// 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/cpu"
	"runtime/internal/atomic"
	"runtime/internal/sys"
	"unsafe"
)

// Functions called by C code.
//go:linkname main
//go:linkname goparkunlock
//go:linkname newextram
//go:linkname acquirep
//go:linkname releasep
//go:linkname incidlelocked
//go:linkname ginit
//go:linkname schedinit
//go:linkname ready
//go:linkname stopm
//go:linkname handoffp
//go:linkname wakep
//go:linkname stoplockedm
//go:linkname schedule
//go:linkname execute
//go:linkname goexit1
//go:linkname reentersyscall
//go:linkname reentersyscallblock
//go:linkname exitsyscall
//go:linkname gfget
//go:linkname kickoff
//go:linkname mstart1
//go:linkname mexit
//go:linkname globrunqput
//go:linkname pidleget

// Exported for test (see runtime/testdata/testprogcgo/dropm_stub.go).
//go:linkname getm

// Function called by misc/cgo/test.
//go:linkname lockedOSThread

// C functions for thread and context management.
func newosproc(*m)

//go:noescape
func malg(bool, bool, *unsafe.Pointer, *uintptr) *g

//go:noescape
func resetNewG(*g, *unsafe.Pointer, *uintptr)
func gogo(*g)
func setGContext()
func makeGContext(*g, unsafe.Pointer, uintptr)
func getTraceback(me, gp *g)
func gtraceback(*g)
func _cgo_notify_runtime_init_done()
func alreadyInCallers() bool
func stackfree(*g)

// Functions created by the compiler.
//extern __go_init_main
func main_init()

//extern main.main
func main_main()

var buildVersion = sys.TheVersion

// 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:
// We unpark an additional thread when we ready a goroutine if (1) there is an
// idle P and 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 global run queue/
// 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 do some
// spinning looking for work in per-P run queues 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 readying goroutines. 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 a new goroutine, 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 goroutine readying is: submit a goroutine
// to local work queue, #StoreLoad-style memory barrier, check sched.nmspinning.
// The general pattern for spinning->non-spinning transition is: decrement nmspinning,
// #StoreLoad-style memory barrier, check all per-P work queues 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.

var (
	m0           m
	g0           g
	raceprocctx0 uintptr
)

// main_init_done is a signal used by cgocallbackg that initialization
// has been completed. It is made before _cgo_notify_runtime_init_done,
// so all cgo calls can rely on it existing. When main_init is complete,
// it is closed, meaning cgocallbackg can reliably receive from it.
var main_init_done chan bool

// 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(unsafe.Pointer) {
	g := getg()

	// 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 sys.PtrSize == 8 {
		maxstacksize = 1000000000
	} else {
		maxstacksize = 250000000
	}

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

	if GOARCH != "wasm" { // no threads on wasm yet, so no sysmon
		systemstack(func() {
			newm(sysmon, nil)
		})
	}

	// 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 g.m != &m0 {
		throw("runtime.main not on m0")
	}

	if nanotime() == 0 {
		throw("nanotime returning zero")
	}

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

	// Record when the world started.
	runtimeInitTime = nanotime()

	main_init_done = make(chan bool)
	if iscgo {
		// Start the template thread in case we enter Go from
		// a C-created thread and need to create a new thread.
		startTemplateThread()
		_cgo_notify_runtime_init_done()
	}

	fn := main_init // make an indirect call, as the linker doesn't know the address of the main package when laying down the runtime
	fn()
	createGcRootsIndex()
	close(main_init_done)

	// For gccgo we have to wait until after main is initialized
	// to enable GC, because initializing main registers the GC roots.
	gcenable()

	needUnlock = false
	unlockOSThread()

	if isarchive || islibrary {
		// A program compiled with -buildmode=c-archive or c-shared
		// has a main, but it is not executed.
		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()
	if raceenabled {
		racefini()
	}

	// 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 atomic.Load(&runningPanicDefers) != 0 {
		// Running deferred functions should not take long.
		for c := 0; c < 1000; c++ {
			if atomic.Load(&runningPanicDefers) == 0 {
				break
			}
			Gosched()
		}
	}
	if atomic.Load(&panicking) != 0 {
		gopark(nil, nil, waitReasonPanicWait, traceEvGoStop, 1)
	}

	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() {
	if raceenabled {
		racefini()
	}
}

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

func forcegchelper() {
	setSystemGoroutine()

	forcegc.g = getg()
	for {
		lock(&forcegc.lock)
		if forcegc.idle != 0 {
			throw("forcegc: phase error")
		}
		atomic.Store(&forcegc.idle, 1)
		goparkunlock(&forcegc.lock, waitReasonForceGGIdle, traceEvGoBlock, 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()})
	}
}

//go:nosplit

// Gosched yields the processor, allowing other goroutines to run. It does not
// suspend the current goroutine, so execution resumes automatically.
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)
}

// Puts the current goroutine into a waiting state and calls unlockf.
// 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.
// 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.
func gopark(unlockf func(*g, unsafe.Pointer) bool, lock unsafe.Pointer, reason waitReason, traceEv byte, 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.waittraceev = traceEv
	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, traceEv byte, traceskip int) {
	gopark(parkunlock_c, unsafe.Pointer(lock), reason, traceEv, traceskip)
}

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 != nil {
		throw("acquireSudog: found s.elem != nil in cache")
	}
	releasem(mp)
	return s
}

//go:nosplit
func releaseSudog(s *sudog) {
	if s.elem != 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 != 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)
}

// funcPC returns the entry PC of the function f.
// It assumes that f is a func value. Otherwise the behavior is undefined.
// CAREFUL: In programs with plugins, funcPC can return different values
// for the same function (because there are actually multiple copies of
// the same function in the address space). To be safe, don't use the
// results of this function in any == expression. It is only safe to
// use the result as an address at which to start executing code.
//
// For gccgo note that this differs from the gc implementation; the gc
// implementation adds sys.PtrSize to the address of the interface
// value, but GCC's alias analysis decides that that can not be a
// reference to the second field of the interface, and in some cases
// it drops the initialization of the second field as a dead store.
//go:nosplit
func funcPC(f interface{}) uintptr {
	i := (*iface)(unsafe.Pointer(&f))
	r := *(*uintptr)(i.data)
	if cpu.FunctionDescriptors {
		// With PPC64 ELF ABI v1 function descriptors the
		// function address is a pointer to a struct whose
		// first field is the actual PC.
		r = *(*uintptr)(unsafe.Pointer(r))
	}
	return r
}

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

var (
	allgs    []*g
	allglock mutex
)

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

	lock(&allglock)
	allgs = append(allgs, gp)
	allglen = uintptr(len(allgs))
	unlock(&allglock)
}

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 extracts the environment variable GODEBUG from the environment on
// Unix-like operating systems and calls internal/cpu.Initialize.
func cpuinit() {
	const prefix = "GODEBUG="
	var env string

	switch GOOS {
	case "aix", "darwin", "dragonfly", "freebsd", "netbsd", "openbsd", "illumos", "solaris", "linux":
		cpu.DebugOptions = true

