
	if !sched.runq.empty() {
		unlock(&sched.lock)
		startm(pp, false, false)
		return
	}
	// If this is the last running P and nobody is polling network,
	// need to wakeup another M to poll network.
	if sched.npidle.Load() == gomaxprocs-1 && sched.lastpoll.Load() != 0 {
		unlock(&sched.lock)
		startm(pp, false, false)
		return
	}

	// The scheduler lock cannot be held when calling wakeNetPoller below
	// because wakeNetPoller may call wakep which may call startm.
	when := pp.timers.wakeTime()
	pidleput(pp, 0)
	unlock(&sched.lock)

	if when != 0 {
		wakeNetPoller(when)
	}
}

// Tries to add one more P to execute G's.
// Called when a G is made runnable (newproc, ready).
// Must be called with a P.
//
// wakep should be an internal detail,
// but widely used packages access it using linkname.
// Notable members of the hall of shame include:
//   - gvisor.dev/gvisor
//
// Do not remove or change the type signature.
// See go.dev/issue/67401.
//
//go:linkname wakep
func wakep() {
	// Be conservative about spinning threads, only start one if none exist
	// already.
	if sched.nmspinning.Load() != 0 || !sched.nmspinning.CompareAndSwap(0, 1) {
		return
	}

	// Disable preemption until ownership of pp transfers to the next M in
	// startm. Otherwise preemption here would leave pp stuck waiting to
	// enter _Pgcstop.
	//
	// See preemption comment on acquirem in startm for more details.
	mp := acquirem()

	var pp *p
	lock(&sched.lock)
	pp, _ = pidlegetSpinning(0)
	if pp == nil {
		if sched.nmspinning.Add(-1) < 0 {
			throw("wakep: negative nmspinning")
		}
		unlock(&sched.lock)
		releasem(mp)
		return
	}
	// Since we always have a P, the race in the "No M is available"
	// comment in startm doesn't apply during the small window between the
	// unlock here and lock in startm. A checkdead in between will always
	// see at least one running M (ours).
	unlock(&sched.lock)

	startm(pp, true, false)

	releasem(mp)
}

// Stops execution of the current m that is locked to a g until the g is runnable again.
// Returns with acquired P.
func stoplockedm() {
	gp := getg()

	if gp.m.lockedg == 0 || gp.m.lockedg.ptr().lockedm.ptr() != gp.m {
		throw("stoplockedm: inconsistent locking")
	}
	if gp.m.p != 0 {
		// Schedule another M to run this p.
		pp := releasep()
		handoffp(pp)
	}
	incidlelocked(1)
	// Wait until another thread schedules lockedg again.
	mPark()
	status := readgstatus(gp.m.lockedg.ptr())
	if status&^_Gscan != _Grunnable {
		print("runtime:stoplockedm: lockedg (atomicstatus=", status, ") is not Grunnable or Gscanrunnable\n")
		dumpgstatus(gp.m.lockedg.ptr())
		throw("stoplockedm: not runnable")
	}
	acquirep(gp.m.nextp.ptr())
	gp.m.nextp = 0
}

// Schedules the locked m to run the locked gp.
// May run during STW, so write barriers are not allowed.
//
//go:nowritebarrierrec
func startlockedm(gp *g) {
	mp := gp.lockedm.ptr()
	if mp == getg().m {
		throw("startlockedm: locked to me")
	}
	if mp.nextp != 0 {
		throw("startlockedm: m has p")
	}
	// directly handoff current P to the locked m
	incidlelocked(-1)
	pp := releasep()
	mp.nextp.set(pp)
	notewakeup(&mp.park)
	stopm()
}

// Stops the current m for stopTheWorld.
// Returns when the world is restarted.
func gcstopm() {
	gp := getg()

	if !sched.gcwaiting.Load() {
		throw("gcstopm: not waiting for gc")
	}
	if gp.m.spinning {
		gp.m.spinning = false
		// OK to just drop nmspinning here,
		// startTheWorld will unpark threads as necessary.
		if sched.nmspinning.Add(-1) < 0 {
			throw("gcstopm: negative nmspinning")
		}
	}
	pp := releasep()
	lock(&sched.lock)
	pp.status = _Pgcstop
	pp.gcStopTime = nanotime()
	sched.stopwait--
	if sched.stopwait == 0 {
		notewakeup(&sched.stopnote)
	}
	unlock(&sched.lock)
	stopm()
}

// Schedules gp to run on the current M.
// If inheritTime is true, gp inherits the remaining time in the
// current time slice. Otherwise, it starts a new time slice.
// Never returns.
//
// Write barriers are allowed because this is called immediately after
// acquiring a P in several places.
//
//go:yeswritebarrierrec
func execute(gp *g, inheritTime bool) {
	mp := getg().m

	if goroutineProfile.active {
		// Make sure that gp has had its stack written out to the goroutine
		// profile, exactly as it was when the goroutine profiler first stopped
		// the world.
		tryRecordGoroutineProfile(gp, nil, osyield)
	}

	// Assign gp.m before entering _Grunning so running Gs have an M.
	mp.curg = gp
	gp.m = mp
	gp.syncSafePoint = false // Clear the flag, which may have been set by morestack.
	casgstatus(gp, _Grunnable, _Grunning)
	gp.waitsince = 0
	gp.preempt = false
	gp.stackguard0 = gp.stack.lo + stackGuard
	if !inheritTime {
		mp.p.ptr().schedtick++
	}

	if sys.DITSupported && debug.dataindependenttiming != 1 {
		if gp.ditWanted && !mp.ditEnabled {
			// The current M doesn't have DIT enabled, but the goroutine we're
			// executing does need it, so turn it on.
			sys.EnableDIT()
			mp.ditEnabled = true
		} else if !gp.ditWanted && mp.ditEnabled {
			// The current M has DIT enabled, but the goroutine we're executing does
			// not need it, so turn it off.
			// NOTE: turning off DIT here means that the scheduler will have DIT enabled
			// when it runs after this goroutine yields or is preempted. This may have
			// a minor performance impact on the scheduler.
			sys.DisableDIT()
			mp.ditEnabled = false
		}
	}

	// Check whether the profiler needs to be turned on or off.
	hz := sched.profilehz
	if mp.profilehz != hz {
		setThreadCPUProfiler(hz)
	}

	trace := traceAcquire()
	if trace.ok() {
		trace.GoStart()
		traceRelease(trace)
	}

	gogo(&gp.sched)
}

// Finds a runnable goroutine to execute.
// Tries to steal from other P's, get g from local or global queue, poll network.
// tryWakeP indicates that the returned goroutine is not normal (GC worker, trace
// reader) so the caller should try to wake a P.
func findRunnable() (gp *g, inheritTime, tryWakeP bool) {
	mp := getg().m

	// The conditions here and in handoffp must agree: if
	// findRunnable would return a G to run, handoffp must start
	// an M.

top:
	// We may have collected an allp snapshot below. The snapshot is only
	// required in each loop iteration. Clear it to all GC to collect the
	// slice.
	mp.clearAllpSnapshot()

	pp := mp.p.ptr()
	if sched.gcwaiting.Load() {
		gcstopm()
		goto top
	}
	if pp.runSafePointFn != 0 {
		runSafePointFn()
	}

	// now and pollUntil are saved for work stealing later,
	// which may steal timers. It's important that between now
	// and then, nothing blocks, so these numbers remain mostly
	// relevant.
	now, pollUntil, _ := pp.timers.check(0, nil)

	// Try to schedule the trace reader.
	if traceEnabled() || traceShuttingDown() {
		gp := traceReader()
		if gp != nil {
			trace := traceAcquire()
			casgstatus(gp, _Gwaiting, _Grunnable)
			if trace.ok() {
				trace.GoUnpark(gp, 0)
				traceRelease(trace)
			}
			return gp, false, true
		}
	}

	// Try to schedule a GC worker.
	if gcBlackenEnabled != 0 {
		gp, tnow := gcController.findRunnableGCWorker(pp, now)
		if gp != nil {
			return gp, false, true
		}
		now = tnow
	}

	// Check the global runnable queue once in a while to ensure fairness.
	// Otherwise two goroutines can completely occupy the local runqueue
	// by constantly respawning each other.
	if pp.schedtick%61 == 0 && !sched.runq.empty() {
		lock(&sched.lock)
		gp := globrunqget()
		unlock(&sched.lock)
		if gp != nil {
			return gp, false, false
		}
	}

	// Wake up the finalizer G.
	if fingStatus.Load()&(fingWait|fingWake) == fingWait|fingWake {
		if gp := wakefing(); gp != nil {
			ready(gp, 0, true)
		}
	}

	// Wake up one or more cleanup Gs.
	if gcCleanups.needsWake() {
		gcCleanups.wake()
	}

	if *cgo_yield != nil {
		asmcgocall(*cgo_yield, nil)
	}

	// local runq
	if gp, inheritTime := runqget(pp); gp != nil {
		return gp, inheritTime, false
	}

	// global runq
	if !sched.runq.empty() {
		lock(&sched.lock)
		gp, q := globrunqgetbatch(int32(len(pp.runq)) / 2)
		unlock(&sched.lock)
		if gp != nil {
			if runqputbatch(pp, &q); !q.empty() {
				throw("Couldn't put Gs into empty local runq")
			}
			return gp, false, false
		}
	}

	// Poll network.
	// This netpoll is only an optimization before we resort to stealing.
	// We can safely skip it if there are no waiters or a thread is blocked
	// in netpoll already. If there is any kind of logical race with that
	// blocked thread (e.g. it has already returned from netpoll, but does
	// not set lastpoll yet), this thread will do blocking netpoll below
	// anyway.
	// We only poll from one thread at a time to avoid kernel contention
	// on machines with many cores.
	if netpollinited() && netpollAnyWaiters() && sched.lastpoll.Load() != 0 && sched.pollingNet.Swap(1) == 0 {
		list, delta := netpoll(0)
		sched.pollingNet.Store(0)
		if !list.empty() { // non-blocking
			gp := list.pop()
			injectglist(&list)
			netpollAdjustWaiters(delta)
			trace := traceAcquire()
			casgstatus(gp, _Gwaiting, _Grunnable)
			if trace.ok() {
				trace.GoUnpark(gp, 0)
				traceRelease(trace)
			}
			return gp, false, false
		}
	}

	// Spinning Ms: steal work from other Ps.
	//
	// Limit the number of spinning Ms to half the number of busy Ps.
	// This is necessary to prevent excessive CPU consumption when
	// GOMAXPROCS>>1 but the program parallelism is low.
	if mp.spinning || 2*sched.nmspinning.Load() < gomaxprocs-sched.npidle.Load() {
		if !mp.spinning {
			mp.becomeSpinning()
		}

		gp, inheritTime, tnow, w, newWork := stealWork(now)
		if gp != nil {
			// Successfully stole.
			return gp, inheritTime, false
		}
		if newWork {
			// There may be new timer or GC work; restart to
			// discover.
			goto top
		}

		now = tnow
		if w != 0 && (pollUntil == 0 || w < pollUntil) {
			// Earlier timer to wait for.
			pollUntil = w
		}
	}

	// We have nothing to do.
	//
	// If we're in the GC mark phase, can safely scan and blacken objects,
	// and have work to do, run idle-time marking rather than give up the P.
	if gcBlackenEnabled != 0 && gcShouldScheduleWorker(pp) && gcController.addIdleMarkWorker() {
		node := (*gcBgMarkWorkerNode)(gcBgMarkWorkerPool.pop())
		if node != nil {
			pp.gcMarkWorkerMode = gcMarkWorkerIdleMode
			gp := node.gp.ptr()

			trace := traceAcquire()
			casgstatus(gp, _Gwaiting, _Grunnable)
			if trace.ok() {
				trace.GoUnpark(gp, 0)
				traceRelease(trace)
			}
			return gp, false, false
		}
		gcController.removeIdleMarkWorker()
	}

	// wasm only:
	// If a callback returned and no other goroutine is awake,
	// then wake event handler goroutine which pauses execution
	// until a callback was triggered.
	gp, otherReady := beforeIdle(now, pollUntil)
	if gp != nil {
		trace := traceAcquire()
		casgstatus(gp, _Gwaiting, _Grunnable)
		if trace.ok() {
			trace.GoUnpark(gp, 0)
			traceRelease(trace)
		}
		return gp, false, false
	}
	if otherReady {
		goto top
	}

	// Before we drop our P, make a snapshot of the allp slice,
	// which can change underfoot once we no longer block
	// safe-points. We don't need to snapshot the contents because
	// everything up to cap(allp) is immutable.
	//
	// We clear the snapshot from the M after return via
	// mp.clearAllpSnapshop (in schedule) and on each iteration of the top
	// loop.
	allpSnapshot := mp.snapshotAllp()
	// Also snapshot masks. Value changes are OK, but we can't allow
	// len to change out from under us.
	idlepMaskSnapshot := idlepMask
	timerpMaskSnapshot := timerpMask

	// return P and block
	lock(&sched.lock)
	if sched.gcwaiting.Load() || pp.runSafePointFn != 0 {
		unlock(&sched.lock)
		goto top
	}
	if !sched.runq.empty() {
		gp, q := globrunqgetbatch(int32(len(pp.runq)) / 2)
		unlock(&sched.lock)
		if gp == nil {
			throw("global runq empty with non-zero runqsize")
		}
		if runqputbatch(pp, &q); !q.empty() {
			throw("Couldn't put Gs into empty local runq")
		}
		return gp, false, false
	}
	if !mp.spinning && sched.needspinning.Load() == 1 {
		// See "Delicate dance" comment below.
		mp.becomeSpinning()
		unlock(&sched.lock)
		goto top
	}
	if releasep() != pp {
		throw("findRunnable: wrong p")
	}
	now = pidleput(pp, now)
	unlock(&sched.lock)

	// Delicate dance: thread transitions from spinning to non-spinning
	// state, potentially concurrently with submission of new work. We must
	// drop nmspinning first and then check all sources again (with
	// #StoreLoad memory barrier in between). If we do it the other way
	// around, another thread can submit work after we've checked all
	// sources but before we drop nmspinning; as a result nobody will
	// unpark a thread to run the work.
	//
	// This applies to the following sources of work:
	//
	// * Goroutines added to the global or a per-P run queue.
	// * New/modified-earlier timers on a per-P timer heap.
	// * Idle-priority GC work (barring golang.org/issue/19112).
	//
	// If we discover new work below, we need to restore m.spinning as a
	// signal for resetspinning to unpark a new worker thread (because
	// there can be more than one starving goroutine).
	//
	// However, if after discovering new work we also observe no idle Ps
	// (either here or in resetspinning), we have a problem. We may be
	// racing with a non-spinning M in the block above, having found no
	// work and preparing to release its P and park. Allowing that P to go
	// idle will result in loss of work conservation (idle P while there is
	// runnable work). This could result in complete deadlock in the
	// unlikely event that we discover new work (from netpoll) right as we
	// are racing with _all_ other Ps going idle.
	//
	// We use sched.needspinning to synchronize with non-spinning Ms going
	// idle. If needspinning is set when they are about to drop their P,
	// they abort the drop and instead become a new spinning M on our
	// behalf. If we are not racing and the system is truly fully loaded
	// then no spinning threads are required, and the next thread to
	// naturally become spinning will clear the flag.
	//
	// Also see "Worker thread parking/unparking" comment at the top of the
	// file.
	wasSpinning := mp.spinning
	if mp.spinning {
		mp.spinning = false
		if sched.nmspinning.Add(-1) < 0 {
			throw("findRunnable: negative nmspinning")
		}

		// Note the for correctness, only the last M transitioning from
		// spinning to non-spinning must perform these rechecks to
		// ensure no missed work. However, the runtime has some cases
		// of transient increments of nmspinning that are decremented
		// without going through this path, so we must be conservative
		// and perform the check on all spinning Ms.
		//
		// See https://go.dev/issue/43997.

		// Check global and P runqueues again.

		lock(&sched.lock)
		if !sched.runq.empty() {
			pp, _ := pidlegetSpinning(0)
			if pp != nil {
				gp, q := globrunqgetbatch(int32(len(pp.runq)) / 2)
				unlock(&sched.lock)
				if gp == nil {
					throw("global runq empty with non-zero runqsize")
				}
				if runqputbatch(pp, &q); !q.empty() {
					throw("Couldn't put Gs into empty local runq")
				}
				acquirep(pp)
				mp.becomeSpinning()
				return gp, false, false
			}
		}
		unlock(&sched.lock)

		pp := checkRunqsNoP(allpSnapshot, idlepMaskSnapshot)
		if pp != nil {
			acquirep(pp)
			mp.becomeSpinning()
			goto top
		}

		// Check for idle-priority GC work again.
		pp, gp := checkIdleGCNoP()
		if pp != nil {
			acquirep(pp)
			mp.becomeSpinning()

			// Run the idle worker.
			pp.gcMarkWorkerMode = gcMarkWorkerIdleMode
			trace := traceAcquire()
			casgstatus(gp, _Gwaiting, _Grunnable)
			if trace.ok() {
				trace.GoUnpark(gp, 0)
				traceRelease(trace)
			}
			return gp, false, false
		}

		// Finally, check for timer creation or expiry concurrently with
		// transitioning from spinning to non-spinning.
		//
		// Note that we cannot use checkTimers here because it calls
		// adjusttimers which may need to allocate memory, and that isn't
		// allowed when we don't have an active P.
		pollUntil = checkTimersNoP(allpSnapshot, timerpMaskSnapshot, pollUntil)
	}

	// We don't need allp anymore at this pointer, but can't clear the
	// snapshot without a P for the write barrier..