		// 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 := *(*string)(unsafe.Pointer(&stringStruct{unsafe.Pointer(p), findnull(p)}))

			if hasPrefix(s, prefix) {
				env = gostring(p)[len(prefix):]
				break
			}
		}
	}

	cpu.Initialize(env)
}

func ginit() {
	_m_ := &m0
	_g_ := &g0
	_m_.g0 = _g_
	_m_.curg = _g_
	_g_.m = _m_
	setg(_g_)
}

// The bootstrap sequence is:
//
//	call osinit
//	call schedinit
//	make & queue new G
//	call runtime·mstart
//
// The new G calls runtime·main.
func schedinit() {
	_g_ := getg()
	sched.maxmcount = 10000

	usestackmaps = probestackmaps()

	mallocinit()
	fastrandinit() // must run before mcommoninit
	mcommoninit(_g_.m)
	cpuinit() // must run before alginit
	alginit() // maps must not be used before this call

	msigsave(_g_.m)
	initSigmask = _g_.m.sigmask

	goargs()
	goenvs()
	parsedebugvars()
	gcinit()

	sched.lastpoll = uint64(nanotime())
	procs := ncpu

	// In 32-bit mode, we can burn a lot of memory on thread stacks.
	// Try to avoid this by limiting the number of threads we run
	// by default.
	if sys.PtrSize == 4 && procs > 32 {
		procs = 32
	}

	if n, ok := atoi32(gogetenv("GOMAXPROCS")); ok && n > 0 {
		procs = n
	}
	if procresize(procs) != nil {
		throw("unknown runnable goroutine during bootstrap")
	}

	// For cgocheck > 1, we turn on the write barrier at all times
	// and check all pointer writes. We can't do this until after
	// procresize because the write barrier needs a P.
	if debug.cgocheck > 1 {
		writeBarrier.cgo = true
		writeBarrier.enabled = true
		for _, p := range allp {
			p.wbBuf.reset()
		}
	}

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

func checkmcount() {
	// sched lock is held
	if mcount() > sched.maxmcount {
		print("runtime: program exceeds ", sched.maxmcount, "-thread limit\n")
		throw("thread exhaustion")
	}
}

func mcommoninit(mp *m) {
	_g_ := getg()

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

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

	mp.fastrand[0] = uint32(int64Hash(uint64(mp.id), fastrandseed))
	mp.fastrand[1] = uint32(int64Hash(uint64(cputicks()), ^fastrandseed))
	if mp.fastrand[0]|mp.fastrand[1] == 0 {
		mp.fastrand[1] = 1
	}

	mpreinit(mp)

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

var fastrandseed uintptr

func fastrandinit() {
	s := (*[unsafe.Sizeof(fastrandseed)]byte)(unsafe.Pointer(&fastrandseed))[:]
	getRandomData(s)
}

// Mark gp ready to run.
func ready(gp *g, traceskip int, next bool) {
	if trace.enabled {
		traceGoUnpark(gp, traceskip)
	}

	status := readgstatus(gp)

	// Mark runnable.
	_g_ := getg()
	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
	casgstatus(gp, _Gwaiting, _Grunnable)
	runqput(_g_.m.p.ptr(), gp, next)
	if atomic.Load(&sched.npidle) != 0 && atomic.Load(&sched.nmspinning) == 0 {
		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 uint32

// 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() {
	atomic.Store(&freezing, 1)
	// 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
		atomic.Store(&sched.gcwaiting, 1)
		// 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 atomic.Load(&gp.atomicstatus)
}

// 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,
		_Gscanpreempted:
		if newval == oldval&^_Gscan {
			success = atomic.Cas(&gp.atomicstatus, 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")
	}
}

// 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,
		_Gsyscall:
		if newval == oldval|_Gscan {
			return atomic.Cas(&gp.atomicstatus, oldval, newval)
		}
	}
	print("runtime: castogscanstatus oldval=", hex(oldval), " newval=", hex(newval), "\n")
	throw("castogscanstatus")
	panic("not reached")
}

// 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() {
			print("runtime: casgstatus: oldval=", hex(oldval), " newval=", hex(newval), "\n")
			throw("casgstatus: bad incoming values")
		})
	}

	// 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; !atomic.Cas(&gp.atomicstatus, oldval, newval); i++ {
		if oldval == _Gwaiting && gp.atomicstatus == _Grunnable {
			throw("casgstatus: waiting for Gwaiting but is Grunnable")
		}
		if i == 0 {
			nextYield = nanotime() + yieldDelay
		}
		if nanotime() < nextYield {
			for x := 0; x < 10 && gp.atomicstatus != oldval; x++ {
				procyield(1)
			}
		} else {
			osyield()
			nextYield = nanotime() + yieldDelay/2
		}
	}
}

// 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")
	}
	for !atomic.Cas(&gp.atomicstatus, _Grunning, _Gscan|_Gpreempted) {
	}
}

// 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")
	}
	return atomic.Cas(&gp.atomicstatus, _Gpreempted, _Gwaiting)
}

// 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.
func stopTheWorld(reason string) {
	semacquire(&worldsema)
	getg().m.preemptoff = reason
	systemstack(stopTheWorldWithSema)
}

// startTheWorld undoes the effects of stopTheWorld.
func startTheWorld() {
	systemstack(func() { startTheWorldWithSema(false) })
	// worldsema must be held over startTheWorldWithSema to ensure
	// gomaxprocs cannot change while worldsema is held.
	semrelease(&worldsema)
	getg().m.preemptoff = ""
}

// Holding worldsema grants an M the right to try to stop the world
// and prevents gomaxprocs from changing concurrently.
var worldsema 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"
//	systemstack(stopTheWorldWithSema)
//
// When finished, the caller must either call startTheWorld or undo
// these three operations separately:
//
//	m.preemptoff = ""
//	systemstack(startTheWorldWithSema)
//	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.
func stopTheWorldWithSema() {
	_g_ := 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 _g_.m.locks > 0 {
		throw("stopTheWorld: holding locks")
	}

	lock(&sched.lock)
	sched.stopwait = gomaxprocs
	atomic.Store(&sched.gcwaiting, 1)
	preemptall()
	// stop current P
	_g_.m.p.ptr().status = _Pgcstop // Pgcstop is only diagnostic.
	sched.stopwait--
	// try to retake all P's in Psyscall status
	for _, p := range allp {
		s := p.status
		if s == _Psyscall && atomic.Cas(&p.status, s, _Pgcstop) {
			if trace.enabled {
				traceGoSysBlock(p)
				traceProcStop(p)
			}
			p.syscalltick++
			sched.stopwait--
		}
	}
	// stop idle P's
	for {
		p := pidleget()
		if p == nil {
			break
		}
		p.status = _Pgcstop
		sched.stopwait--
	}
	wait := sched.stopwait > 0
	unlock(&sched.lock)

	// wait for remaining P's 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()
		}
	}

	// sanity checks
	bad := ""
	if sched.stopwait != 0 {
		bad = "stopTheWorld: not stopped (stopwait != 0)"
	} else {
		for _, p := range allp {
			if p.status != _Pgcstop {
				bad = "stopTheWorld: not stopped (status != _Pgcstop)"
			}
		}
	}
	if atomic.Load(&freezing) != 0 {
		// 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)
	}
}

func startTheWorldWithSema(emitTraceEvent bool) int64 {
	mp := acquirem() // disable preemption because it can be holding p in a local var
	if netpollinited() {
		list := netpoll(0) // non-blocking
		injectglist(&list)
	}
	lock(&sched.lock)