	// Poll network until next timer.
	if netpollinited() && (netpollAnyWaiters() || pollUntil != 0) && sched.lastpoll.Swap(0) != 0 {
		sched.pollUntil.Store(pollUntil)
		if mp.p != 0 {
			throw("findRunnable: netpoll with p")
		}
		if mp.spinning {
			throw("findRunnable: netpoll with spinning")
		}
		delay := int64(-1)
		if pollUntil != 0 {
			if now == 0 {
				now = nanotime()
			}
			delay = pollUntil - now
			if delay < 0 {
				delay = 0
			}
		}
		if faketime != 0 {
			// When using fake time, just poll.
			delay = 0
		}
		list, delta := netpoll(delay) // block until new work is available
		// Refresh now again, after potentially blocking.
		now = nanotime()
		sched.pollUntil.Store(0)
		sched.lastpoll.Store(now)
		if faketime != 0 && list.empty() {
			// Using fake time and nothing is ready; stop M.
			// When all M's stop, checkdead will call timejump.
			stopm()
			goto top
		}
		lock(&sched.lock)
		pp, _ := pidleget(now)
		unlock(&sched.lock)
		if pp == nil {
			injectglist(&list)
			netpollAdjustWaiters(delta)
		} else {
			acquirep(pp)
			if !list.empty() {
				gp := list.pop()
				injectglist(&list)
				netpollAdjustWaiters(delta)
				trace := traceAcquire()
				casgstatus(gp, _Gwaiting, _Grunnable)
				if trace.ok() {
					trace.GoUnpark(gp, 0)
					traceRelease(trace)
				}
				return gp, false, false
			}
			if wasSpinning {
				mp.becomeSpinning()
			}
			goto top
		}
	} else if pollUntil != 0 && netpollinited() {
		pollerPollUntil := sched.pollUntil.Load()
		if pollerPollUntil == 0 || pollerPollUntil > pollUntil {
			netpollBreak()
		}
	}
	stopm()
	goto top
}

// pollWork reports whether there is non-background work this P could
// be doing. This is a fairly lightweight check to be used for
// background work loops, like idle GC. It checks a subset of the
// conditions checked by the actual scheduler.
func pollWork() bool {
	if !sched.runq.empty() {
		return true
	}
	p := getg().m.p.ptr()
	if !runqempty(p) {
		return true
	}
	if netpollinited() && netpollAnyWaiters() && sched.lastpoll.Load() != 0 {
		if list, delta := netpoll(0); !list.empty() {
			injectglist(&list)
			netpollAdjustWaiters(delta)
			return true
		}
	}
	return false
}

// stealWork attempts to steal a runnable goroutine or timer from any P.
//
// If newWork is true, new work may have been readied.
//
// If now is not 0 it is the current time. stealWork returns the passed time or
// the current time if now was passed as 0.
func stealWork(now int64) (gp *g, inheritTime bool, rnow, pollUntil int64, newWork bool) {
	pp := getg().m.p.ptr()

	ranTimer := false

	const stealTries = 4
	for i := 0; i < stealTries; i++ {
		stealTimersOrRunNextG := i == stealTries-1

		for enum := stealOrder.start(cheaprand()); !enum.done(); enum.next() {
			if sched.gcwaiting.Load() {
				// GC work may be available.
				return nil, false, now, pollUntil, true
			}
			p2 := allp[enum.position()]
			if pp == p2 {
				continue
			}

			// Steal timers from p2. This call to checkTimers is the only place
			// where we might hold a lock on a different P's timers. We do this
			// once on the last pass before checking runnext because stealing
			// from the other P's runnext should be the last resort, so if there
			// are timers to steal do that first.
			//
			// We only check timers on one of the stealing iterations because
			// the time stored in now doesn't change in this loop and checking
			// the timers for each P more than once with the same value of now
			// is probably a waste of time.
			//
			// timerpMask tells us whether the P may have timers at all. If it
			// can't, no need to check at all.
			if stealTimersOrRunNextG && timerpMask.read(enum.position()) {
				tnow, w, ran := p2.timers.check(now, nil)
				now = tnow
				if w != 0 && (pollUntil == 0 || w < pollUntil) {
					pollUntil = w
				}
				if ran {
					// Running the timers may have
					// made an arbitrary number of G's
					// ready and added them to this P's
					// local run queue. That invalidates
					// the assumption of runqsteal
					// that it always has room to add
					// stolen G's. So check now if there
					// is a local G to run.
					if gp, inheritTime := runqget(pp); gp != nil {
						return gp, inheritTime, now, pollUntil, ranTimer
					}
					ranTimer = true
				}
			}

			// Don't bother to attempt to steal if p2 is idle.
			if !idlepMask.read(enum.position()) {
				if gp := runqsteal(pp, p2, stealTimersOrRunNextG); gp != nil {
					return gp, false, now, pollUntil, ranTimer
				}
			}
		}
	}

	// No goroutines found to steal. Regardless, running a timer may have
	// made some goroutine ready that we missed. Indicate the next timer to
	// wait for.
	return nil, false, now, pollUntil, ranTimer
}

// Check all Ps for a runnable G to steal.
//
// On entry we have no P. If a G is available to steal and a P is available,
// the P is returned which the caller should acquire and attempt to steal the
// work to.
func checkRunqsNoP(allpSnapshot []*p, idlepMaskSnapshot pMask) *p {
	for id, p2 := range allpSnapshot {
		if !idlepMaskSnapshot.read(uint32(id)) && !runqempty(p2) {
			lock(&sched.lock)
			pp, _ := pidlegetSpinning(0)
			if pp == nil {
				// Can't get a P, don't bother checking remaining Ps.
				unlock(&sched.lock)
				return nil
			}
			unlock(&sched.lock)
			return pp
		}
	}

	// No work available.
	return nil
}

// Check all Ps for a timer expiring sooner than pollUntil.
//
// Returns updated pollUntil value.
func checkTimersNoP(allpSnapshot []*p, timerpMaskSnapshot pMask, pollUntil int64) int64 {
	for id, p2 := range allpSnapshot {
		if timerpMaskSnapshot.read(uint32(id)) {
			w := p2.timers.wakeTime()
			if w != 0 && (pollUntil == 0 || w < pollUntil) {
				pollUntil = w
			}
		}
	}

	return pollUntil
}

// Check for idle-priority GC, without a P on entry.
//
// If some GC work, a P, and a worker G are all available, the P and G will be
// returned. The returned P has not been wired yet.
func checkIdleGCNoP() (*p, *g) {
	// N.B. Since we have no P, gcBlackenEnabled may change at any time; we
	// must check again after acquiring a P. As an optimization, we also check
	// if an idle mark worker is needed at all. This is OK here, because if we
	// observe that one isn't needed, at least one is currently running. Even if
	// it stops running, its own journey into the scheduler should schedule it
	// again, if need be (at which point, this check will pass, if relevant).
	if atomic.Load(&gcBlackenEnabled) == 0 || !gcController.needIdleMarkWorker() {
		return nil, nil
	}
	if !gcShouldScheduleWorker(nil) {
		return nil, nil
	}

	// Work is available; we can start an idle GC worker only if there is
	// an available P and available worker G.
	//
	// We can attempt to acquire these in either order, though both have
	// synchronization concerns (see below). Workers are almost always
	// available (see comment in findRunnableGCWorker for the one case
	// there may be none). Since we're slightly less likely to find a P,
	// check for that first.
	//
	// Synchronization: note that we must hold sched.lock until we are
	// committed to keeping it. Otherwise we cannot put the unnecessary P
	// back in sched.pidle without performing the full set of idle
	// transition checks.
	//
	// If we were to check gcBgMarkWorkerPool first, we must somehow handle
	// the assumption in gcControllerState.findRunnableGCWorker that an
	// empty gcBgMarkWorkerPool is only possible if gcMarkDone is running.
	lock(&sched.lock)
	pp, now := pidlegetSpinning(0)
	if pp == nil {
		unlock(&sched.lock)
		return nil, nil
	}

	// Now that we own a P, gcBlackenEnabled can't change (as it requires STW).
	if gcBlackenEnabled == 0 || !gcController.addIdleMarkWorker() {
		pidleput(pp, now)
		unlock(&sched.lock)
		return nil, nil
	}

	node := (*gcBgMarkWorkerNode)(gcBgMarkWorkerPool.pop())
	if node == nil {
		pidleput(pp, now)
		unlock(&sched.lock)
		gcController.removeIdleMarkWorker()
		return nil, nil
	}

	unlock(&sched.lock)

	return pp, node.gp.ptr()
}

// wakeNetPoller wakes up the thread sleeping in the network poller if it isn't
// going to wake up before the when argument; or it wakes an idle P to service
// timers and the network poller if there isn't one already.
func wakeNetPoller(when int64) {
	if sched.lastpoll.Load() == 0 {
		// In findRunnable we ensure that when polling the pollUntil
		// field is either zero or the time to which the current
		// poll is expected to run. This can have a spurious wakeup
		// but should never miss a wakeup.
		pollerPollUntil := sched.pollUntil.Load()
		if pollerPollUntil == 0 || pollerPollUntil > when {
			netpollBreak()
		}
	} else {
		// There are no threads in the network poller, try to get
		// one there so it can handle new timers.
		if GOOS != "plan9" { // Temporary workaround - see issue #42303.
			wakep()
		}
	}
}

func resetspinning() {
	gp := getg()
	if !gp.m.spinning {
		throw("resetspinning: not a spinning m")
	}
	gp.m.spinning = false
	nmspinning := sched.nmspinning.Add(-1)
	if nmspinning < 0 {
		throw("findRunnable: negative nmspinning")
	}
	// M wakeup policy is deliberately somewhat conservative, so check if we
	// need to wakeup another P here. See "Worker thread parking/unparking"
	// comment at the top of the file for details.
	wakep()
}

// injectglist adds each runnable G on the list to some run queue,
// and clears glist. If there is no current P, they are added to the
// global queue, and up to npidle M's are started to run them.
// Otherwise, for each idle P, this adds a G to the global queue
// and starts an M. Any remaining G's are added to the current P's
// local run queue.
// This may temporarily acquire sched.lock.
// Can run concurrently with GC.
func injectglist(glist *gList) {
	if glist.empty() {
		return
	}

	// Mark all the goroutines as runnable before we put them
	// on the run queues.
	var tail *g
	trace := traceAcquire()
	for gp := glist.head.ptr(); gp != nil; gp = gp.schedlink.ptr() {
		tail = gp
		casgstatus(gp, _Gwaiting, _Grunnable)
		if trace.ok() {
			trace.GoUnpark(gp, 0)
		}
	}
	if trace.ok() {
		traceRelease(trace)
	}

	// Turn the gList into a gQueue.
	q := gQueue{glist.head, tail.guintptr(), glist.size}
	*glist = gList{}

	startIdle := func(n int32) {
		for ; n > 0; n-- {
			mp := acquirem() // See comment in startm.
			lock(&sched.lock)

			pp, _ := pidlegetSpinning(0)
			if pp == nil {
				unlock(&sched.lock)
				releasem(mp)
				break
			}

			startm(pp, false, true)
			unlock(&sched.lock)
			releasem(mp)
		}
	}

	pp := getg().m.p.ptr()
	if pp == nil {
		n := q.size
		lock(&sched.lock)
		globrunqputbatch(&q)
		unlock(&sched.lock)
		startIdle(n)
		return
	}

	var globq gQueue
	npidle := sched.npidle.Load()
	for ; npidle > 0 && !q.empty(); npidle-- {
		g := q.pop()
		globq.pushBack(g)
	}
	if !globq.empty() {
		n := globq.size
		lock(&sched.lock)
		globrunqputbatch(&globq)
		unlock(&sched.lock)
		startIdle(n)
	}

	if runqputbatch(pp, &q); !q.empty() {
		lock(&sched.lock)
		globrunqputbatch(&q)
		unlock(&sched.lock)
	}

	// Some P's might have become idle after we loaded `sched.npidle`
	// but before any goroutines were added to the queue, which could
	// lead to idle P's when there is work available in the global queue.
	// That could potentially last until other goroutines become ready
	// to run. That said, we need to find a way to hedge
	//
	// Calling wakep() here is the best bet, it will do nothing in the
	// common case (no racing on `sched.npidle`), while it could wake one
	// more P to execute G's, which might end up with >1 P's: the first one
	// wakes another P and so forth until there is no more work, but this
	// ought to be an extremely rare case.
	//
	// Also see "Worker thread parking/unparking" comment at the top of the file for details.
	wakep()
}

// One round of scheduler: find a runnable goroutine and execute it.
// Never returns.
func schedule() {
	mp := getg().m

	if mp.locks != 0 {
		throw("schedule: holding locks")
	}

	if mp.lockedg != 0 {
		stoplockedm()
		execute(mp.lockedg.ptr(), false) // Never returns.
	}

	// We should not schedule away from a g that is executing a cgo call,
	// since the cgo call is using the m's g0 stack.
	if mp.incgo {
		throw("schedule: in cgo")
	}

top:
	pp := mp.p.ptr()
	pp.preempt = false

	// Safety check: if we are spinning, the run queue should be empty.
	// Check this before calling checkTimers, as that might call
	// goready to put a ready goroutine on the local run queue.
	if mp.spinning && (pp.runnext != 0 || pp.runqhead != pp.runqtail) {
		throw("schedule: spinning with local work")
	}

	gp, inheritTime, tryWakeP := findRunnable() // blocks until work is available

	// May be on a new P.
	pp = mp.p.ptr()

	// findRunnable may have collected an allp snapshot. The snapshot is
	// only required within findRunnable. Clear it to all GC to collect the
	// slice.
	mp.clearAllpSnapshot()

	// If the P was assigned a next GC mark worker but findRunnable
	// selected anything else, release the worker so another P may run it.
	//
	// N.B. If this occurs because a higher-priority goroutine was selected
	// (trace reader), then tryWakeP is set, which will wake another P to
	// run the worker. If this occurs because the GC is no longer active,
	// there is no need to wakep.
	gcController.releaseNextGCMarkWorker(pp)

	if debug.dontfreezetheworld > 0 && freezing.Load() {
		// See comment in freezetheworld. We don't want to perturb
		// scheduler state, so we didn't gcstopm in findRunnable, but
		// also don't want to allow new goroutines to run.
		//
		// Deadlock here rather than in the findRunnable loop so if
		// findRunnable is stuck in a loop we don't perturb that
		// either.
		lock(&deadlock)
		lock(&deadlock)
	}

	// This thread is going to run a goroutine and is not spinning anymore,
	// so if it was marked as spinning we need to reset it now and potentially
	// start a new spinning M.
	if mp.spinning {
		resetspinning()
	}

	if sched.disable.user && !schedEnabled(gp) {
		// Scheduling of this goroutine is disabled. Put it on
		// the list of pending runnable goroutines for when we
		// re-enable user scheduling and look again.
		lock(&sched.lock)
		if schedEnabled(gp) {
			// Something re-enabled scheduling while we
			// were acquiring the lock.
			unlock(&sched.lock)
		} else {
			sched.disable.runnable.pushBack(gp)
			unlock(&sched.lock)
			goto top
		}
	}

	// If about to schedule a not-normal goroutine (a GCworker or tracereader),
	// wake a P if there is one.
	if tryWakeP {
		wakep()
	}
	if gp.lockedm != 0 {
		// Hands off own p to the locked m,
		// then blocks waiting for a new p.
		startlockedm(gp)
		goto top
	}

	execute(gp, inheritTime)
}

// dropg removes the association between m and the current goroutine m->curg (gp for short).
// Typically a caller sets gp's status away from Grunning and then
// immediately calls dropg to finish the job. The caller is also responsible
// for arranging that gp will be restarted using ready at an
// appropriate time. After calling dropg and arranging for gp to be
// readied later, the caller can do other work but eventually should
// call schedule to restart the scheduling of goroutines on this m.
func dropg() {
	gp := getg()

	setMNoWB(&gp.m.curg.m, nil)
	setGNoWB(&gp.m.curg, nil)
}

func parkunlock_c(gp *g, lock unsafe.Pointer) bool {
	unlock((*mutex)(lock))
	return true
}

// park continuation on g0.
func park_m(gp *g) {
	mp := getg().m

	trace := traceAcquire()

	// If g is in a synctest group, we don't want to let the group
	// become idle until after the waitunlockf (if any) has confirmed
	// that the park is happening.
	// We need to record gp.bubble here, since waitunlockf can change it.
	bubble := gp.bubble
	if bubble != nil {
		bubble.incActive()
	}

	if trace.ok() {
		// Trace the event before the transition. It may take a
		// stack trace, but we won't own the stack after the
		// transition anymore.
		trace.GoPark(mp.waitTraceBlockReason, mp.waitTraceSkip)
	}
	// N.B. Not using casGToWaiting here because the waitreason is
	// set by park_m's caller.
	casgstatus(gp, _Grunning, _Gwaiting)
	if trace.ok() {
		traceRelease(trace)
	}

	dropg()

	if fn := mp.waitunlockf; fn != nil {
		ok := fn(gp, mp.waitlock)
		mp.waitunlockf = nil
		mp.waitlock = nil
		if !ok {
			trace := traceAcquire()
			casgstatus(gp, _Gwaiting, _Grunnable)
			if bubble != nil {
				bubble.decActive()
			}
			if trace.ok() {
				trace.GoUnpark(gp, 2)
				traceRelease(trace)
			}
			execute(gp, true) // Schedule it back, never returns.
		}
	}

	if bubble != nil {
		bubble.decActive()
	}

	schedule()
}

func goschedImpl(gp *g, preempted bool) {
	pp := gp.m.p.ptr()
	trace := traceAcquire()
	status := readgstatus(gp)
	if status&^_Gscan != _Grunning {
		dumpgstatus(gp)
		throw("bad g status")
	}
	if trace.ok() {
		// Trace the event before the transition. It may take a
		// stack trace, but we won't own the stack after the
		// transition anymore.
		if preempted {
			trace.GoPreempt()
		} else {
			trace.GoSched()
		}
	}
	casgstatus(gp, _Grunning, _Grunnable)
	if trace.ok() {
		traceRelease(trace)
	}

	dropg()
	if preempted && sched.gcwaiting.Load() {
		// If preempted for STW, keep the G on the local P in runnext
		// so it can keep running immediately after the STW.
		runqput(pp, gp, true)
	} else {
		lock(&sched.lock)
		globrunqput(gp)
		unlock(&sched.lock)
	}

	if mainStarted {
		wakep()
	}

	schedule()
}

// Gosched continuation on g0.
func gosched_m(gp *g) {
	goschedImpl(gp, false)
}

// goschedguarded is a forbidden-states-avoided version of gosched_m.
func goschedguarded_m(gp *g) {
	if !canPreemptM(gp.m) {
		gogo(&gp.sched) // never return
	}
	goschedImpl(gp, false)
}

func gopreempt_m(gp *g) {
	goschedImpl(gp, true)
}

// preemptPark parks gp and puts it in _Gpreempted.
//
//go:systemstack
func preemptPark(gp *g) {
	status := readgstatus(gp)
	if status&^_Gscan != _Grunning {
		dumpgstatus(gp)
		throw("bad g status")
	}

	if gp.asyncSafePoint {
		// Double-check that async preemption does not
		// happen in SPWRITE assembly functions.
		// isAsyncSafePoint must exclude this case.
		f := findfunc(gp.sched.pc)
		if !f.valid() {
			throw("preempt at unknown pc")
		}
		if f.flag&abi.FuncFlagSPWrite != 0 {
			println("runtime: unexpected SPWRITE function", funcname(f), "in async preempt")
			throw("preempt SPWRITE")
		}
	}

	// Transition from _Grunning to _Gscan|_Gpreempted. We can't
	// be in _Grunning when we dropg because then we'd be running
	// without an M, but the moment we're in _Gpreempted,
	// something could claim this G before we've fully cleaned it
	// up. Hence, we set the scan bit to lock down further
	// transitions until we can dropg.
	casGToPreemptScan(gp, _Grunning, _Gscan|_Gpreempted)

	// Be careful about ownership as we trace this next event.
	//
	// According to the tracer invariants (trace.go) it's unsafe
	// for us to emit an event for a goroutine we do not own.
	// The moment we CAS into _Gpreempted, suspendG could CAS the
	// goroutine to _Gwaiting, effectively taking ownership. All of
	// this could happen before we even get the chance to emit
	// an event. The end result is that the events could appear
	// out of order, and the tracer generally assumes the scheduler
	// takes care of the ordering between GoPark and GoUnpark.
	//
	// The answer here is simple: emit the event while we still hold
	// the _Gscan bit on the goroutine, since the _Gscan bit means
	// ownership over transitions.
	//
	// We still need to traceAcquire and traceRelease across the CAS
	// because the tracer could be what's calling suspendG in the first
	// place. This also upholds the tracer invariant that we must hold
	// traceAcquire/traceRelease across the transition. However, we
	// specifically *only* emit the event while we still have ownership.
	trace := traceAcquire()
	if trace.ok() {
		trace.GoPark(traceBlockPreempted, 0)
	}

	// Drop the goroutine from the M. Only do this after the tracer has
	// emitted an event, because it needs the association for GoPark to
	// work correctly.
	dropg()

	// Drop the scan bit and release the trace locker if necessary.
	casfrom_Gscanstatus(gp, _Gscan|_Gpreempted, _Gpreempted)
	if trace.ok() {
		traceRelease(trace)
	}

	// All done.
	schedule()
}

// goyield is like Gosched, but it:
// - emits a GoPreempt trace event instead of a GoSched trace event
// - puts the current G on the runq of the current P instead of the globrunq
//
// goyield should be an internal detail,
// but widely used packages access it using linkname.
// Notable members of the hall of shame include:
//   - gvisor.dev/gvisor
//   - github.com/sagernet/gvisor
//
// Do not remove or change the type signature.
// See go.dev/issue/67401.
//
//go:linkname goyield
func goyield() {
	checkTimeouts()
	mcall(goyield_m)
}

func goyield_m(gp *g) {
	trace := traceAcquire()
	pp := gp.m.p.ptr()
	if trace.ok() {
		// Trace the event before the transition. It may take a
		// stack trace, but we won't own the stack after the
		// transition anymore.
		trace.GoPreempt()
	}
	casgstatus(gp, _Grunning, _Grunnable)
	if trace.ok() {
		traceRelease(trace)
	}
	dropg()
	runqput(pp, gp, false)
	schedule()
}

// Finishes execution of the current goroutine.
func goexit1() {
	if raceenabled {
		if gp := getg(); gp.bubble != nil {
			racereleasemergeg(gp, gp.bubble.raceaddr())
		}
		racegoend()
	}
	trace := traceAcquire()
	if trace.ok() {
		trace.GoEnd()
		traceRelease(trace)
	}
	mcall(goexit0)
}

// goexit continuation on g0.
func goexit0(gp *g) {
	if goexperiment.RuntimeSecret && gp.secret > 0 {
		// Erase the whole stack. This path only occurs when
		// runtime.Goexit is called from within a runtime/secret.Do call.
		memclrNoHeapPointers(unsafe.Pointer(gp.stack.lo), gp.stack.hi-gp.stack.lo)
		// Since this is running on g0, our registers are already zeroed from going through
		// mcall in secret mode.
	}
	gdestroy(gp)
	schedule()
}

func gdestroy(gp *g) {
	mp := getg().m
	pp := mp.p.ptr()

	casgstatus(gp, _Grunning, _Gdead)
	gcController.addScannableStack(pp, -int64(gp.stack.hi-gp.stack.lo))
	if isSystemGoroutine(gp, false) {
		sched.ngsys.Add(-1)
	}
	gp.m = nil
	locked := gp.lockedm != 0
	gp.lockedm = 0
	mp.lockedg = 0
	gp.preemptStop = false
	gp.paniconfault = false
	gp._defer = nil // should be true already but just in case.
	gp._panic = nil // non-nil for Goexit during panic. points at stack-allocated data.
	gp.writebuf = nil
	gp.waitreason = waitReasonZero
	gp.param = nil
	gp.labels = nil
	gp.timer = nil
	gp.bubble = nil
	gp.fipsOnlyBypass = false
	gp.secret = 0

	if gcBlackenEnabled != 0 && gp.gcAssistBytes > 0 {
		// Flush assist credit to the global pool. This gives
		// better information to pacing if the application is
		// rapidly creating an exiting goroutines.
		assistWorkPerByte := gcController.assistWorkPerByte.Load()
		scanCredit := int64(assistWorkPerByte * float64(gp.gcAssistBytes))
		gcController.bgScanCredit.Add(scanCredit)
		gp.gcAssistBytes = 0
	}

	dropg()

	if GOARCH == "wasm" { // no threads yet on wasm
		gfput(pp, gp)
		return
	}

	if locked && mp.lockedInt != 0 {
		print("runtime: mp.lockedInt = ", mp.lockedInt, "\n")
		if mp.isextra {
			throw("runtime.Goexit called in a thread that was not created by the Go runtime")
		}
		throw("exited a goroutine internally locked to the OS thread")
	}
	gfput(pp, gp)
	if locked {
		// The goroutine may have locked this thread because
		// it put it in an unusual kernel state. Kill it
		// rather than returning it to the thread pool.

		// Return to mstart, which will release the P and exit
		// the thread.
		if GOOS != "plan9" { // See golang.org/issue/22227.
			gogo(&mp.g0.sched)
		} else {
			// Clear lockedExt on plan9 since we may end up re-using
			// this thread.
			mp.lockedExt = 0
		}
	}
}

// save updates getg().sched to refer to pc and sp so that a following
// gogo will restore pc and sp.
//
// save must not have write barriers because invoking a write barrier
// can clobber getg().sched.
//
//go:nosplit
//go:nowritebarrierrec
func save(pc, sp, bp uintptr) {
	gp := getg()

	if gp == gp.m.g0 || gp == gp.m.gsignal {
		// m.g0.sched is special and must describe the context
		// for exiting the thread. mstart1 writes to it directly.
		// m.gsignal.sched should not be used at all.
		// This check makes sure save calls do not accidentally
		// run in contexts where they'd write to system g's.
		throw("save on system g not allowed")
	}

	gp.sched.pc = pc
	gp.sched.sp = sp
	gp.sched.lr = 0
	gp.sched.bp = bp
	// We need to ensure ctxt is zero, but can't have a write
	// barrier here. However, it should always already be zero.
	// Assert that.
	if gp.sched.ctxt != nil {
		badctxt()
	}
}

// The goroutine g is about to enter a system call.
// Record that it's not using the cpu anymore.
// This is called only from the go syscall library and cgocall,
// not from the low-level system calls used by the runtime.
//
// Entersyscall cannot split the stack: the save must
// make g->sched refer to the caller's stack segment, because
// entersyscall is going to return immediately after.
//
// Nothing entersyscall calls can split the stack either.
// We cannot safely move the stack during an active call to syscall,
// because we do not know which of the uintptr arguments are
// really pointers (back into the stack).
// In practice, this means that we make the fast path run through
// entersyscall doing no-split things, and the slow path has to use systemstack
// to run bigger things on the system stack.
//
// reentersyscall is the entry point used by cgo callbacks, where explicitly
// saved SP and PC are restored. This is needed when exitsyscall will be called
// from a function further up in the call stack than the parent, as g->syscallsp
// must always point to a valid stack frame. entersyscall below is the normal
// entry point for syscalls, which obtains the SP and PC from the caller.
//
//go:nosplit
func reentersyscall(pc, sp, bp uintptr) {
	gp := getg()

	// Disable preemption because during this function g is in Gsyscall status,
	// but can have inconsistent g->sched, do not let GC observe it.
	gp.m.locks++

	// This M may have a signal stack that is dirtied with secret information
	// (see package "runtime/secret"). Since it's about to go into a syscall for
	// an arbitrary amount of time and the G that put the secret info there
	// might have returned from secret.Do, we have to zero it out now, lest we
	// break the guarantee that secrets are purged by the next GC after a return
	// to secret.Do.
	//
	// It might be tempting to think that we only need to zero out this if we're
	// not running in secret mode anymore, but that leaves an ABA problem. The G
	// that put the secrets onto our signal stack may not be the one that is
	// currently executing.
	//
	// Logically, we should erase this when we lose our P, not when we enter the
	// syscall. This would avoid a zeroing in the case where the call returns
	// almost immediately. Since we use this path for cgo calls as well, these
	// fast "syscalls" are quite common. However, since we only erase the signal
	// stack if we were delivered a signal in secret mode and considering the
	// cross-thread synchronization cost for the P, it hardly seems worth it.
	//
	// TODO(dmo): can we encode the goid into mp.signalSecret and avoid the ABA problem?
	if goexperiment.RuntimeSecret {
		eraseSecretsSignalStk()
	}

	// Entersyscall must not call any function that might split/grow the stack.
	// (See details in comment above.)
	// Catch calls that might, by replacing the stack guard with something that
	// will trip any stack check and leaving a flag to tell newstack to die.
	gp.stackguard0 = stackPreempt
	gp.throwsplit = true

	// Copy the syscalltick over so we can identify if the P got stolen later.
	gp.m.syscalltick = gp.m.p.ptr().syscalltick

	pp := gp.m.p.ptr()
	if pp.runSafePointFn != 0 {
		// runSafePointFn may stack split if run on this stack
		systemstack(runSafePointFn)
	}
	gp.m.oldp.set(pp)

	// Leave SP around for GC and traceback.
	save(pc, sp, bp)
	gp.syscallsp = sp
	gp.syscallpc = pc
	gp.syscallbp = bp

	// Double-check sp and bp.
	if gp.syscallsp < gp.stack.lo || gp.stack.hi < gp.syscallsp {
		systemstack(func() {
			print("entersyscall inconsistent sp ", hex(gp.syscallsp), " [", hex(gp.stack.lo), ",", hex(gp.stack.hi), "]\n")
			throw("entersyscall")
		})
	}
	if gp.syscallbp != 0 && gp.syscallbp < gp.stack.lo || gp.stack.hi < gp.syscallbp {
		systemstack(func() {
			print("entersyscall inconsistent bp ", hex(gp.syscallbp), " [", hex(gp.stack.lo), ",", hex(gp.stack.hi), "]\n")
			throw("entersyscall")
		})
	}
	trace := traceAcquire()
	if trace.ok() {
		// Emit a trace event. Notably, actually emitting the event must happen before
		// the casgstatus because it mutates the P, but the traceLocker must be held
		// across the casgstatus since we're transitioning out of _Grunning
		// (see trace.go invariants).
		systemstack(func() {
			trace.GoSysCall()
		})
		// systemstack clobbered gp.sched, so restore it.
		save(pc, sp, bp)
	}
	if sched.gcwaiting.Load() {
		// Optimization: If there's a pending STW, do the equivalent of
		// entersyscallblock here at the last minute and immediately give
		// away our P.
		systemstack(func() {
			entersyscallHandleGCWait(trace)
		})
		// systemstack clobbered gp.sched, so restore it.
		save(pc, sp, bp)
	}
	// As soon as we switch to _Gsyscall, we are in danger of losing our P.
	// We must not touch it after this point.
	//
	// Try to do a quick CAS to avoid calling into casgstatus in the common case.
	// If we have a bubble, we need to fall into casgstatus.
	if gp.bubble != nil || !gp.atomicstatus.CompareAndSwap(_Grunning, _Gsyscall) {
		casgstatus(gp, _Grunning, _Gsyscall)
	}
	if staticLockRanking {
		// casgstatus clobbers gp.sched via systemstack under staticLockRanking. Restore it.
		save(pc, sp, bp)
	}
	if trace.ok() {
		// N.B. We don't need to go on the systemstack because traceRelease is very
		// carefully recursively nosplit. This also means we don't need to worry
		// about clobbering gp.sched.
		traceRelease(trace)
	}
	if sched.sysmonwait.Load() {
		systemstack(entersyscallWakeSysmon)
		// systemstack clobbered gp.sched, so restore it.
		save(pc, sp, bp)
	}
	gp.m.locks--
}

// debugExtendGrunningNoP is a debug mode that extends the windows in which
// we're _Grunning without a P in order to try to shake out bugs with code
// assuming this state is impossible.
const debugExtendGrunningNoP = false

// Standard syscall entry used by the go syscall library and normal cgo calls.
//
// This is exported via linkname to assembly in the syscall package and x/sys.
//
// Other packages should not be accessing entersyscall directly,
// but widely used packages access it using linkname.
// Notable members of the hall of shame include:
//   - gvisor.dev/gvisor
//
// Do not remove or change the type signature.
// See go.dev/issue/67401.
//
//go:nosplit
//go:linkname entersyscall
func entersyscall() {
	// N.B. getcallerfp cannot be written directly as argument in the call
	// to reentersyscall because it forces spilling the other arguments to
	// the stack. This results in exceeding the nosplit stack requirements
	// on some platforms.
	fp := getcallerfp()
	reentersyscall(sys.GetCallerPC(), sys.GetCallerSP(), fp)
}

func entersyscallWakeSysmon() {
	lock(&sched.lock)
	if sched.sysmonwait.Load() {
		sched.sysmonwait.Store(false)
		notewakeup(&sched.sysmonnote)
	}
	unlock(&sched.lock)
}

func entersyscallHandleGCWait(trace traceLocker) {
	gp := getg()

	lock(&sched.lock)
	if sched.stopwait > 0 {
		// Set our P to _Pgcstop so the STW can take it.
		pp := gp.m.p.ptr()
		pp.m = 0
		gp.m.p = 0
		atomic.Store(&pp.status, _Pgcstop)

		if trace.ok() {
			trace.ProcStop(pp)
		}
		addGSyscallNoP(gp.m) // We gave up our P voluntarily.
		pp.gcStopTime = nanotime()
		pp.syscalltick++
		if sched.stopwait--; sched.stopwait == 0 {
			notewakeup(&sched.stopnote)
		}
	}
	unlock(&sched.lock)
}

// The same as entersyscall(), but with a hint that the syscall is blocking.

// entersyscallblock should be an internal detail,
// but widely used packages access it using linkname.
// Notable members of the hall of shame include:
//   - gvisor.dev/gvisor
//
// Do not remove or change the type signature.
// See go.dev/issue/67401.
//
//go:linkname entersyscallblock
//go:nosplit
func entersyscallblock() {
	gp := getg()

	gp.m.locks++ // see comment in entersyscall
	gp.throwsplit = true
	gp.stackguard0 = stackPreempt // see comment in entersyscall
	gp.m.syscalltick = gp.m.p.ptr().syscalltick
	gp.m.p.ptr().syscalltick++

	addGSyscallNoP(gp.m) // We're going to give up our P.

	// Leave SP around for GC and traceback.
	pc := sys.GetCallerPC()
	sp := sys.GetCallerSP()
	bp := getcallerfp()
	save(pc, sp, bp)
	gp.syscallsp = gp.sched.sp
	gp.syscallpc = gp.sched.pc
	gp.syscallbp = gp.sched.bp
	if gp.syscallsp < gp.stack.lo || gp.stack.hi < gp.syscallsp {
		sp1 := sp
		sp2 := gp.sched.sp
		sp3 := gp.syscallsp
		systemstack(func() {
			print("entersyscallblock inconsistent sp ", hex(sp1), " ", hex(sp2), " ", hex(sp3), " [", hex(gp.stack.lo), ",", hex(gp.stack.hi), "]\n")
			throw("entersyscallblock")
		})
	}

	// Once we switch to _Gsyscall, we can't safely touch
	// our P anymore, so we need to hand it off beforehand.
	// The tracer also needs to see the syscall before the P
	// handoff, so the order here must be (1) trace,
	// (2) handoff, (3) _Gsyscall switch.
	trace := traceAcquire()
	systemstack(func() {
		if trace.ok() {
			trace.GoSysCall()
		}
		handoffp(releasep())
	})
	// <--
	// Caution: we're in a small window where we are in _Grunning without a P.
	// -->
	if debugExtendGrunningNoP {
		usleep(10)
	}
	casgstatus(gp, _Grunning, _Gsyscall)
	if gp.syscallsp < gp.stack.lo || gp.stack.hi < gp.syscallsp {
		systemstack(func() {
			print("entersyscallblock inconsistent sp ", hex(sp), " ", hex(gp.sched.sp), " ", hex(gp.syscallsp), " [", hex(gp.stack.lo), ",", hex(gp.stack.hi), "]\n")
			throw("entersyscallblock")
		})
	}
	if gp.syscallbp != 0 && gp.syscallbp < gp.stack.lo || gp.stack.hi < gp.syscallbp {
		systemstack(func() {
			print("entersyscallblock inconsistent bp ", hex(bp), " ", hex(gp.sched.bp), " ", hex(gp.syscallbp), " [", hex(gp.stack.lo), ",", hex(gp.stack.hi), "]\n")
			throw("entersyscallblock")
		})
	}
	if trace.ok() {
		systemstack(func() {
			traceRelease(trace)
		})
	}

	// Resave for traceback during blocked call.
	save(sys.GetCallerPC(), sys.GetCallerSP(), getcallerfp())

	gp.m.locks--
}

// The goroutine g exited its system call.
// Arrange for it to run on a cpu again.
// This is called only from the go syscall library, not
// from the low-level system calls used by the runtime.
//
// Write barriers are not allowed because our P may have been stolen.
//
// This is exported via linkname to assembly in the syscall package.
//
// exitsyscall should be an internal detail,
// but widely used packages access it using linkname.
// Notable members of the hall of shame include:
//   - gvisor.dev/gvisor
//
// Do not remove or change the type signature.
// See go.dev/issue/67401.
//
//go:nosplit
//go:nowritebarrierrec
//go:linkname exitsyscall
func exitsyscall() {
	gp := getg()

	gp.m.locks++ // see comment in entersyscall
	if sys.GetCallerSP() > gp.syscallsp {
		throw("exitsyscall: syscall frame is no longer valid")
	}
	gp.waitsince = 0

	if sched.stopwait == freezeStopWait {
		// Wedge ourselves if there's an outstanding freezetheworld.
		// If we transition to running, we might end up with our traceback
		// being taken twice.
		systemstack(func() {
			lock(&deadlock)
			lock(&deadlock)
		})
	}

	// Optimistically assume we're going to keep running, and switch to running.
	// Before this point, our P wiring is not ours. Once we get past this point,
	// we can access our P if we have it, otherwise we lost it.
	//
	// N.B. Because we're transitioning to _Grunning here, traceAcquire doesn't
	// need to be held ahead of time. We're effectively atomic with respect to
	// the tracer because we're non-preemptible and in the runtime. It can't stop
	// us to read a bad status.
	//
	// Try to do a quick CAS to avoid calling into casgstatus in the common case.
	// If we have a bubble, we need to fall into casgstatus.
	if gp.bubble != nil || !gp.atomicstatus.CompareAndSwap(_Gsyscall, _Grunning) {
		casgstatus(gp, _Gsyscall, _Grunning)
	}

	// Caution: we're in a window where we may be in _Grunning without a P.
	// Either we will grab a P or call exitsyscall0, where we'll switch to
	// _Grunnable.
	if debugExtendGrunningNoP {
		usleep(10)
	}

	// Grab and clear our old P.
	oldp := gp.m.oldp.ptr()
	gp.m.oldp.set(nil)

	// Check if we still have a P, and if not, try to acquire an idle P.
	pp := gp.m.p.ptr()
	if pp != nil {
		// Fast path: we still have our P. Just emit a syscall exit event.
		if trace := traceAcquire(); trace.ok() {
			systemstack(func() {
				// The truth is we truly never lost the P, but syscalltick
				// is used to indicate whether the P should be treated as
				// lost anyway. For example, when syscalltick is trashed by
				// dropm.
				//
				// TODO(mknyszek): Consider a more explicit mechanism for this.
				// Then syscalltick doesn't need to be trashed, and can be used
				// exclusively by sysmon for deciding when it's time to retake.
				if pp.syscalltick == gp.m.syscalltick {
					trace.GoSysExit(false)
				} else {
					// Since we need to pretend we lost the P, but nobody ever
					// took it, we need a ProcSteal event to model the loss.
					// Then, continue with everything else we'd do if we lost
					// the P.
					trace.ProcSteal(pp)
					trace.ProcStart()
					trace.GoSysExit(true)
					trace.GoStart()
				}
				traceRelease(trace)
			})
		}
	} else {
		// Slow path: we lost our P. Try to get another one.
		systemstack(func() {
			// Try to get some other P.
			if pp := exitsyscallTryGetP(oldp); pp != nil {
				// Install the P.
				acquirepNoTrace(pp)

				// We're going to start running again, so emit all the relevant events.
				if trace := traceAcquire(); trace.ok() {
					trace.ProcStart()
					trace.GoSysExit(true)
					trace.GoStart()
					traceRelease(trace)
				}
			}
		})
		pp = gp.m.p.ptr()
	}

	// If we have a P, clean up and exit.
	if pp != nil {
		if goroutineProfile.active {
			// Make sure that gp has had its stack written out to the goroutine
			// profile, exactly as it was when the goroutine profiler first
			// stopped the world.
			systemstack(func() {
				tryRecordGoroutineProfileWB(gp)
			})
		}

		// Increment the syscalltick for P, since we're exiting a syscall.
		pp.syscalltick++

		// Garbage collector isn't running (since we are),
		// so okay to clear syscallsp.
		gp.syscallsp = 0
		gp.m.locks--
		if gp.preempt {
			// Restore the preemption request in case we cleared it in newstack.
			gp.stackguard0 = stackPreempt
		} else {
			// Otherwise restore the real stackGuard, we clobbered it in entersyscall/entersyscallblock.
			gp.stackguard0 = gp.stack.lo + stackGuard
		}
		gp.throwsplit = false

		if sched.disable.user && !schedEnabled(gp) {
			// Scheduling of this goroutine is disabled.
			Gosched()
		}
		return
	}
	// Slowest path: We couldn't get a P, so call into the scheduler.
	gp.m.locks--