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

	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)
		}
	}

	// Capture start-the-world time before doing clean-up tasks.
	startTime := nanotime()
	if emitTraceEvent {
		traceGCSTWDone()
	}

	// 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.
	if atomic.Load(&sched.npidle) != 0 && atomic.Load(&sched.nmspinning) == 0 {
		wakep()
	}

	releasem(mp)

	return startTime
}

// First function run by a new goroutine.
// This is passed to makecontext.
func kickoff() {
	gp := getg()

	if gp.traceback != 0 {
		gtraceback(gp)
	}

	fv := gp.entry
	param := gp.param

	// When running on the g0 stack we can wind up here without a p,
	// for example from mcall(exitsyscall0) in exitsyscall, in
	// which case we can not run a write barrier.
	// It is also possible for us to get here from the systemstack
	// call in wbBufFlush, at which point the write barrier buffer
	// is full and we can not run a write barrier.
	// Setting gp.entry = nil or gp.param = nil will try to run a
	// write barrier, so if we are on the g0 stack due to mcall
	// (systemstack calls mcall) then clear the field using uintptr.
	// This is OK when gp.param is gp.m.curg, as curg will be kept
	// alive elsewhere, and gp.entry always points into g, or
	// to a statically allocated value, or (in the case of mcall)
	// to the stack.
	if gp == gp.m.g0 && gp.param == unsafe.Pointer(gp.m.curg) {
		*(*uintptr)(unsafe.Pointer(&gp.entry)) = 0
		*(*uintptr)(unsafe.Pointer(&gp.param)) = 0
	} else if gp.m.p == 0 {
		throw("no p in kickoff")
	} else {
		gp.entry = nil
		gp.param = nil
	}

	// Record the entry SP to help stack scan.
	gp.entrysp = getsp()

	fv(param)
	goexit1()
}

func mstart1() {
	_g_ := getg()

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

	asminit()

	// Install signal handlers; after minit so that minit can
	// prepare the thread to be able to handle the signals.
	// For gccgo minit was called by C code.
	if _g_.m == &m0 {
		mstartm0()
	}

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

	if _g_.m != &m0 {
		acquirep(_g_.m.nextp.ptr())
		_g_.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)
}

// 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(&_g_.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) {
	g := getg()
	m := g.m

	if m == &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)
		notesleep(&m.park)
		throw("locked m0 woke up")
	}

	sigblock()
	unminit()

	// Free the gsignal stack.
	if m.gsignal != nil {
		stackfree(m.gsignal)
		// 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.
		m.gsignal = nil
	}

	// Remove m from allm.
	lock(&sched.lock)
	for pprev := &allm; *pprev != nil; pprev = &(*pprev).alllink {
		if *pprev == m {
			*pprev = m.alllink
			goto found
		}
	}
	throw("m not found in allm")
found:
	if !osStack {
		// Delay reaping m until it's done with the stack.
		//
		// If this is using an OS stack, the OS will free it
		// so there's no need for reaping.
		atomic.Store(&m.freeWait, 1)
		// Put m on the free list, though it will not be reaped until
		// freeWait is 0. Note that the free list must not be linked
		// through alllink because some functions walk allm without
		// locking, so may be using alllink.
		m.freelink = sched.freem
		sched.freem = m
	}
	unlock(&sched.lock)

	// 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 osStack {
		// 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(&m.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.
//
//go:systemstack
func forEachP(fn func(*p)) {
	mp := acquirem()
	_p_ := 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 _, p := range allp {
		if p != _p_ {
			atomic.Store(&p.runSafePointFn, 1)
		}
	}
	preemptall()

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

	// 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(_p_)

	// Force Ps currently in _Psyscall into _Pidle and hand them
	// off to induce safe point function execution.
	for _, p := range allp {
		s := p.status
		if s == _Psyscall && p.runSafePointFn == 1 && atomic.Cas(&p.status, s, _Pidle) {
			if trace.enabled {
				traceGoSysBlock(p)
				traceProcStop(p)
			}
			p.syscalltick++
			handoffp(p)
		}
	}

	// 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 _, p := range allp {
		if p.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
// _Psyscall to avoid a race where forEachP sees that the P is running
// just before the P goes into _Pidle/_Psyscall 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)
}

// 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.
//
// This function is allowed to have write barriers even if the caller
// isn't because it borrows _p_.
//
//go:yeswritebarrierrec
func allocm(_p_ *p, fn func(), allocatestack bool) (mp *m, g0Stack unsafe.Pointer, g0StackSize uintptr) {
	_g_ := getg()
	acquirem() // disable GC because it can be called from sysmon
	if _g_.m.p == 0 {
		acquirep(_p_) // 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; {
			if freem.freeWait != 0 {
				next := freem.freelink
				freem.freelink = newList
				newList = freem
				freem = next
				continue
			}
			stackfree(freem.g0)
			freem = freem.freelink
		}
		sched.freem = newList
		unlock(&sched.lock)
	}

	mp = new(m)
	mp.mstartfn = fn
	mcommoninit(mp)

	mp.g0 = malg(allocatestack, false, &g0Stack, &g0StackSize)
	mp.g0.m = mp

	if _p_ == _g_.m.p.ptr() {
		releasep()
	}
	releasem(_g_.m)

	return mp, g0Stack, g0StackSize
}

// 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.
//
// When the callback is done with the m, it calls dropm to
// put the m back on the list.
//go:nosplit
func needm(x byte) {
	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.
		write(2, unsafe.Pointer(&earlycgocallback[0]), int32(len(earlycgocallback)))
		exit(1)
	}

	// Lock extra list, take head, unlock popped list.
	// nilokay=false is safe here because of the invariant above,
	// that the extra list always contains or will soon contain
	// at least one m.
	mp := lockextra(false)

	// 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 = mp.schedlink == 0
	extraMCount--
	unlockextra(mp.schedlink.ptr())

	// Save and block signals before installing g.
	// 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.
	msigsave(mp)
	sigblock()

	// Install g (= m->curg).
	setg(mp.curg)

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

	setGContext()

	// mp.curg is now a real goroutine.
	casgstatus(mp.curg, _Gdead, _Gsyscall)
	atomic.Xadd(&sched.ngsys, -1)
}

var earlycgocallback = []byte("fatal error: cgo callback before cgo call\n")

// 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 := atomic.Xchg(&extraMWaiters, 0)
	if c > 0 {
		for i := uint32(0); i < c; i++ {
			oneNewExtraM()
		}
	} else {
		// Make sure there is at least one extra M.
		mp := lockextra(true)
		unlockextra(mp)
		if mp == nil {
			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, g0SP, g0SPSize := allocm(nil, nil, true)
	gp := malg(true, false, nil, nil)
	// malg returns status as _Gidle. Change to _Gdead before
	// adding to allg where GC can see it. We use _Gdead to hide
	// this from tracebacks and stack scans since it isn't a
	// "real" goroutine until needm grabs it.
	casgstatus(gp, _Gidle, _Gdead)
	gp.m = mp
	mp.curg = gp
	mp.lockedInt++
	mp.lockedg.set(gp)
	gp.lockedm.set(mp)
	gp.goid = int64(atomic.Xadd64(&sched.goidgen, 1))
	// put on allg for garbage collector
	allgadd(gp)