	// Call the scheduler.
	mcall(exitsyscallNoP)

	// Scheduler returned, so we're allowed to run now.
	// Delete the syscallsp information that we left for
	// the garbage collector during the system call.
	// Must wait until now because until gosched returns
	// we don't know for sure that the garbage collector
	// is not running.
	gp.syscallsp = 0
	gp.m.p.ptr().syscalltick++
	gp.throwsplit = false
}

// exitsyscall's attempt to try to get any P, if it's missing one.
// Returns true on success.
//
// Must execute on the systemstack because exitsyscall is nosplit.
//
//go:systemstack
func exitsyscallTryGetP(oldp *p) *p {
	// Try to steal our old P back.
	if oldp != nil {
		if thread, ok := setBlockOnExitSyscall(oldp); ok {
			thread.takeP()
			decGSyscallNoP(getg().m) // We got a P for ourselves.
			thread.resume()
			return oldp
		}
	}

	// Try to get an idle P.
	if sched.pidle != 0 {
		lock(&sched.lock)
		pp, _ := pidleget(0)
		if pp != nil && sched.sysmonwait.Load() {
			sched.sysmonwait.Store(false)
			notewakeup(&sched.sysmonnote)
		}
		unlock(&sched.lock)
		if pp != nil {
			decGSyscallNoP(getg().m) // We got a P for ourselves.
			return pp
		}
	}
	return nil
}

// exitsyscall slow path on g0.
// Failed to acquire P, enqueue gp as runnable.
//
// Called via mcall, so gp is the calling g from this M.
//
//go:nowritebarrierrec
func exitsyscallNoP(gp *g) {
	traceExitingSyscall()
	trace := traceAcquire()
	casgstatus(gp, _Grunning, _Grunnable)
	traceExitedSyscall()
	if trace.ok() {
		// Write out syscall exit eagerly.
		//
		// It's important that we write this *after* we know whether we
		// lost our P or not (determined by exitsyscallfast).
		trace.GoSysExit(true)
		traceRelease(trace)
	}
	decGSyscallNoP(getg().m)
	dropg()
	lock(&sched.lock)
	var pp *p
	if schedEnabled(gp) {
		pp, _ = pidleget(0)
	}
	var locked bool
	if pp == nil {
		globrunqput(gp)

		// Below, we stoplockedm if gp is locked. globrunqput releases
		// ownership of gp, so we must check if gp is locked prior to
		// committing the release by unlocking sched.lock, otherwise we
		// could race with another M transitioning gp from unlocked to
		// locked.
		locked = gp.lockedm != 0
	} else if sched.sysmonwait.Load() {
		sched.sysmonwait.Store(false)
		notewakeup(&sched.sysmonnote)
	}
	unlock(&sched.lock)
	if pp != nil {
		acquirep(pp)
		execute(gp, false) // Never returns.
	}
	if locked {
		// Wait until another thread schedules gp and so m again.
		//
		// N.B. lockedm must be this M, as this g was running on this M
		// before entersyscall.
		stoplockedm()
		execute(gp, false) // Never returns.
	}
	stopm()
	schedule() // Never returns.
}

// addGSyscallNoP must be called when a goroutine in a syscall loses its P.
// This function updates all relevant accounting.
//
// nosplit because it's called on the syscall paths.
//
//go:nosplit
func addGSyscallNoP(mp *m) {
	// It's safe to read isExtraInC here because it's only mutated
	// outside of _Gsyscall, and we know this thread is attached
	// to a goroutine in _Gsyscall and blocked from exiting.
	if !mp.isExtraInC {
		// Increment nGsyscallNoP since we're taking away a P
		// from a _Gsyscall goroutine, but only if isExtraInC
		// is not set on the M. If it is, then this thread is
		// back to being a full C thread, and will just inflate
		// the count of not-in-go goroutines. See go.dev/issue/76435.
		sched.nGsyscallNoP.Add(1)
	}
}

// decGSsyscallNoP must be called whenever a goroutine in a syscall without
// a P exits the system call. This function updates all relevant accounting.
//
// nosplit because it's called from dropm.
//
//go:nosplit
func decGSyscallNoP(mp *m) {
	// Update nGsyscallNoP, but only if this is not a thread coming
	// out of C. See the comment in addGSyscallNoP. This logic must match,
	// to avoid unmatched increments and decrements.
	if !mp.isExtraInC {
		sched.nGsyscallNoP.Add(-1)
	}
}

// Called from syscall package before fork.
//
// syscall_runtime_BeforeFork is for package syscall,
// but widely used packages access it using linkname.
// Notable members of the hall of shame include:
//   - gvisor.dev/gvisor
//
// Do not remove or change the type signature.
// See go.dev/issue/67401.
//
//go:linkname syscall_runtime_BeforeFork syscall.runtime_BeforeFork
//go:nosplit
func syscall_runtime_BeforeFork() {
	gp := getg().m.curg

	// Block signals during a fork, so that the child does not run
	// a signal handler before exec if a signal is sent to the process
	// group. See issue #18600.
	gp.m.locks++
	sigsave(&gp.m.sigmask)
	sigblock(false)

	// This function is called before fork in syscall package.
	// Code between fork and exec must not allocate memory nor even try to grow stack.
	// Here we spoil g.stackguard0 to reliably detect any attempts to grow stack.
	// runtime_AfterFork will undo this in parent process, but not in child.
	gp.stackguard0 = stackFork
}

// Called from syscall package after fork in parent.
//
// syscall_runtime_AfterFork is for package syscall,
// but widely used packages access it using linkname.
// Notable members of the hall of shame include:
//   - gvisor.dev/gvisor
//
// Do not remove or change the type signature.
// See go.dev/issue/67401.
//
//go:linkname syscall_runtime_AfterFork syscall.runtime_AfterFork
//go:nosplit
func syscall_runtime_AfterFork() {
	gp := getg().m.curg

	// See the comments in beforefork.
	gp.stackguard0 = gp.stack.lo + stackGuard

	msigrestore(gp.m.sigmask)

	gp.m.locks--
}

// inForkedChild is true while manipulating signals in the child process.
// This is used to avoid calling libc functions in case we are using vfork.
var inForkedChild bool

// Called from syscall package after fork in child.
// It resets non-sigignored signals to the default handler, and
// restores the signal mask in preparation for the exec.
//
// Because this might be called during a vfork, and therefore may be
// temporarily sharing address space with the parent process, this must
// not change any global variables or calling into C code that may do so.
//
// syscall_runtime_AfterForkInChild is for package syscall,
// but widely used packages access it using linkname.
// Notable members of the hall of shame include:
//   - gvisor.dev/gvisor
//
// Do not remove or change the type signature.
// See go.dev/issue/67401.
//
//go:linkname syscall_runtime_AfterForkInChild syscall.runtime_AfterForkInChild
//go:nosplit
//go:nowritebarrierrec
func syscall_runtime_AfterForkInChild() {
	// It's OK to change the global variable inForkedChild here
	// because we are going to change it back. There is no race here,
	// because if we are sharing address space with the parent process,
	// then the parent process can not be running concurrently.
	inForkedChild = true

	clearSignalHandlers()

	// When we are the child we are the only thread running,
	// so we know that nothing else has changed gp.m.sigmask.
	msigrestore(getg().m.sigmask)

	inForkedChild = false
}

// pendingPreemptSignals is the number of preemption signals
// that have been sent but not received. This is only used on Darwin.
// For #41702.
var pendingPreemptSignals atomic.Int32

// Called from syscall package before Exec.
//
//go:linkname syscall_runtime_BeforeExec syscall.runtime_BeforeExec
func syscall_runtime_BeforeExec() {
	// Prevent thread creation during exec.
	execLock.lock()

	// On Darwin, wait for all pending preemption signals to
	// be received. See issue #41702.
	if GOOS == "darwin" || GOOS == "ios" {
		for pendingPreemptSignals.Load() > 0 {
			osyield()
		}
	}
}

// Called from syscall package after Exec.
//
//go:linkname syscall_runtime_AfterExec syscall.runtime_AfterExec
func syscall_runtime_AfterExec() {
	execLock.unlock()
}

// Allocate a new g, with a stack big enough for stacksize bytes.
func malg(stacksize int32) *g {
	newg := new(g)
	if stacksize >= 0 {
		stacksize = round2(stackSystem + stacksize)
		systemstack(func() {
			newg.stack = stackalloc(uint32(stacksize))
			if valgrindenabled {
				newg.valgrindStackID = valgrindRegisterStack(unsafe.Pointer(newg.stack.lo), unsafe.Pointer(newg.stack.hi))
			}
		})
		newg.stackguard0 = newg.stack.lo + stackGuard
		newg.stackguard1 = ^uintptr(0)
		// Clear the bottom word of the stack. We record g
		// there on gsignal stack during VDSO on ARM and ARM64.
		*(*uintptr)(unsafe.Pointer(newg.stack.lo)) = 0
	}
	return newg
}

// Create a new g running fn.
// Put it on the queue of g's waiting to run.
// The compiler turns a go statement into a call to this.
func newproc(fn *funcval) {
	gp := getg()
	pc := sys.GetCallerPC()
	systemstack(func() {
		newg := newproc1(fn, gp, pc, false, waitReasonZero)

		pp := getg().m.p.ptr()
		runqput(pp, newg, true)

		if mainStarted {
			wakep()
		}
	})
}

// Create a new g in state _Grunnable (or _Gwaiting if parked is true), starting at fn.
// callerpc is the address of the go statement that created this. The caller is responsible
// for adding the new g to the scheduler. If parked is true, waitreason must be non-zero.
func newproc1(fn *funcval, callergp *g, callerpc uintptr, parked bool, waitreason waitReason) *g {
	if fn == nil {
		fatal("go of nil func value")
	}

	mp := acquirem() // disable preemption because we hold M and P in local vars.
	pp := mp.p.ptr()
	newg := gfget(pp)
	if newg == nil {
		newg = malg(stackMin)
		casgstatus(newg, _Gidle, _Gdead)
		allgadd(newg) // publishes with a g->status of Gdead so GC scanner doesn't look at uninitialized stack.
	}
	if newg.stack.hi == 0 {
		throw("newproc1: newg missing stack")
	}

	if readgstatus(newg) != _Gdead {
		throw("newproc1: new g is not Gdead")
	}

	totalSize := uintptr(4*goarch.PtrSize + sys.MinFrameSize) // extra space in case of reads slightly beyond frame
	totalSize = alignUp(totalSize, sys.StackAlign)
	sp := newg.stack.hi - totalSize
	if usesLR {
		// caller's LR
		*(*uintptr)(unsafe.Pointer(sp)) = 0
		prepGoExitFrame(sp)
	}
	if GOARCH == "arm64" {
		// caller's FP
		*(*uintptr)(unsafe.Pointer(sp - goarch.PtrSize)) = 0
	}

	memclrNoHeapPointers(unsafe.Pointer(&newg.sched), unsafe.Sizeof(newg.sched))
	newg.sched.sp = sp
	newg.stktopsp = sp
	newg.sched.pc = abi.FuncPCABI0(goexit) + sys.PCQuantum // +PCQuantum so that previous instruction is in same function
	newg.sched.g = guintptr(unsafe.Pointer(newg))
	gostartcallfn(&newg.sched, fn)
	newg.parentGoid = callergp.goid
	newg.gopc = callerpc
	newg.ancestors = saveAncestors(callergp)
	newg.startpc = fn.fn
	newg.runningCleanups.Store(false)
	if isSystemGoroutine(newg, false) {
		sched.ngsys.Add(1)
	} else {
		// Only user goroutines inherit synctest groups and pprof labels.
		newg.bubble = callergp.bubble
		if mp.curg != nil {
			newg.labels = mp.curg.labels
		}
		if goroutineProfile.active {
			// A concurrent goroutine profile is running. It should include
			// exactly the set of goroutines that were alive when the goroutine
			// profiler first stopped the world. That does not include newg, so
			// mark it as not needing a profile before transitioning it from
			// _Gdead.
			newg.goroutineProfiled.Store(goroutineProfileSatisfied)
		}
	}
	// Track initial transition?
	newg.trackingSeq = uint8(cheaprand())
	if newg.trackingSeq%gTrackingPeriod == 0 {
		newg.tracking = true
	}
	gcController.addScannableStack(pp, int64(newg.stack.hi-newg.stack.lo))

	// Get a goid and switch to runnable. This needs to happen under traceAcquire
	// since it's a goroutine transition. See tracer invariants in trace.go.
	trace := traceAcquire()
	var status uint32 = _Grunnable
	if parked {
		status = _Gwaiting
		newg.waitreason = waitreason
	}
	if pp.goidcache == pp.goidcacheend {
		// Sched.goidgen is the last allocated id,
		// this batch must be [sched.goidgen+1, sched.goidgen+GoidCacheBatch].
		// At startup sched.goidgen=0, so main goroutine receives goid=1.
		pp.goidcache = sched.goidgen.Add(_GoidCacheBatch)
		pp.goidcache -= _GoidCacheBatch - 1
		pp.goidcacheend = pp.goidcache + _GoidCacheBatch
	}
	newg.goid = pp.goidcache
	casgstatus(newg, _Gdead, status)
	pp.goidcache++
	newg.trace.reset()
	if trace.ok() {
		trace.GoCreate(newg, newg.startpc, parked)
		traceRelease(trace)
	}

	// fips140 bubble
	newg.fipsOnlyBypass = callergp.fipsOnlyBypass

	// dit bubble
	newg.ditWanted = callergp.ditWanted

	if goexperiment.RuntimeSecret && callergp.secret > 0 {
		// while it might seem weird to have a non-zero gp.secret value
		// with no calls to secret.Do on the stack, this case is handled
		// just fine by the cleanup logic in goexit0
		// TODO: secret mode is invisible to the user if they don't ask about it via secret.Enabled
		// and can have severe performance penalties (at time of writing, wrapping the entire
		// tls handshake resulted in a 30% slowdown of the benchmarks).
		// Whether a goroutine is running in secret mode should be more visible,
		// maybe with a stack frame or some sort of bubble inspecting mechanism
		newg.secret = 1
	}

	// Set up race context.
	if raceenabled {
		newg.racectx = racegostart(callerpc)
		newg.raceignore = 0
		if newg.labels != nil {
			// See note in proflabel.go on labelSync's role in synchronizing
			// with the reads in the signal handler.
			racereleasemergeg(newg, unsafe.Pointer(&labelSync))
		}
	}
	pp.goroutinesCreated++
	releasem(mp)

	return newg
}

// saveAncestors copies previous ancestors of the given caller g and
// includes info for the current caller into a new set of tracebacks for
// a g being created.
func saveAncestors(callergp *g) *[]ancestorInfo {
	// Copy all prior info, except for the root goroutine (goid 0).
	if debug.tracebackancestors <= 0 || callergp.goid == 0 {
		return nil
	}
	var callerAncestors []ancestorInfo
	if callergp.ancestors != nil {
		callerAncestors = *callergp.ancestors
	}
	n := int32(len(callerAncestors)) + 1
	if n > debug.tracebackancestors {
		n = debug.tracebackancestors
	}
	ancestors := make([]ancestorInfo, n)
	copy(ancestors[1:], callerAncestors)

	var pcs [tracebackInnerFrames]uintptr
	npcs := gcallers(callergp, 0, pcs[:])
	ipcs := make([]uintptr, npcs)
	copy(ipcs, pcs[:])
	ancestors[0] = ancestorInfo{
		pcs:  ipcs,
		goid: callergp.goid,
		gopc: callergp.gopc,
	}

	ancestorsp := new([]ancestorInfo)
	*ancestorsp = ancestors
	return ancestorsp
}

// Put on gfree list.
// If local list is too long, transfer a batch to the global list.
func gfput(pp *p, gp *g) {
	if readgstatus(gp) != _Gdead {
		throw("gfput: bad status (not Gdead)")
	}

	stksize := gp.stack.hi - gp.stack.lo

	if stksize != uintptr(startingStackSize) {
		// non-standard stack size - free it.
		stackfree(gp.stack)
		gp.stack.lo = 0
		gp.stack.hi = 0
		gp.stackguard0 = 0
		if valgrindenabled {
			valgrindDeregisterStack(gp.valgrindStackID)
			gp.valgrindStackID = 0
		}
	}

	pp.gFree.push(gp)
	if pp.gFree.size >= 64 {
		var (
			stackQ   gQueue
			noStackQ gQueue
		)
		for pp.gFree.size >= 32 {
			gp := pp.gFree.pop()
			if gp.stack.lo == 0 {
				noStackQ.push(gp)
			} else {
				stackQ.push(gp)
			}
		}
		lock(&sched.gFree.lock)
		sched.gFree.noStack.pushAll(noStackQ)
		sched.gFree.stack.pushAll(stackQ)
		unlock(&sched.gFree.lock)
	}
}

// Get from gfree list.
// If local list is empty, grab a batch from global list.
func gfget(pp *p) *g {
retry:
	if pp.gFree.empty() && (!sched.gFree.stack.empty() || !sched.gFree.noStack.empty()) {
		lock(&sched.gFree.lock)
		// Move a batch of free Gs to the P.
		for pp.gFree.size < 32 {
			// Prefer Gs with stacks.
			gp := sched.gFree.stack.pop()
			if gp == nil {
				gp = sched.gFree.noStack.pop()
				if gp == nil {
					break
				}
			}
			pp.gFree.push(gp)
		}
		unlock(&sched.gFree.lock)
		goto retry
	}
	gp := pp.gFree.pop()
	if gp == nil {
		return nil
	}
	if gp.stack.lo != 0 && gp.stack.hi-gp.stack.lo != uintptr(startingStackSize) {
		// Deallocate old stack. We kept it in gfput because it was the
		// right size when the goroutine was put on the free list, but
		// the right size has changed since then.
		systemstack(func() {
			stackfree(gp.stack)
			gp.stack.lo = 0
			gp.stack.hi = 0
			gp.stackguard0 = 0
			if valgrindenabled {
				valgrindDeregisterStack(gp.valgrindStackID)
				gp.valgrindStackID = 0
			}
		})
	}
	if gp.stack.lo == 0 {
		// Stack was deallocated in gfput or just above. Allocate a new one.
		systemstack(func() {
			gp.stack = stackalloc(startingStackSize)
			if valgrindenabled {
				gp.valgrindStackID = valgrindRegisterStack(unsafe.Pointer(gp.stack.lo), unsafe.Pointer(gp.stack.hi))
			}
		})
		gp.stackguard0 = gp.stack.lo + stackGuard
	} else {
		if raceenabled {
			racemalloc(unsafe.Pointer(gp.stack.lo), gp.stack.hi-gp.stack.lo)
		}
		if msanenabled {
			msanmalloc(unsafe.Pointer(gp.stack.lo), gp.stack.hi-gp.stack.lo)
		}
		if asanenabled {
			asanunpoison(unsafe.Pointer(gp.stack.lo), gp.stack.hi-gp.stack.lo)
		}
	}
	return gp
}

// Purge all cached G's from gfree list to the global list.
func gfpurge(pp *p) {
	var (
		stackQ   gQueue
		noStackQ gQueue
	)
	for !pp.gFree.empty() {
		gp := pp.gFree.pop()
		if gp.stack.lo == 0 {
			noStackQ.push(gp)
		} else {
			stackQ.push(gp)
		}
	}
	lock(&sched.gFree.lock)
	sched.gFree.noStack.pushAll(noStackQ)
	sched.gFree.stack.pushAll(stackQ)
	unlock(&sched.gFree.lock)
}