	// The context for gp will be set up in needm.
	// Here we need to set the context for g0.
	makeGContext(mp.g0, g0SP, g0SPSize)

	// 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.
	atomic.Xadd(&sched.ngsys, +1)

	// Add m to the extra list.
	mnext := lockextra(true)
	mp.schedlink.set(mnext)
	extraMCount++
	unlockextra(mp)
}

// 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.
// It puts the current m back onto the extra list.
//
// 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.
//
// TODO(rsc): An alternative would be to allocate a dummy pthread per-thread
// variable using pthread_key_create. Unlike the pthread keys we already use
// on OS X, this dummy key would never be read by Go code. It would exist
// only so that we could register at thread-exit-time destructor.
// That destructor would put the m back onto the extra list.
// This is purely a performance optimization. The current version,
// in which dropm happens on each cgo call, is still correct too.
// We may have to keep the current version on systems with cgo
// but without pthreads, like Windows.
//
// CgocallBackDone calls this after releasing p, so no write barriers.
//go:nowritebarrierrec
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

	// Return mp.curg to dead state.
	casgstatus(mp.curg, _Gsyscall, _Gdead)
	mp.curg.preemptStop = false
	atomic.Xadd(&sched.ngsys, +1)

	// 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()
	unminit()

	// gccgo sets the stack to Gdead here, because the splitstack
	// context is not initialized.
	atomic.Store(&mp.curg.atomicstatus, _Gdead)
	mp.curg.gcstack = 0
	mp.curg.gcnextsp = 0

	mnext := lockextra(true)
	extraMCount++
	mp.schedlink.set(mnext)

	setg(nil)

	// Commit the release of mp.
	unlockextra(mp)

	msigrestore(sigmask)
}

// A helper function for EnsureDropM.
func getm() uintptr {
	return uintptr(unsafe.Pointer(getg().m))
}

var extram uintptr
var extraMCount uint32 // Protected by lockextra
var extraMWaiters 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
//go:nowritebarrierrec
func lockextra(nilokay bool) *m {
	const locked = 1

	incr := false
	for {
		old := atomic.Loaduintptr(&extram)
		if old == locked {
			yield := osyield
			yield()
			continue
		}
		if old == 0 && !nilokay {
			if !incr {
				// Add 1 to the number of threads
				// waiting for an M.
				// This is cleared by newextram.
				atomic.Xadd(&extraMWaiters, 1)
				incr = true
			}
			usleep(1)
			continue
		}
		if atomic.Casuintptr(&extram, old, locked) {
			return (*m)(unsafe.Pointer(old))
		}
		yield := osyield
		yield()
		continue
	}
}

//go:nosplit
//go:nowritebarrierrec
func unlockextra(mp *m) {
	atomic.Storeuintptr(&extram, uintptr(unsafe.Pointer(mp)))
}

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

// 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.
//go:nowritebarrierrec
func newm(fn func(), _p_ *p) {
	mp, _, _ := allocm(_p_, fn, false)
	mp.nextp.set(_p_)
	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)
		return
	}
	newm1(mp)
}

func newm1(mp *m) {
	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)
	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() {
	_g_ := getg()

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

	lock(&sched.lock)
	mput(_g_.m)
	unlock(&sched.lock)
	notesleep(&_g_.m.park)
	noteclear(&_g_.m.park)
	acquirep(_g_.m.nextp.ptr())
	_g_.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 startm will
// either decrement nmspinning or set m.spinning in the newly started M.
//go:nowritebarrierrec
func startm(_p_ *p, spinning bool) {
	lock(&sched.lock)
	if _p_ == nil {
		_p_ = pidleget()
		if _p_ == nil {
			unlock(&sched.lock)
			if spinning {
				// The caller incremented nmspinning, but there are no idle Ps,
				// so it's okay to just undo the increment and give up.
				if int32(atomic.Xadd(&sched.nmspinning, -1)) < 0 {
					throw("startm: negative nmspinning")
				}
			}
			return
		}
	}
	mp := mget()
	unlock(&sched.lock)
	if mp == nil {
		var fn func()
		if spinning {
			// The caller incremented nmspinning, so set m.spinning in the new M.
			fn = mspinning
		}
		newm(fn, _p_)
		return
	}
	if mp.spinning {
		throw("startm: m is spinning")
	}
	if mp.nextp != 0 {
		throw("startm: m has p")
	}
	if spinning && !runqempty(_p_) {
		throw("startm: p has runnable gs")
	}
	// The caller incremented nmspinning, so set m.spinning in the new M.
	mp.spinning = spinning
	mp.nextp.set(_p_)
	notewakeup(&mp.park)
}

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

	// if it has local work, start it straight away
	if !runqempty(_p_) || sched.runqsize != 0 {
		startm(_p_, false)
		return
	}
	// if it has GC work, start it straight away
	if gcBlackenEnabled != 0 && gcMarkWorkAvailable(_p_) {
		startm(_p_, false)
		return
	}
	// no local work, check that there are no spinning/idle M's,
	// otherwise our help is not required
	if atomic.Load(&sched.nmspinning)+atomic.Load(&sched.npidle) == 0 && atomic.Cas(&sched.nmspinning, 0, 1) { // TODO: fast atomic
		startm(_p_, true)
		return
	}
	lock(&sched.lock)
	if sched.gcwaiting != 0 {
		_p_.status = _Pgcstop
		sched.stopwait--
		if sched.stopwait == 0 {
			notewakeup(&sched.stopnote)
		}
		unlock(&sched.lock)
		return
	}
	if _p_.runSafePointFn != 0 && atomic.Cas(&_p_.runSafePointFn, 1, 0) {
		sched.safePointFn(_p_)
		sched.safePointWait--
		if sched.safePointWait == 0 {
			notewakeup(&sched.safePointNote)
		}
	}
	if sched.runqsize != 0 {
		unlock(&sched.lock)
		startm(_p_, 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 == uint32(gomaxprocs-1) && atomic.Load64(&sched.lastpoll) != 0 {
		unlock(&sched.lock)
		startm(_p_, false)
		return
	}
	if when := nobarrierWakeTime(_p_); when != 0 {
		wakeNetPoller(when)
	}
	pidleput(_p_)
	unlock(&sched.lock)
}

// Tries to add one more P to execute G's.
// Called when a G is made runnable (newproc, ready).
func wakep() {
	// be conservative about spinning threads
	if !atomic.Cas(&sched.nmspinning, 0, 1) {
		return
	}
	startm(nil, true)
}

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

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

	mp := gp.lockedm.ptr()
	if mp == _g_.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)
	_p_ := releasep()
	mp.nextp.set(_p_)
	notewakeup(&mp.park)
	stopm()
}

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

	if sched.gcwaiting == 0 {
		throw("gcstopm: not waiting for gc")
	}
	if _g_.m.spinning {
		_g_.m.spinning = false
		// OK to just drop nmspinning here,
		// startTheWorld will unpark threads as necessary.
		if int32(atomic.Xadd(&sched.nmspinning, -1)) < 0 {
			throw("gcstopm: negative nmspinning")
		}
	}
	_p_ := releasep()
	lock(&sched.lock)
	_p_.status = _Pgcstop
	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) {
	_g_ := getg()