// Breakpoint executes a breakpoint trap.
func Breakpoint() {
	breakpoint()
}

// dolockOSThread is called by LockOSThread and lockOSThread below
// after they modify m.locked. Do not allow preemption during this call,
// or else the m might be different in this function than in the caller.
//
//go:nosplit
func dolockOSThread() {
	if GOARCH == "wasm" {
		return // no threads on wasm yet
	}
	gp := getg()
	gp.m.lockedg.set(gp)
	gp.lockedm.set(gp.m)
}

// LockOSThread wires the calling goroutine to its current operating system thread.
// The calling goroutine will always execute in that thread,
// and no other goroutine will execute in it,
// until the calling goroutine has made as many calls to
// [UnlockOSThread] as to LockOSThread.
// If the calling goroutine exits without unlocking the thread,
// the thread will be terminated.
//
// All init functions are run on the startup thread. Calling LockOSThread
// from an init function will cause the main function to be invoked on
// that thread.
//
// A goroutine should call LockOSThread before calling OS services or
// non-Go library functions that depend on per-thread state.
//
//go:nosplit
func LockOSThread() {
	if atomic.Load(&newmHandoff.haveTemplateThread) == 0 && GOOS != "plan9" {
		// If we need to start a new thread from the locked
		// thread, we need the template thread. Start it now
		// while we're in a known-good state.
		startTemplateThread()
	}
	gp := getg()
	gp.m.lockedExt++
	if gp.m.lockedExt == 0 {
		gp.m.lockedExt--
		panic("LockOSThread nesting overflow")
	}
	dolockOSThread()
}

//go:nosplit
func lockOSThread() {
	getg().m.lockedInt++
	dolockOSThread()
}

// dounlockOSThread is called by UnlockOSThread and unlockOSThread below
// after they update m->locked. Do not allow preemption during this call,
// or else the m might be in different in this function than in the caller.
//
//go:nosplit
func dounlockOSThread() {
	if GOARCH == "wasm" {
		return // no threads on wasm yet
	}
	gp := getg()
	if gp.m.lockedInt != 0 || gp.m.lockedExt != 0 {
		return
	}
	gp.m.lockedg = 0
	gp.lockedm = 0
}

// UnlockOSThread undoes an earlier call to LockOSThread.
// If this drops the number of active LockOSThread calls on the
// calling goroutine to zero, it unwires the calling goroutine from
// its fixed operating system thread.
// If there are no active LockOSThread calls, this is a no-op.
//
// Before calling UnlockOSThread, the caller must ensure that the OS
// thread is suitable for running other goroutines. If the caller made
// any permanent changes to the state of the thread that would affect
// other goroutines, it should not call this function and thus leave
// the goroutine locked to the OS thread until the goroutine (and
// hence the thread) exits.
//
//go:nosplit
func UnlockOSThread() {
	gp := getg()
	if gp.m.lockedExt == 0 {
		return
	}
	gp.m.lockedExt--
	dounlockOSThread()
}

//go:nosplit
func unlockOSThread() {
	gp := getg()
	if gp.m.lockedInt == 0 {
		systemstack(badunlockosthread)
	}
	gp.m.lockedInt--
	dounlockOSThread()
}

func badunlockosthread() {
	throw("runtime: internal error: misuse of lockOSThread/unlockOSThread")
}

func gcount(includeSys bool) int32 {
	n := int32(atomic.Loaduintptr(&allglen)) - sched.gFree.stack.size - sched.gFree.noStack.size
	if !includeSys {
		n -= sched.ngsys.Load()
	}
	for _, pp := range allp {
		n -= pp.gFree.size
	}

	// All these variables can be changed concurrently, so the result can be inconsistent.
	// But at least the current goroutine is running.
	if n < 1 {
		n = 1
	}
	return n
}

// goroutineleakcount returns the number of leaked goroutines last reported by
// the runtime.
//
//go:linkname goroutineleakcount runtime/pprof.runtime_goroutineleakcount
func goroutineleakcount() int {
	return work.goroutineLeak.count
}

func mcount() int32 {
	return int32(sched.mnext - sched.nmfreed)
}

var prof struct {
	signalLock atomic.Uint32

	// Must hold signalLock to write. Reads may be lock-free, but
	// signalLock should be taken to synchronize with changes.
	hz atomic.Int32
}

func _System()                    { _System() }
func _ExternalCode()              { _ExternalCode() }
func _LostExternalCode()          { _LostExternalCode() }
func _GC()                        { _GC() }
func _LostSIGPROFDuringAtomic64() { _LostSIGPROFDuringAtomic64() }
func _LostContendedRuntimeLock()  { _LostContendedRuntimeLock() }
func _VDSO()                      { _VDSO() }

// Called if we receive a SIGPROF signal.
// Called by the signal handler, may run during STW.
//
//go:nowritebarrierrec
func sigprof(pc, sp, lr uintptr, gp *g, mp *m) {
	if prof.hz.Load() == 0 {
		return
	}

	// If mp.profilehz is 0, then profiling is not enabled for this thread.
	// We must check this to avoid a deadlock between setcpuprofilerate
	// and the call to cpuprof.add, below.
	if mp != nil && mp.profilehz == 0 {
		return
	}

	// On mips{,le}/arm, 64bit atomics are emulated with spinlocks, in
	// internal/runtime/atomic. If SIGPROF arrives while the program is inside
	// the critical section, it creates a deadlock (when writing the sample).
	// As a workaround, create a counter of SIGPROFs while in critical section
	// to store the count, and pass it to sigprof.add() later when SIGPROF is
	// received from somewhere else (with _LostSIGPROFDuringAtomic64 as pc).
	if GOARCH == "mips" || GOARCH == "mipsle" || GOARCH == "arm" {
		if f := findfunc(pc); f.valid() {
			if stringslite.HasPrefix(funcname(f), "internal/runtime/atomic") {
				cpuprof.lostAtomic++
				return
			}
		}
		if GOARCH == "arm" && goarm < 7 && GOOS == "linux" && pc&0xffff0000 == 0xffff0000 {
			// internal/runtime/atomic functions call into kernel
			// helpers on arm < 7. See
			// internal/runtime/atomic/sys_linux_arm.s.
			cpuprof.lostAtomic++
			return
		}
	}

	// Profiling runs concurrently with GC, so it must not allocate.
	// Set a trap in case the code does allocate.
	// Note that on windows, one thread takes profiles of all the
	// other threads, so mp is usually not getg().m.
	// In fact mp may not even be stopped.
	// See golang.org/issue/17165.
	getg().m.mallocing++

	var u unwinder
	var stk [maxCPUProfStack]uintptr
	n := 0
	if mp.ncgo > 0 && mp.curg != nil && mp.curg.syscallpc != 0 && mp.curg.syscallsp != 0 {
		cgoOff := 0
		// Check cgoCallersUse to make sure that we are not
		// interrupting other code that is fiddling with
		// cgoCallers.  We are running in a signal handler
		// with all signals blocked, so we don't have to worry
		// about any other code interrupting us.
		if mp.cgoCallersUse.Load() == 0 && mp.cgoCallers != nil && mp.cgoCallers[0] != 0 {
			for cgoOff < len(mp.cgoCallers) && mp.cgoCallers[cgoOff] != 0 {
				cgoOff++
			}
			n += copy(stk[:], mp.cgoCallers[:cgoOff])
			mp.cgoCallers[0] = 0
		}

		// Collect Go stack that leads to the cgo call.
		u.initAt(mp.curg.syscallpc, mp.curg.syscallsp, 0, mp.curg, unwindSilentErrors)
	} else if usesLibcall() && mp.libcallg != 0 && mp.libcallpc != 0 && mp.libcallsp != 0 {
		// Libcall, i.e. runtime syscall on windows.
		// Collect Go stack that leads to the call.
		u.initAt(mp.libcallpc, mp.libcallsp, 0, mp.libcallg.ptr(), unwindSilentErrors)
	} else if mp != nil && mp.vdsoSP != 0 {
		// VDSO call, e.g. nanotime1 on Linux.
		// Collect Go stack that leads to the call.
		u.initAt(mp.vdsoPC, mp.vdsoSP, 0, gp, unwindSilentErrors|unwindJumpStack)
	} else {
		u.initAt(pc, sp, lr, gp, unwindSilentErrors|unwindTrap|unwindJumpStack)
	}
	n += tracebackPCs(&u, 0, stk[n:])

	if n <= 0 {
		// Normal traceback is impossible or has failed.
		// Account it against abstract "System" or "GC".
		n = 2
		if inVDSOPage(pc) {
			pc = abi.FuncPCABIInternal(_VDSO) + sys.PCQuantum
		} else if pc > firstmoduledata.etext {
			// "ExternalCode" is better than "etext".
			pc = abi.FuncPCABIInternal(_ExternalCode) + sys.PCQuantum
		}
		stk[0] = pc
		if mp.preemptoff != "" {
			stk[1] = abi.FuncPCABIInternal(_GC) + sys.PCQuantum
		} else {
			stk[1] = abi.FuncPCABIInternal(_System) + sys.PCQuantum
		}
	}

	if prof.hz.Load() != 0 {
		// Note: it can happen on Windows that we interrupted a system thread
		// with no g, so gp could nil. The other nil checks are done out of
		// caution, but not expected to be nil in practice.
		var tagPtr *unsafe.Pointer
		if gp != nil && gp.m != nil && gp.m.curg != nil {
			tagPtr = &gp.m.curg.labels
		}
		cpuprof.add(tagPtr, stk[:n])

		gprof := gp
		var mp *m
		var pp *p
		if gp != nil && gp.m != nil {
			if gp.m.curg != nil {
				gprof = gp.m.curg
			}
			mp = gp.m
			pp = gp.m.p.ptr()
		}
		traceCPUSample(gprof, mp, pp, stk[:n])
	}
	getg().m.mallocing--
}

// setcpuprofilerate sets the CPU profiling rate to hz times per second.
// If hz <= 0, setcpuprofilerate turns off CPU profiling.
func setcpuprofilerate(hz int32) {
	// Force sane arguments.
	if hz < 0 {
		hz = 0
	}

	// Disable preemption, otherwise we can be rescheduled to another thread
	// that has profiling enabled.
	gp := getg()
	gp.m.locks++

	// Stop profiler on this thread so that it is safe to lock prof.
	// if a profiling signal came in while we had prof locked,
	// it would deadlock.
	setThreadCPUProfiler(0)

	for !prof.signalLock.CompareAndSwap(0, 1) {
		osyield()
	}
	if prof.hz.Load() != hz {
		setProcessCPUProfiler(hz)
		prof.hz.Store(hz)
	}
	prof.signalLock.Store(0)

	lock(&sched.lock)
	sched.profilehz = hz
	unlock(&sched.lock)

	if hz != 0 {
		setThreadCPUProfiler(hz)
	}

	gp.m.locks--
}

// init initializes pp, which may be a freshly allocated p or a
// previously destroyed p, and transitions it to status _Pgcstop.
func (pp *p) init(id int32) {
	pp.id = id
	pp.gcw.id = id
	pp.status = _Pgcstop
	pp.sudogcache = pp.sudogbuf[:0]
	pp.deferpool = pp.deferpoolbuf[:0]
	pp.wbBuf.reset()
	if pp.mcache == nil {
		if id == 0 {
			if mcache0 == nil {
				throw("missing mcache?")
			}
			// Use the bootstrap mcache0. Only one P will get
			// mcache0: the one with ID 0.
			pp.mcache = mcache0
		} else {
			pp.mcache = allocmcache()
		}
	}
	if raceenabled && pp.raceprocctx == 0 {
		if id == 0 {
			pp.raceprocctx = raceprocctx0
			raceprocctx0 = 0 // bootstrap
		} else {
			pp.raceprocctx = raceproccreate()
		}
	}
	lockInit(&pp.timers.mu, lockRankTimers)

	// This P may get timers when it starts running. Set the mask here
	// since the P may not go through pidleget (notably P 0 on startup).
	timerpMask.set(id)
	// Similarly, we may not go through pidleget before this P starts
	// running if it is P 0 on startup.
	idlepMask.clear(id)
}

// destroy releases all of the resources associated with pp and
// transitions it to status _Pdead.
//
// sched.lock must be held and the world must be stopped.
func (pp *p) destroy() {
	assertLockHeld(&sched.lock)
	assertWorldStopped()

	// Move all runnable goroutines to the global queue
	for pp.runqhead != pp.runqtail {
		// Pop from tail of local queue
		pp.runqtail--
		gp := pp.runq[pp.runqtail%uint32(len(pp.runq))].ptr()
		// Push onto head of global queue
		globrunqputhead(gp)
	}
	if pp.runnext != 0 {
		globrunqputhead(pp.runnext.ptr())
		pp.runnext = 0
	}

	// Move all timers to the local P.
	getg().m.p.ptr().timers.take(&pp.timers)

	// No need to flush p's write barrier buffer or span queue, as Ps
	// cannot be destroyed during the mark phase.
	if phase := gcphase; phase != _GCoff {
		println("runtime: p id", pp.id, "destroyed during GC phase", phase)
		throw("P destroyed while GC is running")
	}
	// We should free the queues though.
	pp.gcw.spanq.destroy()

	clear(pp.sudogbuf[:])
	pp.sudogcache = pp.sudogbuf[:0]
	pp.pinnerCache = nil
	clear(pp.deferpoolbuf[:])
	pp.deferpool = pp.deferpoolbuf[:0]
	systemstack(func() {
		for i := 0; i < pp.mspancache.len; i++ {
			// Safe to call since the world is stopped.
			mheap_.spanalloc.free(unsafe.Pointer(pp.mspancache.buf[i]))
		}
		pp.mspancache.len = 0
		lock(&mheap_.lock)
		pp.pcache.flush(&mheap_.pages)
		unlock(&mheap_.lock)
	})
	freemcache(pp.mcache)
	pp.mcache = nil
	gfpurge(pp)
	if raceenabled {
		if pp.timers.raceCtx != 0 {
			// The race detector code uses a callback to fetch
			// the proc context, so arrange for that callback
			// to see the right thing.
			// This hack only works because we are the only
			// thread running.
			mp := getg().m
			phold := mp.p.ptr()
			mp.p.set(pp)

			racectxend(pp.timers.raceCtx)
			pp.timers.raceCtx = 0

			mp.p.set(phold)
		}
		raceprocdestroy(pp.raceprocctx)
		pp.raceprocctx = 0
	}
	pp.gcAssistTime = 0
	gcCleanups.queued += pp.cleanupsQueued
	pp.cleanupsQueued = 0
	sched.goroutinesCreated.Add(int64(pp.goroutinesCreated))
	pp.goroutinesCreated = 0
	pp.xRegs.free()
	pp.status = _Pdead
}

// Change number of processors.
//
// sched.lock must be held, and the world must be stopped.
//
// gcworkbufs must not be being modified by either the GC or the write barrier
// code, so the GC must not be running if the number of Ps actually changes.
//
// Returns list of Ps with local work, they need to be scheduled by the caller.
func procresize(nprocs int32) *p {
	assertLockHeld(&sched.lock)
	assertWorldStopped()

	old := gomaxprocs
	if old < 0 || nprocs <= 0 {
		throw("procresize: invalid arg")
	}
	trace := traceAcquire()
	if trace.ok() {
		trace.Gomaxprocs(nprocs)
		traceRelease(trace)
	}

	// update statistics
	now := nanotime()
	if sched.procresizetime != 0 {
		sched.totaltime += int64(old) * (now - sched.procresizetime)
	}
	sched.procresizetime = now

	// Grow allp if necessary.
	if nprocs > int32(len(allp)) {
		// Synchronize with retake, which could be running
		// concurrently since it doesn't run on a P.
		lock(&allpLock)
		if nprocs <= int32(cap(allp)) {
			allp = allp[:nprocs]
		} else {
			nallp := make([]*p, nprocs)
			// Copy everything up to allp's cap so we
			// never lose old allocated Ps.
			copy(nallp, allp[:cap(allp)])
			allp = nallp
		}

		idlepMask = idlepMask.resize(nprocs)
		timerpMask = timerpMask.resize(nprocs)
		work.spanqMask = work.spanqMask.resize(nprocs)
		unlock(&allpLock)
	}

	// initialize new P's
	for i := old; i < nprocs; i++ {
		pp := allp[i]
		if pp == nil {
			pp = new(p)
		}
		pp.init(i)
		atomicstorep(unsafe.Pointer(&allp[i]), unsafe.Pointer(pp))
	}

	gp := getg()
	if gp.m.p != 0 && gp.m.p.ptr().id < nprocs {
		// continue to use the current P
		gp.m.p.ptr().status = _Prunning
		gp.m.p.ptr().mcache.prepareForSweep()
	} else {
		// release the current P and acquire allp[0].
		//
		// We must do this before destroying our current P
		// because p.destroy itself has write barriers, so we
		// need to do that from a valid P.
		if gp.m.p != 0 {
			trace := traceAcquire()
			if trace.ok() {
				// Pretend that we were descheduled
				// and then scheduled again to keep
				// the trace consistent.
				trace.GoSched()
				trace.ProcStop(gp.m.p.ptr())
				traceRelease(trace)
			}
			gp.m.p.ptr().m = 0
		}
		gp.m.p = 0
		pp := allp[0]
		pp.m = 0
		pp.status = _Pidle
		acquirep(pp)
		trace := traceAcquire()
		if trace.ok() {
			trace.GoStart()
			traceRelease(trace)
		}
	}

	// g.m.p is now set, so we no longer need mcache0 for bootstrapping.
	mcache0 = nil

	// release resources from unused P's
	for i := nprocs; i < old; i++ {
		pp := allp[i]
		pp.destroy()
		// can't free P itself because it can be referenced by an M in syscall
	}

	// Trim allp.
	if int32(len(allp)) != nprocs {
		lock(&allpLock)
		allp = allp[:nprocs]
		idlepMask = idlepMask.resize(nprocs)
		timerpMask = timerpMask.resize(nprocs)
		work.spanqMask = work.spanqMask.resize(nprocs)
		unlock(&allpLock)
	}

	// Assign Ms to Ps with runnable goroutines.
	var runnablePs *p
	var runnablePsNeedM *p
	var idlePs *p
	for i := nprocs - 1; i >= 0; i-- {
		pp := allp[i]
		if gp.m.p.ptr() == pp {
			continue
		}
		pp.status = _Pidle
		if runqempty(pp) {
			pp.link.set(idlePs)
			idlePs = pp
			continue
		}

		// Prefer to run on the most recent M if it is
		// available.
		//
		// Ps with no oldm (or for which oldm is already taken
		// by an earlier P), we delay until all oldm Ps are
		// handled. Otherwise, mget may return an M that a
		// later P has in oldm.
		var mp *m
		if oldm := pp.oldm.get(); oldm != nil {
			// Returns nil if oldm is not idle.
			mp = mgetSpecific(oldm)
		}
		if mp == nil {
			// Call mget later.
			pp.link.set(runnablePsNeedM)
			runnablePsNeedM = pp
			continue
		}
		pp.m.set(mp)
		pp.link.set(runnablePs)
		runnablePs = pp
	}
	// Assign Ms to remaining runnable Ps without usable oldm. See comment
	// above.
	for runnablePsNeedM != nil {
		pp := runnablePsNeedM
		runnablePsNeedM = pp.link.ptr()

		mp := mget()
		pp.m.set(mp)
		pp.link.set(runnablePs)
		runnablePs = pp
	}

	// Now that we've assigned Ms to Ps with runnable goroutines, assign GC
	// mark workers to remaining idle Ps, if needed.
	//
	// By assigning GC workers to Ps here, we slightly speed up starting
	// the world, as we will start enough Ps to run all of the user
	// goroutines and GC mark workers all at once, rather than using a
	// sequence of wakep calls as each P's findRunnable realizes it needs
	// to run a mark worker instead of a user goroutine.
	//
	// By assigning GC workers to Ps only _after_ previously-running Ps are
	// assigned Ms, we ensure that goroutines previously running on a P
	// continue to run on the same P, with GC mark workers preferring
	// previously-idle Ps. This helps prevent goroutines from shuffling
	// around too much across STW.
	//
	// N.B., if there aren't enough Ps left in idlePs for all of the GC
	// mark workers, then findRunnable will still choose to run mark
	// workers on Ps assigned above.
	//
	// N.B., we do this during any STW in the mark phase, not just the
	// sweep termination STW that starts the mark phase. gcBgMarkWorker
	// always preempts by removing itself from the P, so even unrelated
	// STWs during the mark require that Ps reselect mark workers upon
	// restart.
	if gcBlackenEnabled != 0 {
		for idlePs != nil {
			pp := idlePs

			ok, _ := gcController.assignWaitingGCWorker(pp, now)
			if !ok {
				// No more mark workers needed.
				break
			}

			// Got a worker, P is now runnable.
			//
			// mget may return nil if there aren't enough Ms, in
			// which case startTheWorldWithSema will start one.
			//
			// N.B. findRunnableGCWorker will make the worker G
			// itself runnable.
			idlePs = pp.link.ptr()
			mp := mget()
			pp.m.set(mp)
			pp.link.set(runnablePs)
			runnablePs = pp
		}
	}

	// Finally, any remaining Ps are truly idle.
	for idlePs != nil {
		pp := idlePs
		idlePs = pp.link.ptr()
		pidleput(pp, now)
	}

	stealOrder.reset(uint32(nprocs))
	var int32p *int32 = &gomaxprocs // make compiler check that gomaxprocs is an int32
	atomic.Store((*uint32)(unsafe.Pointer(int32p)), uint32(nprocs))
	if old != nprocs {
		// Notify the limiter that the amount of procs has changed.
		gcCPULimiter.resetCapacity(now, nprocs)
	}
	return runnablePs
}

// Associate p and the current m.
//
// This function is allowed to have write barriers even if the caller
// isn't because it immediately acquires pp.
//
//go:yeswritebarrierrec
func acquirep(pp *p) {
	// Do the work.
	acquirepNoTrace(pp)

	// Emit the event.
	trace := traceAcquire()
	if trace.ok() {
		trace.ProcStart()
		traceRelease(trace)
	}
}

// Internals of acquirep, just skipping the trace events.
//
//go:yeswritebarrierrec
func acquirepNoTrace(pp *p) {
	// Do the part that isn't allowed to have write barriers.
	wirep(pp)

	// Have p; write barriers now allowed.