	// Assign gp.m before entering _Grunning so running Gs have an
	// M.
	_g_.m.curg = gp
	gp.m = _g_.m
	casgstatus(gp, _Grunnable, _Grunning)
	gp.waitsince = 0
	gp.preempt = false
	if !inheritTime {
		_g_.m.p.ptr().schedtick++
	}

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

	if trace.enabled {
		// GoSysExit has to happen when we have a P, but before GoStart.
		// So we emit it here.
		if gp.syscallsp != 0 && gp.sysblocktraced {
			traceGoSysExit(gp.sysexitticks)
		}
		traceGoStart()
	}

	gogo(gp)
}

// Finds a runnable goroutine to execute.
// Tries to steal from other P's, get g from local or global queue, poll network.
func findrunnable() (gp *g, inheritTime bool) {
	_g_ := getg()

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

top:
	_p_ := _g_.m.p.ptr()
	if sched.gcwaiting != 0 {
		gcstopm()
		goto top
	}
	if _p_.runSafePointFn != 0 {
		runSafePointFn()
	}

	now, pollUntil, _ := checkTimers(_p_, 0)

	if fingwait && fingwake {
		if gp := wakefing(); gp != nil {
			ready(gp, 0, true)
		}
	}
	if *cgo_yield != nil {
		asmcgocall(*cgo_yield, nil)
	}

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

	// global runq
	if sched.runqsize != 0 {
		lock(&sched.lock)
		gp := globrunqget(_p_, 0)
		unlock(&sched.lock)
		if gp != nil {
			return gp, 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.
	if netpollinited() && atomic.Load(&netpollWaiters) > 0 && atomic.Load64(&sched.lastpoll) != 0 {
		if list := netpoll(0); !list.empty() { // non-blocking
			gp := list.pop()
			injectglist(&list)
			casgstatus(gp, _Gwaiting, _Grunnable)
			if trace.enabled {
				traceGoUnpark(gp, 0)
			}
			return gp, false
		}
	}

	// Steal work from other P's.
	procs := uint32(gomaxprocs)
	ranTimer := false
	// If number of spinning M's >= number of busy P's, block.
	// This is necessary to prevent excessive CPU consumption
	// when GOMAXPROCS>>1 but the program parallelism is low.
	if !_g_.m.spinning && 2*atomic.Load(&sched.nmspinning) >= procs-atomic.Load(&sched.npidle) {
		goto stop
	}
	if !_g_.m.spinning {
		_g_.m.spinning = true
		atomic.Xadd(&sched.nmspinning, 1)
	}
	for i := 0; i < 4; i++ {
		for enum := stealOrder.start(fastrand()); !enum.done(); enum.next() {
			if sched.gcwaiting != 0 {
				goto top
			}
			stealRunNextG := i > 2 // first look for ready queues with more than 1 g
			p2 := allp[enum.position()]
			if _p_ == p2 {
				continue
			}
			if gp := runqsteal(_p_, p2, stealRunNextG); gp != nil {
				return gp, false
			}

			// Consider stealing timers from p2.
			// This call to checkTimers is the only place where
			// we hold a lock on a different P's timers.
			// Lock contention can be a problem here, so avoid
			// grabbing the lock if p2 is running and not marked
			// for preemption. If p2 is running and not being
			// preempted we assume it will handle its own timers.
			if i > 2 && shouldStealTimers(p2) {
				tnow, w, ran := checkTimers(p2, now)
				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 is always has room to add
					// stolen G's. So check now if there
					// is a local G to run.
					if gp, inheritTime := runqget(_p_); gp != nil {
						return gp, inheritTime
					}
					ranTimer = true
				}
			}
		}
	}
	if ranTimer {
		// Running a timer may have made some goroutine ready.
		goto top
	}

stop:

	// 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 && _p_.gcBgMarkWorker != 0 && gcMarkWorkAvailable(_p_) {
		_p_.gcMarkWorkerMode = gcMarkWorkerIdleMode
		gp := _p_.gcBgMarkWorker.ptr()
		casgstatus(gp, _Gwaiting, _Grunnable)
		if trace.enabled {
			traceGoUnpark(gp, 0)
		}
		return gp, false
	}

	delta := int64(-1)
	if pollUntil != 0 {
		// checkTimers ensures that polluntil > now.
		delta = pollUntil - now
	}

	// wasm only:
	// If a callback returned and no other goroutine is awake,
	// then pause execution until a callback was triggered.
	if beforeIdle(delta) {
		// At least one goroutine got woken.
		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.
	allpSnapshot := allp

	// return P and block
	lock(&sched.lock)
	if sched.gcwaiting != 0 || _p_.runSafePointFn != 0 {
		unlock(&sched.lock)
		goto top
	}
	if sched.runqsize != 0 {
		gp := globrunqget(_p_, 0)
		unlock(&sched.lock)
		return gp, false
	}
	if releasep() != _p_ {
		throw("findrunnable: wrong p")
	}
	pidleput(_p_)
	unlock(&sched.lock)

	// Delicate dance: thread transitions from spinning to non-spinning state,
	// potentially concurrently with submission of new goroutines. We must
	// drop nmspinning first and then check all per-P queues again (with
	// #StoreLoad memory barrier in between). If we do it the other way around,
	// another thread can submit a goroutine after we've checked all run queues
	// but before we drop nmspinning; as the result nobody will unpark a thread
	// to run the goroutine.
	// 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, it is OK to just park the current thread:
	// the system is fully loaded so no spinning threads are required.
	// Also see "Worker thread parking/unparking" comment at the top of the file.
	wasSpinning := _g_.m.spinning
	if _g_.m.spinning {
		_g_.m.spinning = false
		if int32(atomic.Xadd(&sched.nmspinning, -1)) < 0 {
			throw("findrunnable: negative nmspinning")
		}
	}

	// check all runqueues once again
	for _, _p_ := range allpSnapshot {
		if !runqempty(_p_) {
			lock(&sched.lock)
			_p_ = pidleget()
			unlock(&sched.lock)
			if _p_ != nil {
				acquirep(_p_)
				if wasSpinning {
					_g_.m.spinning = true
					atomic.Xadd(&sched.nmspinning, 1)
				}
				goto top
			}
			break
		}
	}

	// Check for idle-priority GC work again.
	if gcBlackenEnabled != 0 && gcMarkWorkAvailable(nil) {
		lock(&sched.lock)
		_p_ = pidleget()
		if _p_ != nil && _p_.gcBgMarkWorker == 0 {
			pidleput(_p_)
			_p_ = nil
		}
		unlock(&sched.lock)
		if _p_ != nil {
			acquirep(_p_)
			if wasSpinning {
				_g_.m.spinning = true
				atomic.Xadd(&sched.nmspinning, 1)
			}
			// Go back to idle GC check.
			goto stop
		}
	}

	// poll network
	if netpollinited() && (atomic.Load(&netpollWaiters) > 0 || pollUntil != 0) && atomic.Xchg64(&sched.lastpoll, 0) != 0 {
		atomic.Store64(&sched.pollUntil, uint64(pollUntil))
		if _g_.m.p != 0 {
			throw("findrunnable: netpoll with p")
		}
		if _g_.m.spinning {
			throw("findrunnable: netpoll with spinning")
		}
		if faketime != 0 {
			// When using fake time, just poll.
			delta = 0
		}
		list := netpoll(delta) // block until new work is available
		atomic.Store64(&sched.pollUntil, 0)
		atomic.Store64(&sched.lastpoll, uint64(nanotime()))
		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)
		_p_ = pidleget()
		unlock(&sched.lock)
		if _p_ == nil {
			injectglist(&list)
		} else {
			acquirep(_p_)
			if !list.empty() {
				gp := list.pop()
				injectglist(&list)
				casgstatus(gp, _Gwaiting, _Grunnable)
				if trace.enabled {
					traceGoUnpark(gp, 0)
				}
				return gp, false
			}
			if wasSpinning {
				_g_.m.spinning = true
				atomic.Xadd(&sched.nmspinning, 1)
			}
			goto top
		}
	} else if pollUntil != 0 && netpollinited() {
		pollerPollUntil := int64(atomic.Load64(&sched.pollUntil))
		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.runqsize != 0 {
		return true
	}
	p := getg().m.p.ptr()
	if !runqempty(p) {
		return true
	}
	if netpollinited() && atomic.Load(&netpollWaiters) > 0 && sched.lastpoll != 0 {
		if list := netpoll(0); !list.empty() {
			injectglist(&list)
			return true
		}
	}
	return false
}

// wakeNetPoller wakes up the thread sleeping in the network poller,
// if there is one, and if it isn't going to wake up anyhow before
// the when argument.
func wakeNetPoller(when int64) {
	if atomic.Load64(&sched.lastpoll) == 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 := int64(atomic.Load64(&sched.pollUntil))
		if pollerPollUntil == 0 || pollerPollUntil > when {
			netpollBreak()
		}
	}
}

func resetspinning() {
	_g_ := getg()
	if !_g_.m.spinning {
		throw("resetspinning: not a spinning m")
	}
	_g_.m.spinning = false
	nmspinning := atomic.Xadd(&sched.nmspinning, -1)
	if int32(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.
	if nmspinning == 0 && atomic.Load(&sched.npidle) > 0 {
		wakep()
	}
}

// Injects the list of runnable G's into the scheduler and clears glist.
// Can run concurrently with GC.
func injectglist(glist *gList) {
	if glist.empty() {
		return
	}
	if trace.enabled {
		for gp := glist.head.ptr(); gp != nil; gp = gp.schedlink.ptr() {
			traceGoUnpark(gp, 0)
		}
	}
	lock(&sched.lock)
	var n int
	for n = 0; !glist.empty(); n++ {
		gp := glist.pop()
		casgstatus(gp, _Gwaiting, _Grunnable)
		globrunqput(gp)
	}
	unlock(&sched.lock)
	for ; n != 0 && sched.npidle != 0; n-- {
		startm(nil, false)
	}
	*glist = gList{}
}

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

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

	if _g_.m.lockedg != 0 {
		stoplockedm()
		execute(_g_.m.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 _g_.m.incgo {
		throw("schedule: in cgo")
	}

top:
	pp := _g_.m.p.ptr()
	pp.preempt = false

	if sched.gcwaiting != 0 {
		gcstopm()
		goto top
	}
	if pp.runSafePointFn != 0 {
		runSafePointFn()
	}

	// Sanity 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 _g_.m.spinning && (pp.runnext != 0 || pp.runqhead != pp.runqtail) {
		throw("schedule: spinning with local work")
	}

	checkTimers(pp, 0)

	var gp *g
	var inheritTime bool

	// Normal goroutines will check for need to wakeP in ready,
	// but GCworkers and tracereaders will not, so the check must
	// be done here instead.
	tryWakeP := false
	if trace.enabled || trace.shutdown {
		gp = traceReader()
		if gp != nil {
			casgstatus(gp, _Gwaiting, _Grunnable)
			traceGoUnpark(gp, 0)
			tryWakeP = true
		}
	}
	if gp == nil && gcBlackenEnabled != 0 {
		gp = gcController.findRunnableGCWorker(_g_.m.p.ptr())
		tryWakeP = tryWakeP || gp != nil
	}
	if gp == nil {
		// 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 _g_.m.p.ptr().schedtick%61 == 0 && sched.runqsize > 0 {
			lock(&sched.lock)
			gp = globrunqget(_g_.m.p.ptr(), 1)
			unlock(&sched.lock)
		}
	}
	if gp == nil {
		gp, inheritTime = runqget(_g_.m.p.ptr())
		// We can see gp != nil here even if the M is spinning,
		// if checkTimers added a local goroutine via goready.

		// Because gccgo does not implement preemption as a stack check,
		// we need to check for preemption here for fairness.
		// Otherwise goroutines on the local queue may starve
		// goroutines on the global queue.
		// Since we preempt by storing the goroutine on the global
		// queue, this is the only place we need to check preempt.
		// This does not call checkPreempt because gp is not running.
		if gp != nil && gp.preempt {
			gp.preempt = false
			lock(&sched.lock)
			globrunqput(gp)
			unlock(&sched.lock)
			goto top
		}
	}
	if gp == nil {
		gp, inheritTime = findrunnable() // blocks until work is available
	}

	// 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 _g_.m.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)
			sched.disable.n++
			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 {
		if atomic.Load(&sched.npidle) != 0 && atomic.Load(&sched.nmspinning) == 0 {
			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() {
	_g_ := getg()

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

// checkTimers runs any timers for the P that are ready.
// If now is not 0 it is the current time.
// It returns the current time or 0 if it is not known,
// and the time when the next timer should run or 0 if there is no next timer,
// and reports whether it ran any timers.
// If the time when the next timer should run is not 0,
// it is always larger than the returned time.
// We pass now in and out to avoid extra calls of nanotime.
//go:yeswritebarrierrec
func checkTimers(pp *p, now int64) (rnow, pollUntil int64, ran bool) {
	// If there are no timers to adjust, and the first timer on
	// the heap is not yet ready to run, then there is nothing to do.
	if atomic.Load(&pp.adjustTimers) == 0 {
		next := int64(atomic.Load64(&pp.timer0When))
		if next == 0 {
			return now, 0, false
		}
		if now == 0 {
			now = nanotime()
		}
		if now < next {
			// Next timer is not ready to run.
			// But keep going if we would clear deleted timers.
			// This corresponds to the condition below where
			// we decide whether to call clearDeletedTimers.
			if pp != getg().m.p.ptr() || int(atomic.Load(&pp.deletedTimers)) <= int(atomic.Load(&pp.numTimers)/4) {
				return now, next, false
			}
		}
	}

	lock(&pp.timersLock)

	adjusttimers(pp)

	rnow = now
	if len(pp.timers) > 0 {
		if rnow == 0 {
			rnow = nanotime()
		}
		for len(pp.timers) > 0 {
			// Note that runtimer may temporarily unlock
			// pp.timersLock.
			if tw := runtimer(pp, rnow); tw != 0 {
				if tw > 0 {
					pollUntil = tw
				}
				break
			}
			ran = true
		}
	}