	// The M we're associating with will be the old M after the next
	// releasep. We must set this here because write barriers are not
	// allowed in releasep.
	pp.oldm = pp.m.ptr().self

	// Perform deferred mcache flush before this P can allocate
	// from a potentially stale mcache.
	pp.mcache.prepareForSweep()
}

// wirep is the first step of acquirep, which actually associates the
// current M to pp. This is broken out so we can disallow write
// barriers for this part, since we don't yet have a P.
//
//go:nowritebarrierrec
//go:nosplit
func wirep(pp *p) {
	gp := getg()

	if gp.m.p != 0 {
		// Call on the systemstack to avoid a nosplit overflow build failure
		// on some platforms when built with -N -l. See #64113.
		systemstack(func() {
			throw("wirep: already in go")
		})
	}
	if pp.m != 0 || pp.status != _Pidle {
		// Call on the systemstack to avoid a nosplit overflow build failure
		// on some platforms when built with -N -l. See #64113.
		systemstack(func() {
			id := int64(0)
			if pp.m != 0 {
				id = pp.m.ptr().id
			}
			print("wirep: p->m=", pp.m, "(", id, ") p->status=", pp.status, "\n")
			throw("wirep: invalid p state")
		})
	}
	gp.m.p.set(pp)
	pp.m.set(gp.m)
	pp.status = _Prunning
}

// Disassociate p and the current m.
func releasep() *p {
	trace := traceAcquire()
	if trace.ok() {
		trace.ProcStop(getg().m.p.ptr())
		traceRelease(trace)
	}
	return releasepNoTrace()
}

// Disassociate p and the current m without tracing an event.
func releasepNoTrace() *p {
	gp := getg()

	if gp.m.p == 0 {
		throw("releasep: invalid arg")
	}
	pp := gp.m.p.ptr()
	if pp.m.ptr() != gp.m || pp.status != _Prunning {
		print("releasep: m=", gp.m, " m->p=", gp.m.p.ptr(), " p->m=", hex(pp.m), " p->status=", pp.status, "\n")
		throw("releasep: invalid p state")
	}

	// P must clear if nextGCMarkWorker if it stops.
	gcController.releaseNextGCMarkWorker(pp)

	gp.m.p = 0
	pp.m = 0
	pp.status = _Pidle
	return pp
}

func incidlelocked(v int32) {
	lock(&sched.lock)
	sched.nmidlelocked += v
	if v > 0 {
		checkdead()
	}
	unlock(&sched.lock)
}

// Check for deadlock situation.
// The check is based on number of running M's, if 0 -> deadlock.
// sched.lock must be held.
func checkdead() {
	assertLockHeld(&sched.lock)

	// For -buildmode=c-shared or -buildmode=c-archive it's OK if
	// there are no running goroutines. The calling program is
	// assumed to be running.
	// One exception is Wasm, which is single-threaded. If we are
	// in Go and all goroutines are blocked, it deadlocks.
	if (islibrary || isarchive) && GOARCH != "wasm" {
		return
	}

	// If we are dying because of a signal caught on an already idle thread,
	// freezetheworld will cause all running threads to block.
	// And runtime will essentially enter into deadlock state,
	// except that there is a thread that will call exit soon.
	if panicking.Load() > 0 {
		return
	}

	// If we are not running under cgo, but we have an extra M then account
	// for it. (It is possible to have an extra M on Windows without cgo to
	// accommodate callbacks created by syscall.NewCallback. See issue #6751
	// for details.)
	var run0 int32
	if !iscgo && cgoHasExtraM && extraMLength.Load() > 0 {
		run0 = 1
	}

	run := mcount() - sched.nmidle - sched.nmidlelocked - sched.nmsys
	if run > run0 {
		return
	}
	if run < 0 {
		print("runtime: checkdead: nmidle=", sched.nmidle, " nmidlelocked=", sched.nmidlelocked, " mcount=", mcount(), " nmsys=", sched.nmsys, "\n")
		unlock(&sched.lock)
		throw("checkdead: inconsistent counts")
	}

	grunning := 0
	forEachG(func(gp *g) {
		if isSystemGoroutine(gp, false) {
			return
		}
		s := readgstatus(gp)
		switch s &^ _Gscan {
		case _Gwaiting,
			_Gpreempted:
			grunning++
		case _Grunnable,
			_Grunning,
			_Gsyscall:
			print("runtime: checkdead: find g ", gp.goid, " in status ", s, "\n")
			unlock(&sched.lock)
			throw("checkdead: runnable g")
		}
	})
	if grunning == 0 { // possible if main goroutine calls runtime·Goexit()
		unlock(&sched.lock) // unlock so that GODEBUG=scheddetail=1 doesn't hang
		fatal("no goroutines (main called runtime.Goexit) - deadlock!")
	}

	// Maybe jump time forward for playground.
	if faketime != 0 {
		if when := timeSleepUntil(); when < maxWhen {
			faketime = when

			// Start an M to steal the timer.
			pp, _ := pidleget(faketime)
			if pp == nil {
				// There should always be a free P since
				// nothing is running.
				unlock(&sched.lock)
				throw("checkdead: no p for timer")
			}
			mp := mget()
			if mp == nil {
				// There should always be a free M since
				// nothing is running.
				unlock(&sched.lock)
				throw("checkdead: no m for timer")
			}
			// M must be spinning to steal. We set this to be
			// explicit, but since this is the only M it would
			// become spinning on its own anyways.
			sched.nmspinning.Add(1)
			mp.spinning = true
			mp.nextp.set(pp)
			notewakeup(&mp.park)
			return
		}
	}

	// There are no goroutines running, so we can look at the P's.
	for _, pp := range allp {
		if len(pp.timers.heap) > 0 {
			return
		}
	}

	unlock(&sched.lock) // unlock so that GODEBUG=scheddetail=1 doesn't hang
	fatal("all goroutines are asleep - deadlock!")
}

// forcegcperiod is the maximum time in nanoseconds between garbage
// collections. If we go this long without a garbage collection, one
// is forced to run.
//
// This is a variable for testing purposes. It normally doesn't change.
var forcegcperiod int64 = 2 * 60 * 1e9

// haveSysmon indicates whether there is sysmon thread support.
//
// No threads on wasm yet, so no sysmon.
const haveSysmon = GOARCH != "wasm"

// Always runs without a P, so write barriers are not allowed.
//
//go:nowritebarrierrec
func sysmon() {
	lock(&sched.lock)
	sched.nmsys++
	checkdead()
	unlock(&sched.lock)

	lastgomaxprocs := int64(0)
	lasttrace := int64(0)
	idle := 0 // how many cycles in succession we had not wokeup somebody
	delay := uint32(0)

	for {
		if idle == 0 { // start with 20us sleep...
			delay = 20
		} else if idle > 50 { // start doubling the sleep after 1ms...
			delay *= 2
		}
		if delay > 10*1000 { // up to 10ms
			delay = 10 * 1000
		}
		usleep(delay)

		// sysmon should not enter deep sleep if schedtrace is enabled so that
		// it can print that information at the right time.
		//
		// It should also not enter deep sleep if there are any active P's so
		// that it can retake P's from syscalls, preempt long running G's, and
		// poll the network if all P's are busy for long stretches.
		//
		// It should wakeup from deep sleep if any P's become active either due
		// to exiting a syscall or waking up due to a timer expiring so that it
		// can resume performing those duties. If it wakes from a syscall it
		// resets idle and delay as a bet that since it had retaken a P from a
		// syscall before, it may need to do it again shortly after the
		// application starts work again. It does not reset idle when waking
		// from a timer to avoid adding system load to applications that spend
		// most of their time sleeping.
		now := nanotime()
		if debug.schedtrace <= 0 && (sched.gcwaiting.Load() || sched.npidle.Load() == gomaxprocs) {
			lock(&sched.lock)
			if sched.gcwaiting.Load() || sched.npidle.Load() == gomaxprocs {
				syscallWake := false
				next := timeSleepUntil()
				if next > now {
					sched.sysmonwait.Store(true)
					unlock(&sched.lock)
					// Make wake-up period small enough
					// for the sampling to be correct.
					sleep := forcegcperiod / 2
					if next-now < sleep {
						sleep = next - now
					}
					shouldRelax := sleep >= osRelaxMinNS
					if shouldRelax {
						osRelax(true)
					}
					syscallWake = notetsleep(&sched.sysmonnote, sleep)
					if shouldRelax {
						osRelax(false)
					}
					lock(&sched.lock)
					sched.sysmonwait.Store(false)
					noteclear(&sched.sysmonnote)
				}
				if syscallWake {
					idle = 0
					delay = 20
				}
			}
			unlock(&sched.lock)
		}

		lock(&sched.sysmonlock)
		// Update now in case we blocked on sysmonnote or spent a long time
		// blocked on schedlock or sysmonlock above.
		now = nanotime()

		// trigger libc interceptors if needed
		if *cgo_yield != nil {
			asmcgocall(*cgo_yield, nil)
		}
		// poll network if not polled for more than 10ms
		lastpoll := sched.lastpoll.Load()
		if netpollinited() && lastpoll != 0 && lastpoll+10*1000*1000 < now {
			sched.lastpoll.CompareAndSwap(lastpoll, now)
			list, delta := netpoll(0) // non-blocking - returns list of goroutines
			if !list.empty() {
				// Need to decrement number of idle locked M's
				// (pretending that one more is running) before injectglist.
				// Otherwise it can lead to the following situation:
				// injectglist grabs all P's but before it starts M's to run the P's,
				// another M returns from syscall, finishes running its G,
				// observes that there is no work to do and no other running M's
				// and reports deadlock.
				incidlelocked(-1)
				injectglist(&list)
				incidlelocked(1)
				netpollAdjustWaiters(delta)
			}
		}
		// Check if we need to update GOMAXPROCS at most once per second.
		if debug.updatemaxprocs != 0 && lastgomaxprocs+1e9 <= now {
			sysmonUpdateGOMAXPROCS()
			lastgomaxprocs = now
		}
		if scavenger.sysmonWake.Load() != 0 {
			// Kick the scavenger awake if someone requested it.
			scavenger.wake()
		}
		// retake P's blocked in syscalls
		// and preempt long running G's
		if retake(now) != 0 {
			idle = 0
		} else {
			idle++
		}
		// check if we need to force a GC
		if t := (gcTrigger{kind: gcTriggerTime, now: now}); t.test() && forcegc.idle.Load() {
			lock(&forcegc.lock)
			forcegc.idle.Store(false)
			var list gList
			list.push(forcegc.g)
			injectglist(&list)
			unlock(&forcegc.lock)
		}
		if debug.schedtrace > 0 && lasttrace+int64(debug.schedtrace)*1000000 <= now {
			lasttrace = now
			schedtrace(debug.scheddetail > 0)
		}
		unlock(&sched.sysmonlock)
	}
}

type sysmontick struct {
	schedtick   uint32
	syscalltick uint32
	schedwhen   int64
	syscallwhen int64
}

// forcePreemptNS is the time slice given to a G before it is
// preempted.
const forcePreemptNS = 10 * 1000 * 1000 // 10ms

func retake(now int64) uint32 {
	n := 0
	// Prevent allp slice changes. This lock will be completely
	// uncontended unless we're already stopping the world.
	lock(&allpLock)
	// We can't use a range loop over allp because we may
	// temporarily drop the allpLock. Hence, we need to re-fetch
	// allp each time around the loop.
	for i := 0; i < len(allp); i++ {
		// Quickly filter out non-running Ps. Running Ps are either
		// in a syscall or are actually executing. Idle Ps don't
		// need to be retaken.
		//
		// This is best-effort, so it's OK that it's racy. Our target
		// is to retake Ps that have been running or in a syscall for
		// a long time (milliseconds), so the state has plenty of time
		// to stabilize.
		pp := allp[i]
		if pp == nil || atomic.Load(&pp.status) != _Prunning {
			// pp can be nil if procresize has grown
			// allp but not yet created new Ps.
			continue
		}
		pd := &pp.sysmontick
		sysretake := false

		// Preempt G if it's running on the same schedtick for
		// too long. This could be from a single long-running
		// goroutine or a sequence of goroutines run via
		// runnext, which share a single schedtick time slice.
		schedt := int64(pp.schedtick)
		if int64(pd.schedtick) != schedt {
			pd.schedtick = uint32(schedt)
			pd.schedwhen = now
		} else if pd.schedwhen+forcePreemptNS <= now {
			preemptone(pp)
			// If pp is in a syscall, preemptone doesn't work.
			// The goroutine nor the thread can respond to a
			// preemption request because they're not in Go code,
			// so we need to take the P ourselves.
			sysretake = true
		}

		// Drop allpLock so we can take sched.lock.
		unlock(&allpLock)

		// Need to decrement number of idle locked M's (pretending that
		// one more is running) before we take the P and resume.
		// Otherwise the M from which we retake can exit the syscall,
		// increment nmidle and report deadlock.
		//
		// Can't call incidlelocked once we setBlockOnExitSyscall, due
		// to a lock ordering violation between sched.lock and _Gscan.
		incidlelocked(-1)

		// Try to prevent the P from continuing in the syscall, if it's in one at all.
		thread, ok := setBlockOnExitSyscall(pp)
		if !ok {
			// Not in a syscall, or something changed out from under us.
			goto done
		}

		// Retake the P if it's there for more than 1 sysmon tick (at least 20us).
		if syst := int64(pp.syscalltick); !sysretake && int64(pd.syscalltick) != syst {
			pd.syscalltick = uint32(syst)
			pd.syscallwhen = now
			thread.resume()
			goto done
		}

		// On the one hand we don't want to retake Ps if there is no other work to do,
		// but on the other hand we want to retake them eventually
		// because they can prevent the sysmon thread from deep sleep.
		if runqempty(pp) && sched.nmspinning.Load()+sched.npidle.Load() > 0 && pd.syscallwhen+10*1000*1000 > now {
			thread.resume()
			goto done
		}

		// Take the P. Note: because we have the scan bit, the goroutine
		// is at worst stuck spinning in exitsyscall.
		thread.takeP()
		thread.resume()
		n++

		// Handoff the P for some other thread to run it.
		handoffp(pp)

		// The P has been handed off to another thread, so risk of a false
		// deadlock report while we hold onto it is gone.
	done:
		incidlelocked(1)
		lock(&allpLock)
	}
	unlock(&allpLock)
	return uint32(n)
}

// syscallingThread represents a thread in a system call that temporarily
// cannot advance out of the system call.
type syscallingThread struct {
	gp     *g
	mp     *m
	pp     *p
	status uint32
}

// setBlockOnExitSyscall prevents pp's thread from advancing out of
// exitsyscall. On success, returns the g/m/p state of the thread
// and true. At that point, the caller owns the g/m/p links referenced,
// the goroutine is in _Gsyscall, and prevented from transitioning out
// of it. On failure, it returns false, and none of these guarantees are
// made.
//
// Callers must call resume on the resulting thread state once
// they're done with thread, otherwise it will remain blocked forever.
//
// This function races with state changes on pp, and thus may fail
// if pp is not in a system call, or exits a system call concurrently
// with this function. However, this function is safe to call without
// any additional synchronization.
func setBlockOnExitSyscall(pp *p) (syscallingThread, bool) {
	if pp.status != _Prunning {
		return syscallingThread{}, false
	}
	// Be very careful here, these reads are intentionally racy.
	// Once we notice the G is in _Gsyscall, acquire its scan bit,
	// and validate that it's still connected to the *same* M and P,
	// we can actually get to work. Holding the scan bit will prevent
	// the G from exiting the syscall.
	//
	// Our goal here is to interrupt long syscalls. If it turns out
	// that we're wrong and the G switched to another syscall while
	// we were trying to do this, that's completely fine. It's
	// probably making more frequent syscalls and the typical
	// preemption paths should be effective.
	mp := pp.m.ptr()
	if mp == nil {
		// Nothing to do.
		return syscallingThread{}, false
	}
	gp := mp.curg
	if gp == nil {
		// Nothing to do.
		return syscallingThread{}, false
	}
	status := readgstatus(gp) &^ _Gscan

	// A goroutine is considered in a syscall, and may have a corresponding
	// P, if it's in _Gsyscall *or* _Gdeadextra. In the latter case, it's an
	// extra M goroutine.
	if status != _Gsyscall && status != _Gdeadextra {
		// Not in a syscall, nothing to do.
		return syscallingThread{}, false
	}
	if !castogscanstatus(gp, status, status|_Gscan) {
		// Not in _Gsyscall or _Gdeadextra anymore. Nothing to do.
		return syscallingThread{}, false
	}
	if gp.m != mp || gp.m.p.ptr() != pp {
		// This is not what we originally observed. Nothing to do.
		casfrom_Gscanstatus(gp, status|_Gscan, status)
		return syscallingThread{}, false
	}
	return syscallingThread{gp, mp, pp, status}, true
}

// gcstopP unwires the P attached to the syscalling thread
// and moves it into the _Pgcstop state.
//
// The caller must be stopping the world.
func (s syscallingThread) gcstopP() {
	assertLockHeld(&sched.lock)

	s.releaseP(_Pgcstop)
	s.pp.gcStopTime = nanotime()
	sched.stopwait--
}

// takeP unwires the P attached to the syscalling thread
// and moves it into the _Pidle state.
func (s syscallingThread) takeP() {
	s.releaseP(_Pidle)
}

// releaseP unwires the P from the syscalling thread, moving
// it to the provided state. Callers should prefer to use
// takeP and gcstopP.
func (s syscallingThread) releaseP(state uint32) {
	if state != _Pidle && state != _Pgcstop {
		throw("attempted to release P into a bad state")
	}
	trace := traceAcquire()
	s.pp.m = 0
	s.mp.p = 0
	atomic.Store(&s.pp.status, state)
	if trace.ok() {
		trace.ProcSteal(s.pp)
		traceRelease(trace)
	}
	addGSyscallNoP(s.mp)
	s.pp.syscalltick++
}

// resume allows a syscalling thread to advance beyond exitsyscall.
func (s syscallingThread) resume() {
	casfrom_Gscanstatus(s.gp, s.status|_Gscan, s.status)
}

// Tell all goroutines that they have been preempted and they should stop.
// This function is purely best-effort. It can fail to inform a goroutine if a
// processor just started running it.
// No locks need to be held.
// Returns true if preemption request was issued to at least one goroutine.
func preemptall() bool {
	res := false
	for _, pp := range allp {
		if pp.status != _Prunning {
			continue
		}
		if preemptone(pp) {
			res = true
		}
	}
	return res
}

// Tell the goroutine running on processor P to stop.
// This function is purely best-effort. It can incorrectly fail to inform the
// goroutine. It can inform the wrong goroutine. Even if it informs the
// correct goroutine, that goroutine might ignore the request if it is
// simultaneously executing newstack.
// No lock needs to be held.
// Returns true if preemption request was issued.
// The actual preemption will happen at some point in the future
// and will be indicated by the gp->status no longer being
// Grunning
func preemptone(pp *p) bool {
	mp := pp.m.ptr()
	if mp == nil || mp == getg().m {
		return false
	}
	gp := mp.curg
	if gp == nil || gp == mp.g0 {
		return false
	}
	if readgstatus(gp)&^_Gscan == _Gsyscall {
		// Don't bother trying to preempt a goroutine in a syscall.
		return false
	}

	gp.preempt = true

	// Every call in a goroutine checks for stack overflow by
	// comparing the current stack pointer to gp->stackguard0.
	// Setting gp->stackguard0 to StackPreempt folds
	// preemption into the normal stack overflow check.
	gp.stackguard0 = stackPreempt

	// Request an async preemption of this P.
	if preemptMSupported && debug.asyncpreemptoff == 0 {
		pp.preempt = true
		preemptM(mp)
	}

	return true
}

var starttime int64

func schedtrace(detailed bool) {
	now := nanotime()
	if starttime == 0 {
		starttime = now
	}

	lock(&sched.lock)
	print("SCHED ", (now-starttime)/1e6, "ms: gomaxprocs=", gomaxprocs, " idleprocs=", sched.npidle.Load(), " threads=", mcount(), " spinningthreads=", sched.nmspinning.Load(), " needspinning=", sched.needspinning.Load(), " idlethreads=", sched.nmidle, " runqueue=", sched.runq.size)
	if detailed {
		print(" gcwaiting=", sched.gcwaiting.Load(), " nmidlelocked=", sched.nmidlelocked, " stopwait=", sched.stopwait, " sysmonwait=", sched.sysmonwait.Load(), "\n")
	}
	// We must be careful while reading data from P's, M's and G's.
	// Even if we hold schedlock, most data can be changed concurrently.
	// E.g. (p->m ? p->m->id : -1) can crash if p->m changes from non-nil to nil.
	for i, pp := range allp {
		h := atomic.Load(&pp.runqhead)
		t := atomic.Load(&pp.runqtail)
		if detailed {
			print("  P", i, ": status=", pp.status, " schedtick=", pp.schedtick, " syscalltick=", pp.syscalltick, " m=")
			mp := pp.m.ptr()
			if mp != nil {
				print(mp.id)
			} else {
				print("nil")
			}
			print(" runqsize=", t-h, " gfreecnt=", pp.gFree.size, " timerslen=", len(pp.timers.heap), "\n")
		} else {
			// In non-detailed mode format lengths of per-P run queues as:
			// [ len1 len2 len3 len4 ]
			print(" ")
			if i == 0 {
				print("[ ")
			}
			print(t - h)
			if i == len(allp)-1 {
				print(" ]")
			}
		}
	}

	if !detailed {
		// Format per-P schedticks as: schedticks=[ tick1 tick2 tick3 tick4 ].
		print(" schedticks=[ ")
		for _, pp := range allp {
			print(pp.schedtick)
			print(" ")
		}
		print("]\n")
	}

	if !detailed {
		unlock(&sched.lock)
		return
	}

	for mp := allm; mp != nil; mp = mp.alllink {
		pp := mp.p.ptr()
		print("  M", mp.id, ": p=")
		if pp != nil {
			print(pp.id)
		} else {
			print("nil")
		}
		print(" curg=")
		if mp.curg != nil {
			print(mp.curg.goid)
		} else {
			print("nil")
		}
		print(" mallocing=", mp.mallocing, " throwing=", mp.throwing, " preemptoff=", mp.preemptoff, " locks=", mp.locks, " dying=", mp.dying, " spinning=", mp.spinning, " blocked=", mp.blocked, " lockedg=")
		if lockedg := mp.lockedg.ptr(); lockedg != nil {
			print(lockedg.goid)
		} else {
			print("nil")
		}
		print("\n")
	}

	forEachG(func(gp *g) {
		print("  G", gp.goid, ": status=", readgstatus(gp), "(", gp.waitreason.String(), ") m=")
		if gp.m != nil {
			print(gp.m.id)
		} else {
			print("nil")
		}
		print(" lockedm=")
		if lockedm := gp.lockedm.ptr(); lockedm != nil {
			print(lockedm.id)
		} else {
			print("nil")
		}
		print("\n")
	})
	unlock(&sched.lock)
}

type updateMaxProcsGState struct {
	lock mutex
	g    *g
	idle atomic.Bool

	// Readable when idle == false, writable when idle == true.
	procs int32 // new GOMAXPROCS value
}

var (
	// GOMAXPROCS update godebug metric. Incremented if automatic
	// GOMAXPROCS updates actually change the value of GOMAXPROCS.
	updatemaxprocs = &godebugInc{name: "updatemaxprocs"}

	// Synchronization and state between updateMaxProcsGoroutine and
	// sysmon.
	updateMaxProcsG updateMaxProcsGState

	// Synchronization between GOMAXPROCS and sysmon.
	//
	// Setting GOMAXPROCS via a call to GOMAXPROCS disables automatic
	// GOMAXPROCS updates.
	//
	// We want to make two guarantees to callers of GOMAXPROCS. After
	// GOMAXPROCS returns:
	//
	// 1. The runtime will not make any automatic changes to GOMAXPROCS.
	//
	// 2. The runtime will not perform any of the system calls used to
	//    determine the appropriate value of GOMAXPROCS (i.e., it won't
	//    call defaultGOMAXPROCS).
	//
	// (1) is the baseline guarantee that everyone needs. The GOMAXPROCS
	// API isn't useful to anyone if automatic updates may occur after it
	// returns. This is easily achieved by double-checking the state under
	// STW before committing an automatic GOMAXPROCS update.
	//
	// (2) doesn't matter to most users, as it is isn't observable as long
	// as (1) holds. However, it can be important to users sandboxing Go.
	// They want disable these system calls and need some way to know when
	// they are guaranteed the calls will stop.
	//
	// This would be simple to achieve if we simply called
	// defaultGOMAXPROCS under STW in updateMaxProcsGoroutine below.
	// However, we would like to avoid scheduling this goroutine every
	// second when it will almost never do anything. Instead, sysmon calls
	// defaultGOMAXPROCS to decide whether to schedule
	// updateMaxProcsGoroutine. Thus we need to synchronize between sysmon
	// and GOMAXPROCS calls.
	//
	// GOMAXPROCS can't hold a runtime mutex across STW. It could hold a
	// semaphore, but sysmon cannot take semaphores. Instead, we have a
	// more complex scheme:
	//
	// * sysmon holds computeMaxProcsLock while calling defaultGOMAXPROCS.
	// * sysmon skips the current update if sched.customGOMAXPROCS is
	//   set.
	// * GOMAXPROCS sets sched.customGOMAXPROCS once it is committed to
	//   changing GOMAXPROCS.
	// * GOMAXPROCS takes computeMaxProcsLock to wait for outstanding
	//   defaultGOMAXPROCS calls to complete.
	//
	// N.B. computeMaxProcsLock could simply be sched.lock, but we want to
	// avoid holding that lock during the potentially slow
	// defaultGOMAXPROCS.
	computeMaxProcsLock mutex
)

// Start GOMAXPROCS update helper goroutine.
//
// This is based on forcegchelper.
func defaultGOMAXPROCSUpdateEnable() {
	if debug.updatemaxprocs == 0 {
		// Unconditionally increment the metric when updates are disabled.
		//
		// It would be more descriptive if we did a dry run of the
		// complete update, determining the appropriate value of
		// GOMAXPROCS and the bailing out and just incrementing the
		// metric if a change would occur.
		//
		// Not only is that a lot of ongoing work for a disabled
		// feature, but some users need to be able to completely
		// disable the update system calls (such as sandboxes).
		// Currently, updatemaxprocs=0 serves that purpose.
		updatemaxprocs.IncNonDefault()
		return
	}

	go updateMaxProcsGoroutine()
}

func updateMaxProcsGoroutine() {
	updateMaxProcsG.g = getg()
	lockInit(&updateMaxProcsG.lock, lockRankUpdateMaxProcsG)
	for {
		lock(&updateMaxProcsG.lock)
		if updateMaxProcsG.idle.Load() {
			throw("updateMaxProcsGoroutine: phase error")
		}
		updateMaxProcsG.idle.Store(true)
		goparkunlock(&updateMaxProcsG.lock, waitReasonUpdateGOMAXPROCSIdle, traceBlockSystemGoroutine, 1)
		// This goroutine is explicitly resumed by sysmon.

		stw := stopTheWorldGC(stwGOMAXPROCS)

		// Still OK to update?
		lock(&sched.lock)
		custom := sched.customGOMAXPROCS
		unlock(&sched.lock)
		if custom {
			startTheWorldGC(stw)
			return
		}

		// newprocs will be processed by startTheWorld
		//
		// TODO(prattmic): this could use a nicer API. Perhaps add it to the
		// stw parameter?
		newprocs = updateMaxProcsG.procs
		lock(&sched.lock)
		sched.customGOMAXPROCS = false
		unlock(&sched.lock)

		startTheWorldGC(stw)
	}
}

func sysmonUpdateGOMAXPROCS() {
	// Synchronize with GOMAXPROCS. See comment on computeMaxProcsLock.
	lock(&computeMaxProcsLock)

	// No update if GOMAXPROCS was set manually.
	lock(&sched.lock)
	custom := sched.customGOMAXPROCS
	curr := gomaxprocs
	unlock(&sched.lock)
	if custom {
		unlock(&computeMaxProcsLock)
		return
	}

	// Don't hold sched.lock while we read the filesystem.
	procs := defaultGOMAXPROCS(0)
	unlock(&computeMaxProcsLock)
	if procs == curr {
		// Nothing to do.
		return
	}

	// Sysmon can't directly stop the world. Run the helper to do so on our
	// behalf. If updateGOMAXPROCS.idle is false, then a previous update is
	// still pending.
	if updateMaxProcsG.idle.Load() {
		lock(&updateMaxProcsG.lock)
		updateMaxProcsG.procs = procs
		updateMaxProcsG.idle.Store(false)
		var list gList
		list.push(updateMaxProcsG.g)
		injectglist(&list)
		unlock(&updateMaxProcsG.lock)
	}
}

// schedEnableUser enables or disables the scheduling of user
// goroutines.
//
// This does not stop already running user goroutines, so the caller
// should first stop the world when disabling user goroutines.
func schedEnableUser(enable bool) {
	lock(&sched.lock)
	if sched.disable.user == !enable {
		unlock(&sched.lock)
		return
	}
	sched.disable.user = !enable
	if enable {
		n := sched.disable.runnable.size
		globrunqputbatch(&sched.disable.runnable)
		unlock(&sched.lock)
		for ; n != 0 && sched.npidle.Load() != 0; n-- {
			startm(nil, false, false)
		}
	} else {
		unlock(&sched.lock)
	}
}

// schedEnabled reports whether gp should be scheduled. It returns
// false is scheduling of gp is disabled.
//
// sched.lock must be held.
func schedEnabled(gp *g) bool {
	assertLockHeld(&sched.lock)

	if sched.disable.user {
		return isSystemGoroutine(gp, true)
	}
	return true
}

// Put mp on midle list.
// sched.lock must be held.
// May run during STW, so write barriers are not allowed.
//
//go:nowritebarrierrec
func mput(mp *m) {
	assertLockHeld(&sched.lock)

	sched.midle.push(unsafe.Pointer(mp))
	sched.nmidle++
	checkdead()
}

// Try to get an m from midle list.
// sched.lock must be held.
// May run during STW, so write barriers are not allowed.
//
//go:nowritebarrierrec
func mget() *m {
	assertLockHeld(&sched.lock)

	mp := (*m)(sched.midle.pop())
	if mp != nil {
		sched.nmidle--
	}
	return mp
}

// Try to get a specific m from midle list. Returns nil if it isn't on the
// midle list.
//
// sched.lock must be held.
// May run during STW, so write barriers are not allowed.
//
//go:nowritebarrierrec
func mgetSpecific(mp *m) *m {
	assertLockHeld(&sched.lock)

	if mp.idleNode.prev == 0 && mp.idleNode.next == 0 {
		// Not on the list.
		return nil
	}

	sched.midle.remove(unsafe.Pointer(mp))
	sched.nmidle--

	return mp
}

// Put gp on the global runnable queue.
// sched.lock must be held.
// May run during STW, so write barriers are not allowed.
//
//go:nowritebarrierrec
func globrunqput(gp *g) {
	assertLockHeld(&sched.lock)

	sched.runq.pushBack(gp)
}

// Put gp at the head of the global runnable queue.
// sched.lock must be held.
// May run during STW, so write barriers are not allowed.
//
//go:nowritebarrierrec
func globrunqputhead(gp *g) {
	assertLockHeld(&sched.lock)

	sched.runq.push(gp)
}

// Put a batch of runnable goroutines on the global runnable queue.
// This clears *batch.
// sched.lock must be held.
// May run during STW, so write barriers are not allowed.
//
//go:nowritebarrierrec
func globrunqputbatch(batch *gQueue) {
	assertLockHeld(&sched.lock)

	sched.runq.pushBackAll(*batch)
	*batch = gQueue{}
}

// Try get a single G from the global runnable queue.
// sched.lock must be held.
func globrunqget() *g {
	assertLockHeld(&sched.lock)

	if sched.runq.size == 0 {
		return nil
	}

	return sched.runq.pop()
}

// Try get a batch of G's from the global runnable queue.
// sched.lock must be held.
func globrunqgetbatch(n int32) (gp *g, q gQueue) {
	assertLockHeld(&sched.lock)

	if sched.runq.size == 0 {
		return
	}

	n = min(n, sched.runq.size, sched.runq.size/gomaxprocs+1)

	gp = sched.runq.pop()
	n--

	for ; n > 0; n-- {
		gp1 := sched.runq.pop()
		q.pushBack(gp1)
	}
	return
}

// pMask is an atomic bitstring with one bit per P.
type pMask []uint32

// read returns true if P id's bit is set.
func (p pMask) read(id uint32) bool {
	word := id / 32
	mask := uint32(1) << (id % 32)
	return (atomic.Load(&p[word]) & mask) != 0
}

// set sets P id's bit.
func (p pMask) set(id int32) {
	word := id / 32
	mask := uint32(1) << (id % 32)
	atomic.Or(&p[word], mask)
}

// clear clears P id's bit.
func (p pMask) clear(id int32) {
	word := id / 32
	mask := uint32(1) << (id % 32)
	atomic.And(&p[word], ^mask)
}

// any returns true if any bit in p is set.
func (p pMask) any() bool {
	for i := range p {
		if atomic.Load(&p[i]) != 0 {
			return true
		}
	}
	return false
}

// resize resizes the pMask and returns a new one.
//
// The result may alias p, so callers are encouraged to
// discard p. Not safe for concurrent use.
func (p pMask) resize(nprocs int32) pMask {
	maskWords := (nprocs + 31) / 32

	if maskWords <= int32(cap(p)) {
		return p[:maskWords]
	}
	newMask := make([]uint32, maskWords)
	// No need to copy beyond len, old Ps are irrelevant.
	copy(newMask, p)
	return newMask
}

// pidleput puts p on the _Pidle list. now must be a relatively recent call
// to nanotime or zero. Returns now or the current time if now was zero.
//
// This releases ownership of p. Once sched.lock is released it is no longer
// safe to use p.
//
// sched.lock must be held.
//
// May run during STW, so write barriers are not allowed.
//
//go:nowritebarrierrec
func pidleput(pp *p, now int64) int64 {
	assertLockHeld(&sched.lock)

	if !runqempty(pp) {
		throw("pidleput: P has non-empty run queue")
	}
	if now == 0 {
		now = nanotime()
	}
	if pp.timers.len.Load() == 0 {
		timerpMask.clear(pp.id)
	}
	idlepMask.set(pp.id)
	pp.link = sched.pidle
	sched.pidle.set(pp)
	sched.npidle.Add(1)
	if !pp.limiterEvent.start(limiterEventIdle, now) {
		throw("must be able to track idle limiter event")
	}
	return now
}

// pidleget tries to get a p from the _Pidle list, acquiring ownership.
//
// sched.lock must be held.
//
// May run during STW, so write barriers are not allowed.
//
//go:nowritebarrierrec
func pidleget(now int64) (*p, int64) {
	assertLockHeld(&sched.lock)

	pp := sched.pidle.ptr()
	if pp != nil {
		// Timer may get added at any time now.
		if now == 0 {
			now = nanotime()
		}
		timerpMask.set(pp.id)
		idlepMask.clear(pp.id)
		sched.pidle = pp.link
		sched.npidle.Add(-1)
		pp.limiterEvent.stop(limiterEventIdle, now)
	}
	return pp, now
}

// pidlegetSpinning tries to get a p from the _Pidle list, acquiring ownership.
// This is called by spinning Ms (or callers than need a spinning M) that have
// found work. If no P is available, this must synchronized with non-spinning
// Ms that may be preparing to drop their P without discovering this work.
//
// sched.lock must be held.
//
// May run during STW, so write barriers are not allowed.
//
//go:nowritebarrierrec
func pidlegetSpinning(now int64) (*p, int64) {
	assertLockHeld(&sched.lock)

	pp, now := pidleget(now)
	if pp == nil {
		// See "Delicate dance" comment in findRunnable. We found work
		// that we cannot take, we must synchronize with non-spinning
		// Ms that may be preparing to drop their P.
		sched.needspinning.Store(1)
		return nil, now
	}

	return pp, now
}

// runqempty reports whether pp has no Gs on its local run queue.
// It never returns true spuriously.
func runqempty(pp *p) bool {
	// Defend against a race where 1) pp has G1 in runqnext but runqhead == runqtail,
	// 2) runqput on pp kicks G1 to the runq, 3) runqget on pp empties runqnext.
	// Simply observing that runqhead == runqtail and then observing that runqnext == nil
	// does not mean the queue is empty.
	for {
		head := atomic.Load(&pp.runqhead)
		tail := atomic.Load(&pp.runqtail)
		runnext := atomic.Loaduintptr((*uintptr)(unsafe.Pointer(&pp.runnext)))
		if tail == atomic.Load(&pp.runqtail) {
			return head == tail && runnext == 0
		}
	}
}

// To shake out latent assumptions about scheduling order,
// we introduce some randomness into scheduling decisions
// when running with the race detector.
// The need for this was made obvious by changing the
// (deterministic) scheduling order in Go 1.5 and breaking
// many poorly-written tests.
// With the randomness here, as long as the tests pass
// consistently with -race, they shouldn't have latent scheduling
// assumptions.
const randomizeScheduler = raceenabled