	// If this is the local P, and there are a lot of deleted timers,
	// clear them out. We only do this for the local P to reduce
	// lock contention on timersLock.
	if pp == getg().m.p.ptr() && int(atomic.Load(&pp.deletedTimers)) > len(pp.timers)/4 {
		clearDeletedTimers(pp)
	}

	unlock(&pp.timersLock)

	return rnow, pollUntil, ran
}

// shouldStealTimers reports whether we should try stealing the timers from p2.
// We don't steal timers from a running P that is not marked for preemption,
// on the assumption that it will run its own timers. This reduces
// contention on the timers lock.
func shouldStealTimers(p2 *p) bool {
	if p2.status != _Prunning {
		return true
	}
	mp := p2.m.ptr()
	if mp == nil || mp.locks > 0 {
		return false
	}
	gp := mp.curg
	if gp == nil || gp.atomicstatus != _Grunning || !gp.preempt {
		return false
	}
	return true
}

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

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

	if trace.enabled {
		traceGoPark(_g_.m.waittraceev, _g_.m.waittraceskip)
	}

	casgstatus(gp, _Grunning, _Gwaiting)
	dropg()

	if fn := _g_.m.waitunlockf; fn != nil {
		ok := fn(gp, _g_.m.waitlock)
		_g_.m.waitunlockf = nil
		_g_.m.waitlock = nil
		if !ok {
			if trace.enabled {
				traceGoUnpark(gp, 2)
			}
			casgstatus(gp, _Gwaiting, _Grunnable)
			execute(gp, true) // Schedule it back, never returns.
		}
	}
	schedule()
}

func goschedImpl(gp *g) {
	status := readgstatus(gp)
	if status&^_Gscan != _Grunning {
		dumpgstatus(gp)
		throw("bad g status")
	}
	casgstatus(gp, _Grunning, _Grunnable)
	dropg()
	lock(&sched.lock)
	globrunqput(gp)
	unlock(&sched.lock)

	schedule()
}

// Gosched continuation on g0.
func gosched_m(gp *g) {
	if trace.enabled {
		traceGoSched()
	}
	goschedImpl(gp)
}

// goschedguarded is a forbidden-states-avoided version of gosched_m
func goschedguarded_m(gp *g) {

	if !canPreemptM(gp.m) {
		gogo(gp) // never return
	}

	if trace.enabled {
		traceGoSched()
	}
	goschedImpl(gp)
}

func gopreempt_m(gp *g) {
	if trace.enabled {
		traceGoPreempt()
	}
	goschedImpl(gp)
}

// preemptPark parks gp and puts it in _Gpreempted.
//
//go:systemstack
func preemptPark(gp *g) {
	if trace.enabled {
		traceGoPark(traceEvGoBlock, 0)
	}
	status := readgstatus(gp)
	if status&^_Gscan != _Grunning {
		dumpgstatus(gp)
		throw("bad g status")
	}
	gp.waitreason = waitReasonPreempted
	// 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)
	dropg()
	casfrom_Gscanstatus(gp, _Gscan|_Gpreempted, _Gpreempted)
	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
func goyield() {
	checkTimeouts()
	mcall(goyield_m)
}

func goyield_m(gp *g) {
	if trace.enabled {
		traceGoPreempt()
	}
	pp := gp.m.p.ptr()
	casgstatus(gp, _Grunning, _Grunnable)
	dropg()
	runqput(pp, gp, false)
	schedule()
}

// Finishes execution of the current goroutine.
func goexit1() {
	if trace.enabled {
		traceGoEnd()
	}
	mcall(goexit0)
}

// goexit continuation on g0.
func goexit0(gp *g) {
	_g_ := getg()

	casgstatus(gp, _Grunning, _Gdead)
	if isSystemGoroutine(gp, false) {
		atomic.Xadd(&sched.ngsys, -1)
		gp.isSystemGoroutine = false
	}
	gp.m = nil
	locked := gp.lockedm != 0
	gp.lockedm = 0
	_g_.m.lockedg = 0
	gp.entry = nil
	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 = 0
	gp.param = nil
	gp.labels = nil
	gp.timer = nil

	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.
		scanCredit := int64(gcController.assistWorkPerByte * float64(gp.gcAssistBytes))
		atomic.Xaddint64(&gcController.bgScanCredit, scanCredit)
		gp.gcAssistBytes = 0
	}

	dropg()

	if GOARCH == "wasm" { // no threads yet on wasm
		gfput(_g_.m.p.ptr(), gp)
		schedule() // never returns
	}

	if _g_.m.lockedInt != 0 {
		print("invalid m->lockedInt = ", _g_.m.lockedInt, "\n")
		throw("internal lockOSThread error")
	}
	gfput(_g_.m.p.ptr(), 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.
			_g_.m.exiting = true
			gogo(_g_.m.g0)
		} else {
			// Clear lockedExt on plan9 since we may end up re-using
			// this thread.
			_g_.m.lockedExt = 0
		}
	}
	schedule()
}

// 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.
//
// The entersyscall function is written in C, so that it can save the
// current register context so that the GC will see them.
// It calls reentersyscall.
//
// Syscall tracing:
// At the start of a syscall we emit traceGoSysCall to capture the stack trace.
// If the syscall does not block, that is it, we do not emit any other events.
// If the syscall blocks (that is, P is retaken), retaker emits traceGoSysBlock;
// when syscall returns we emit traceGoSysExit and when the goroutine starts running
// (potentially instantly, if exitsyscallfast returns true) we emit traceGoStart.
// To ensure that traceGoSysExit is emitted strictly after traceGoSysBlock,
// we remember current value of syscalltick in m (_g_.m.syscalltick = _g_.m.p.ptr().syscalltick),
// whoever emits traceGoSysBlock increments p.syscalltick afterwards;
// and we wait for the increment before emitting traceGoSysExit.
// Note that the increment is done even if tracing is not enabled,
// because tracing can be enabled in the middle of syscall. We don't want the wait to hang.
//
//go:nosplit
//go:noinline
func reentersyscall(pc, sp uintptr) {
	_g_ := getg()

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

	_g_.syscallsp = sp
	_g_.syscallpc = pc
	casgstatus(_g_, _Grunning, _Gsyscall)

	if trace.enabled {
		systemstack(traceGoSysCall)
	}

	if atomic.Load(&sched.sysmonwait) != 0 {
		systemstack(entersyscall_sysmon)
	}

	if _g_.m.p.ptr().runSafePointFn != 0 {
		// runSafePointFn may stack split if run on this stack
		systemstack(runSafePointFn)
	}

	_g_.m.syscalltick = _g_.m.p.ptr().syscalltick
	_g_.sysblocktraced = true
	_g_.m.mcache = nil
	pp := _g_.m.p.ptr()
	pp.m = 0
	_g_.m.oldp.set(pp)
	_g_.m.p = 0
	atomic.Store(&pp.status, _Psyscall)
	if sched.gcwaiting != 0 {
		systemstack(entersyscall_gcwait)
	}

	_g_.m.locks--
}

func entersyscall_sysmon() {
	lock(&sched.lock)
	if atomic.Load(&sched.sysmonwait) != 0 {
		atomic.Store(&sched.sysmonwait, 0)
		notewakeup(&sched.sysmonnote)
	}
	unlock(&sched.lock)
}

func entersyscall_gcwait() {
	_g_ := getg()
	_p_ := _g_.m.oldp.ptr()

	lock(&sched.lock)
	if sched.stopwait > 0 && atomic.Cas(&_p_.status, _Psyscall, _Pgcstop) {
		if trace.enabled {
			traceGoSysBlock(_p_)
			traceProcStop(_p_)
		}
		_p_.syscalltick++
		if sched.stopwait--; sched.stopwait == 0 {
			notewakeup(&sched.stopnote)
		}
	}
	unlock(&sched.lock)
}

func reentersyscallblock(pc, sp uintptr) {
	_g_ := getg()