// runqput tries to put g on the local runnable queue.
// If next is false, runqput adds g to the tail of the runnable queue.
// If next is true, runqput puts g in the pp.runnext slot.
// If the run queue is full, runnext puts g on the global queue.
// Executed only by the owner P.
func runqput(pp *p, gp *g, next bool) {
	if !haveSysmon && next {
		// A runnext goroutine shares the same time slice as the
		// current goroutine (inheritTime from runqget). To prevent a
		// ping-pong pair of goroutines from starving all others, we
		// depend on sysmon to preempt "long-running goroutines". That
		// is, any set of goroutines sharing the same time slice.
		//
		// If there is no sysmon, we must avoid runnext entirely or
		// risk starvation.
		next = false
	}
	if randomizeScheduler && next && randn(2) == 0 {
		next = false
	}

	if next {
	retryNext:
		oldnext := pp.runnext
		if !pp.runnext.cas(oldnext, guintptr(unsafe.Pointer(gp))) {
			goto retryNext
		}
		if oldnext == 0 {
			return
		}
		// Kick the old runnext out to the regular run queue.
		gp = oldnext.ptr()
	}

retry:
	h := atomic.LoadAcq(&pp.runqhead) // load-acquire, synchronize with consumers
	t := pp.runqtail
	if t-h < uint32(len(pp.runq)) {
		pp.runq[t%uint32(len(pp.runq))].set(gp)
		atomic.StoreRel(&pp.runqtail, t+1) // store-release, makes the item available for consumption
		return
	}
	if runqputslow(pp, gp, h, t) {
		return
	}
	// the queue is not full, now the put above must succeed
	goto retry
}

// Put g and a batch of work from local runnable queue on global queue.
// Executed only by the owner P.
func runqputslow(pp *p, gp *g, h, t uint32) bool {
	var batch [len(pp.runq)/2 + 1]*g

	// First, grab a batch from local queue.
	n := t - h
	n = n / 2
	if n != uint32(len(pp.runq)/2) {
		throw("runqputslow: queue is not full")
	}
	for i := uint32(0); i < n; i++ {
		batch[i] = pp.runq[(h+i)%uint32(len(pp.runq))].ptr()
	}
	if !atomic.CasRel(&pp.runqhead, h, h+n) { // cas-release, commits consume
		return false
	}
	batch[n] = gp

	if randomizeScheduler {
		for i := uint32(1); i <= n; i++ {
			j := cheaprandn(i + 1)
			batch[i], batch[j] = batch[j], batch[i]
		}
	}

	// Link the goroutines.
	for i := uint32(0); i < n; i++ {
		batch[i].schedlink.set(batch[i+1])
	}

	q := gQueue{batch[0].guintptr(), batch[n].guintptr(), int32(n + 1)}

	// Now put the batch on global queue.
	lock(&sched.lock)
	globrunqputbatch(&q)
	unlock(&sched.lock)
	return true
}

// runqputbatch tries to put all the G's on q on the local runnable queue.
// If the local runq is full the input queue still contains unqueued Gs.
// Executed only by the owner P.
func runqputbatch(pp *p, q *gQueue) {
	if q.empty() {
		return
	}
	h := atomic.LoadAcq(&pp.runqhead)
	t := pp.runqtail
	n := uint32(0)
	for !q.empty() && t-h < uint32(len(pp.runq)) {
		gp := q.pop()
		pp.runq[t%uint32(len(pp.runq))].set(gp)
		t++
		n++
	}

	if randomizeScheduler {
		off := func(o uint32) uint32 {
			return (pp.runqtail + o) % uint32(len(pp.runq))
		}
		for i := uint32(1); i < n; i++ {
			j := cheaprandn(i + 1)
			pp.runq[off(i)], pp.runq[off(j)] = pp.runq[off(j)], pp.runq[off(i)]
		}
	}

	atomic.StoreRel(&pp.runqtail, t)

	return
}

// Get g from local runnable queue.
// If inheritTime is true, gp should inherit the remaining time in the
// current time slice. Otherwise, it should start a new time slice.
// Executed only by the owner P.
func runqget(pp *p) (gp *g, inheritTime bool) {
	// If there's a runnext, it's the next G to run.
	next := pp.runnext
	// If the runnext is non-0 and the CAS fails, it could only have been stolen by another P,
	// because other Ps can race to set runnext to 0, but only the current P can set it to non-0.
	// Hence, there's no need to retry this CAS if it fails.
	if next != 0 && pp.runnext.cas(next, 0) {
		return next.ptr(), true
	}

	for {
		h := atomic.LoadAcq(&pp.runqhead) // load-acquire, synchronize with other consumers
		t := pp.runqtail
		if t == h {
			return nil, false
		}
		gp := pp.runq[h%uint32(len(pp.runq))].ptr()
		if atomic.CasRel(&pp.runqhead, h, h+1) { // cas-release, commits consume
			return gp, false
		}
	}
}

// runqdrain drains the local runnable queue of pp and returns all goroutines in it.
// Executed only by the owner P.
func runqdrain(pp *p) (drainQ gQueue) {
	oldNext := pp.runnext
	if oldNext != 0 && pp.runnext.cas(oldNext, 0) {
		drainQ.pushBack(oldNext.ptr())
	}

retry:
	h := atomic.LoadAcq(&pp.runqhead) // load-acquire, synchronize with other consumers
	t := pp.runqtail
	qn := t - h
	if qn == 0 {
		return
	}
	if qn > uint32(len(pp.runq)) { // read inconsistent h and t
		goto retry
	}

	if !atomic.CasRel(&pp.runqhead, h, h+qn) { // cas-release, commits consume
		goto retry
	}

	// We've inverted the order in which it gets G's from the local P's runnable queue
	// and then advances the head pointer because we don't want to mess up the statuses of G's
	// while runqdrain() and runqsteal() are running in parallel.
	// Thus we should advance the head pointer before draining the local P into a gQueue,
	// so that we can update any gp.schedlink only after we take the full ownership of G,
	// meanwhile, other P's can't access to all G's in local P's runnable queue and steal them.
	// See https://groups.google.com/g/golang-dev/c/0pTKxEKhHSc/m/6Q85QjdVBQAJ for more details.
	for i := uint32(0); i < qn; i++ {
		gp := pp.runq[(h+i)%uint32(len(pp.runq))].ptr()
		drainQ.pushBack(gp)
	}
	return
}

// Grabs a batch of goroutines from pp's runnable queue into batch.
// Batch is a ring buffer starting at batchHead.
// Returns number of grabbed goroutines.
// Can be executed by any P.
func runqgrab(pp *p, batch *[256]guintptr, batchHead uint32, stealRunNextG bool) uint32 {
	for {
		h := atomic.LoadAcq(&pp.runqhead) // load-acquire, synchronize with other consumers
		t := atomic.LoadAcq(&pp.runqtail) // load-acquire, synchronize with the producer
		n := t - h
		n = n - n/2
		if n == 0 {
			if stealRunNextG {
				// Try to steal from pp.runnext.
				if next := pp.runnext; next != 0 {
					if pp.status == _Prunning {
						if mp := pp.m.ptr(); mp != nil {
							if gp := mp.curg; gp == nil || readgstatus(gp)&^_Gscan != _Gsyscall {
								// Sleep to ensure that pp isn't about to run the g
								// we are about to steal.
								// The important use case here is when the g running
								// on pp ready()s another g and then almost
								// immediately blocks. Instead of stealing runnext
								// in this window, back off to give pp a chance to
								// schedule runnext. This will avoid thrashing gs
								// between different Ps.
								// A sync chan send/recv takes ~50ns as of time of
								// writing, so 3us gives ~50x overshoot.
								// If curg is nil, we assume that the P is likely
								// to be in the scheduler. If curg isn't nil and isn't
								// in a syscall, then it's either running, waiting, or
								// runnable. In this case we want to sleep because the
								// P might either call into the scheduler soon (running),
								// or already is (since we found a waiting or runnable
								// goroutine hanging off of a running P, suggesting it
								// either recently transitioned out of running, or will
								// transition to running shortly).
								if !osHasLowResTimer {
									usleep(3)
								} else {
									// On some platforms system timer granularity is
									// 1-15ms, which is way too much for this
									// optimization. So just yield.
									osyield()
								}
							}
						}
					}
					if !pp.runnext.cas(next, 0) {
						continue
					}
					batch[batchHead%uint32(len(batch))] = next
					return 1
				}
			}
			return 0
		}
		if n > uint32(len(pp.runq)/2) { // read inconsistent h and t
			continue
		}
		for i := uint32(0); i < n; i++ {
			g := pp.runq[(h+i)%uint32(len(pp.runq))]
			batch[(batchHead+i)%uint32(len(batch))] = g
		}
		if atomic.CasRel(&pp.runqhead, h, h+n) { // cas-release, commits consume
			return n
		}
	}
}

// Steal half of elements from local runnable queue of p2
// and put onto local runnable queue of p.
// Returns one of the stolen elements (or nil if failed).
func runqsteal(pp, p2 *p, stealRunNextG bool) *g {
	t := pp.runqtail
	n := runqgrab(p2, &pp.runq, t, stealRunNextG)
	if n == 0 {
		return nil
	}
	n--
	gp := pp.runq[(t+n)%uint32(len(pp.runq))].ptr()
	if n == 0 {
		return gp
	}
	h := atomic.LoadAcq(&pp.runqhead) // load-acquire, synchronize with consumers
	if t-h+n >= uint32(len(pp.runq)) {
		throw("runqsteal: runq overflow")
	}
	atomic.StoreRel(&pp.runqtail, t+n) // store-release, makes the item available for consumption
	return gp
}

// A gQueue is a dequeue of Gs linked through g.schedlink. A G can only
// be on one gQueue or gList at a time.
type gQueue struct {
	head guintptr
	tail guintptr
	size int32
}

// empty reports whether q is empty.
func (q *gQueue) empty() bool {
	return q.head == 0
}

// push adds gp to the head of q.
func (q *gQueue) push(gp *g) {
	gp.schedlink = q.head
	q.head.set(gp)
	if q.tail == 0 {
		q.tail.set(gp)
	}
	q.size++
}

// pushBack adds gp to the tail of q.
func (q *gQueue) pushBack(gp *g) {
	gp.schedlink = 0
	if q.tail != 0 {
		q.tail.ptr().schedlink.set(gp)
	} else {
		q.head.set(gp)
	}
	q.tail.set(gp)
	q.size++
}

// pushBackAll adds all Gs in q2 to the tail of q. After this q2 must
// not be used.
func (q *gQueue) pushBackAll(q2 gQueue) {
	if q2.tail == 0 {
		return
	}
	q2.tail.ptr().schedlink = 0
	if q.tail != 0 {
		q.tail.ptr().schedlink = q2.head
	} else {
		q.head = q2.head
	}
	q.tail = q2.tail
	q.size += q2.size
}

// pop removes and returns the head of queue q. It returns nil if
// q is empty.
func (q *gQueue) pop() *g {
	gp := q.head.ptr()
	if gp != nil {
		q.head = gp.schedlink
		if q.head == 0 {
			q.tail = 0
		}
		q.size--
	}
	return gp
}

// popList takes all Gs in q and returns them as a gList.
func (q *gQueue) popList() gList {
	stack := gList{q.head, q.size}
	*q = gQueue{}
	return stack
}

// A gList is a list of Gs linked through g.schedlink. A G can only be
// on one gQueue or gList at a time.
type gList struct {
	head guintptr
	size int32
}

// empty reports whether l is empty.
func (l *gList) empty() bool {
	return l.head == 0
}

// push adds gp to the head of l.
func (l *gList) push(gp *g) {
	gp.schedlink = l.head
	l.head.set(gp)
	l.size++
}

// pushAll prepends all Gs in q to l. After this q must not be used.
func (l *gList) pushAll(q gQueue) {
	if !q.empty() {
		q.tail.ptr().schedlink = l.head
		l.head = q.head
		l.size += q.size
	}
}

// pop removes and returns the head of l. If l is empty, it returns nil.
func (l *gList) pop() *g {
	gp := l.head.ptr()
	if gp != nil {
		l.head = gp.schedlink
		l.size--
	}
	return gp
}

//go:linkname setMaxThreads runtime/debug.setMaxThreads
func setMaxThreads(in int) (out int) {
	lock(&sched.lock)
	out = int(sched.maxmcount)
	if in > 0x7fffffff { // MaxInt32
		sched.maxmcount = 0x7fffffff
	} else {
		sched.maxmcount = int32(in)
	}
	checkmcount()
	unlock(&sched.lock)
	return
}

// procPin should be an internal detail,
// but widely used packages access it using linkname.
// Notable members of the hall of shame include:
//   - github.com/bytedance/gopkg
//   - github.com/choleraehyq/pid
//   - github.com/songzhibin97/gkit
//
// Do not remove or change the type signature.
// See go.dev/issue/67401.
//
//go:linkname procPin
//go:nosplit
func procPin() int {
	gp := getg()
	mp := gp.m

	mp.locks++
	return int(mp.p.ptr().id)
}

// procUnpin should be an internal detail,
// but widely used packages access it using linkname.
// Notable members of the hall of shame include:
//   - github.com/bytedance/gopkg
//   - github.com/choleraehyq/pid
//   - github.com/songzhibin97/gkit
//
// Do not remove or change the type signature.
// See go.dev/issue/67401.
//
//go:linkname procUnpin
//go:nosplit
func procUnpin() {
	gp := getg()
	gp.m.locks--
}

//go:linkname sync_runtime_procPin sync.runtime_procPin
//go:nosplit
func sync_runtime_procPin() int {
	return procPin()
}

//go:linkname sync_runtime_procUnpin sync.runtime_procUnpin
//go:nosplit
func sync_runtime_procUnpin() {
	procUnpin()
}

//go:linkname sync_atomic_runtime_procPin sync/atomic.runtime_procPin
//go:nosplit
func sync_atomic_runtime_procPin() int {
	return procPin()
}

//go:linkname sync_atomic_runtime_procUnpin sync/atomic.runtime_procUnpin
//go:nosplit
func sync_atomic_runtime_procUnpin() {
	procUnpin()
}

// Active spinning for sync.Mutex.
//
//go:linkname internal_sync_runtime_canSpin internal/sync.runtime_canSpin
//go:nosplit
func internal_sync_runtime_canSpin(i int) bool {
	// sync.Mutex is cooperative, so we are conservative with spinning.
	// Spin only few times and only if running on a multicore machine and
	// GOMAXPROCS>1 and there is at least one other running P and local runq is empty.
	// As opposed to runtime mutex we don't do passive spinning here,
	// because there can be work on global runq or on other Ps.
	if i >= active_spin || numCPUStartup <= 1 || gomaxprocs <= sched.npidle.Load()+sched.nmspinning.Load()+1 {
		return false
	}
	if p := getg().m.p.ptr(); !runqempty(p) {
		return false
	}
	return true
}

//go:linkname internal_sync_runtime_doSpin internal/sync.runtime_doSpin
//go:nosplit
func internal_sync_runtime_doSpin() {
	procyield(active_spin_cnt)
}

// Active spinning for sync.Mutex.
//
// sync_runtime_canSpin should be an internal detail,
// but widely used packages access it using linkname.
// Notable members of the hall of shame include:
//   - github.com/livekit/protocol
//   - github.com/sagernet/gvisor
//   - gvisor.dev/gvisor
//
// Do not remove or change the type signature.
// See go.dev/issue/67401.
//
//go:linkname sync_runtime_canSpin sync.runtime_canSpin
//go:nosplit
func sync_runtime_canSpin(i int) bool {
	return internal_sync_runtime_canSpin(i)
}

// sync_runtime_doSpin should be an internal detail,
// but widely used packages access it using linkname.
// Notable members of the hall of shame include:
//   - github.com/livekit/protocol
//   - github.com/sagernet/gvisor
//   - gvisor.dev/gvisor
//
// Do not remove or change the type signature.
// See go.dev/issue/67401.
//
//go:linkname sync_runtime_doSpin sync.runtime_doSpin
//go:nosplit
func sync_runtime_doSpin() {
	internal_sync_runtime_doSpin()
}

var stealOrder randomOrder

// randomOrder/randomEnum are helper types for randomized work stealing.
// They allow to enumerate all Ps in different pseudo-random orders without repetitions.
// The algorithm is based on the fact that if we have X such that X and GOMAXPROCS
// are coprime, then a sequences of (i + X) % GOMAXPROCS gives the required enumeration.
type randomOrder struct {
	count    uint32
	coprimes []uint32
}

type randomEnum struct {
	i     uint32
	count uint32
	pos   uint32
	inc   uint32
}

func (ord *randomOrder) reset(count uint32) {
	ord.count = count
	ord.coprimes = ord.coprimes[:0]
	for i := uint32(1); i <= count; i++ {
		if gcd(i, count) == 1 {
			ord.coprimes = append(ord.coprimes, i)
		}
	}
}

func (ord *randomOrder) start(i uint32) randomEnum {
	return randomEnum{
		count: ord.count,
		pos:   i % ord.count,
		inc:   ord.coprimes[i/ord.count%uint32(len(ord.coprimes))],
	}
}

func (enum *randomEnum) done() bool {
	return enum.i == enum.count
}

func (enum *randomEnum) next() {
	enum.i++
	enum.pos = (enum.pos + enum.inc) % enum.count
}

func (enum *randomEnum) position() uint32 {
	return enum.pos
}

func gcd(a, b uint32) uint32 {
	for b != 0 {
		a, b = b, a%b
	}
	return a
}

// An initTask represents the set of initializations that need to be done for a package.
// Keep in sync with ../../test/noinit.go:initTask
type initTask struct {
	state uint32 // 0 = uninitialized, 1 = in progress, 2 = done
	nfns  uint32
	// followed by nfns pcs, uintptr sized, one per init function to run
}

// inittrace stores statistics for init functions which are
// updated by malloc and newproc when active is true.
var inittrace tracestat

type tracestat struct {
	active bool   // init tracing activation status
	id     uint64 // init goroutine id
	allocs uint64 // heap allocations
	bytes  uint64 // heap allocated bytes
}

func doInit(ts []*initTask) {
	for _, t := range ts {
		doInit1(t)
	}
}

func doInit1(t *initTask) {
	switch t.state {
	case 2: // fully initialized
		return
	case 1: // initialization in progress
		throw("recursive call during initialization - linker skew")
	default: // not initialized yet
		t.state = 1 // initialization in progress

		var (
			start  int64
			before tracestat
		)

		if inittrace.active {
			start = nanotime()
			// Load stats non-atomically since tracinit is updated only by this init goroutine.
			before = inittrace
		}

		if t.nfns == 0 {
			// We should have pruned all of these in the linker.
			throw("inittask with no functions")
		}

		firstFunc := add(unsafe.Pointer(t), 8)
		for i := uint32(0); i < t.nfns; i++ {
			p := add(firstFunc, uintptr(i)*goarch.PtrSize)
			f := *(*func())(unsafe.Pointer(&p))
			f()
		}

		if inittrace.active {
			end := nanotime()
			// Load stats non-atomically since tracinit is updated only by this init goroutine.
			after := inittrace

			f := *(*func())(unsafe.Pointer(&firstFunc))
			pkg := funcpkgpath(findfunc(abi.FuncPCABIInternal(f)))

			var sbuf [24]byte
			print("init ", pkg, " @")
			print(string(fmtNSAsMS(sbuf[:], uint64(start-runtimeInitTime))), " ms, ")
			print(string(fmtNSAsMS(sbuf[:], uint64(end-start))), " ms clock, ")
			print(string(itoa(sbuf[:], after.bytes-before.bytes)), " bytes, ")
			print(string(itoa(sbuf[:], after.allocs-before.allocs)), " allocs")
			print("\n")
		}

		t.state = 2 // initialization done
	}
}