	_g_.m.locks++ // see comment in entersyscall
	_g_.throwsplit = true
	_g_.m.syscalltick = _g_.m.p.ptr().syscalltick
	_g_.sysblocktraced = true
	_g_.m.p.ptr().syscalltick++

	// Leave SP around for GC and traceback.
	_g_.syscallsp = sp
	_g_.syscallpc = pc
	casgstatus(_g_, _Grunning, _Gsyscall)
	systemstack(entersyscallblock_handoff)

	_g_.m.locks--
}

func entersyscallblock_handoff() {
	if trace.enabled {
		traceGoSysCall()
		traceGoSysBlock(getg().m.p.ptr())
	}
	handoffp(releasep())
}

// 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.
//
//go:nosplit
//go:nowritebarrierrec
func exitsyscall() {
	_g_ := getg()

	_g_.m.locks++ // see comment in entersyscall

	_g_.waitsince = 0
	oldp := _g_.m.oldp.ptr()
	_g_.m.oldp = 0
	if exitsyscallfast(oldp) {
		if _g_.m.mcache == nil {
			throw("lost mcache")
		}
		if trace.enabled {
			if oldp != _g_.m.p.ptr() || _g_.m.syscalltick != _g_.m.p.ptr().syscalltick {
				systemstack(traceGoStart)
			}
		}
		// There's a cpu for us, so we can run.
		_g_.m.p.ptr().syscalltick++
		// We need to cas the status and scan before resuming...
		casgstatus(_g_, _Gsyscall, _Grunning)

		exitsyscallclear(_g_)
		_g_.m.locks--
		_g_.throwsplit = false

		// Check preemption, since unlike gc we don't check on
		// every call.
		if getg().preempt {
			checkPreempt()
		}
		_g_.throwsplit = false

		if sched.disable.user && !schedEnabled(_g_) {
			// Scheduling of this goroutine is disabled.
			Gosched()
		}

		return
	}

	_g_.sysexitticks = 0
	if trace.enabled {
		// Wait till traceGoSysBlock event is emitted.
		// This ensures consistency of the trace (the goroutine is started after it is blocked).
		for oldp != nil && oldp.syscalltick == _g_.m.syscalltick {
			osyield()
		}
		// We can't trace syscall exit right now because we don't have a P.
		// Tracing code can invoke write barriers that cannot run without a P.
		// So instead we remember the syscall exit time and emit the event
		// in execute when we have a P.
		_g_.sysexitticks = cputicks()
	}

	_g_.m.locks--

	// Call the scheduler.
	mcall(exitsyscall0)

	if _g_.m.mcache == nil {
		throw("lost mcache")
	}

	// 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.
	exitsyscallclear(_g_)

	_g_.m.p.ptr().syscalltick++
	_g_.throwsplit = false
}

//go:nosplit
func exitsyscallfast(oldp *p) bool {
	_g_ := getg()

	// Freezetheworld sets stopwait but does not retake P's.
	if sched.stopwait == freezeStopWait {
		return false
	}

	// Try to re-acquire the last P.
	if oldp != nil && oldp.status == _Psyscall && atomic.Cas(&oldp.status, _Psyscall, _Pidle) {
		// There's a cpu for us, so we can run.
		wirep(oldp)
		exitsyscallfast_reacquired()
		return true
	}

	// Try to get any other idle P.
	if sched.pidle != 0 {
		var ok bool
		systemstack(func() {
			ok = exitsyscallfast_pidle()
			if ok && trace.enabled {
				if oldp != nil {
					// Wait till traceGoSysBlock event is emitted.
					// This ensures consistency of the trace (the goroutine is started after it is blocked).
					for oldp.syscalltick == _g_.m.syscalltick {
						osyield()
					}
				}
				traceGoSysExit(0)
			}
		})
		if ok {
			return true
		}
	}
	return false
}

// exitsyscallfast_reacquired is the exitsyscall path on which this G
// has successfully reacquired the P it was running on before the
// syscall.
//
//go:nosplit
func exitsyscallfast_reacquired() {
	_g_ := getg()
	if _g_.m.syscalltick != _g_.m.p.ptr().syscalltick {
		if trace.enabled {
			// The p was retaken and then enter into syscall again (since _g_.m.syscalltick has changed).
			// traceGoSysBlock for this syscall was already emitted,
			// but here we effectively retake the p from the new syscall running on the same p.
			systemstack(func() {
				// Denote blocking of the new syscall.
				traceGoSysBlock(_g_.m.p.ptr())
				// Denote completion of the current syscall.
				traceGoSysExit(0)
			})
		}
		_g_.m.p.ptr().syscalltick++
	}
}

func exitsyscallfast_pidle() bool {
	lock(&sched.lock)
	_p_ := pidleget()
	if _p_ != nil && atomic.Load(&sched.sysmonwait) != 0 {
		atomic.Store(&sched.sysmonwait, 0)
		notewakeup(&sched.sysmonnote)
	}
	unlock(&sched.lock)
	if _p_ != nil {
		acquirep(_p_)
		return true
	}
	return false
}

// exitsyscall slow path on g0.
// Failed to acquire P, enqueue gp as runnable.
//
//go:nowritebarrierrec
func exitsyscall0(gp *g) {
	_g_ := getg()

	casgstatus(gp, _Gsyscall, _Gexitingsyscall)
	dropg()
	casgstatus(gp, _Gexitingsyscall, _Grunnable)
	lock(&sched.lock)
	var _p_ *p
	if schedEnabled(_g_) {
		_p_ = pidleget()
	}
	if _p_ == nil {
		globrunqput(gp)
	} else if atomic.Load(&sched.sysmonwait) != 0 {
		atomic.Store(&sched.sysmonwait, 0)
		notewakeup(&sched.sysmonnote)
	}
	unlock(&sched.lock)
	if _p_ != nil {
		acquirep(_p_)
		execute(gp, false) // Never returns.
	}
	if _g_.m.lockedg != 0 {
		// Wait until another thread schedules gp and so m again.
		stoplockedm()
		execute(gp, false) // Never returns.
	}
	stopm()
	schedule() // Never returns.
}

// exitsyscallclear clears GC-related information that we only track
// during a syscall.
func exitsyscallclear(gp *g) {
	// Garbage collector isn't running (since we are), so okay to
	// clear syscallsp.
	gp.syscallsp = 0

	gp.gcstack = 0
	gp.gcnextsp = 0
	memclrNoHeapPointers(unsafe.Pointer(&gp.gcregs), unsafe.Sizeof(gp.gcregs))
}

// Code generated by cgo, and some library code, calls syscall.Entersyscall
// and syscall.Exitsyscall.

//go:linkname syscall_entersyscall syscall.Entersyscall
//go:nosplit
func syscall_entersyscall() {
	entersyscall()
}

//go:linkname syscall_exitsyscall syscall.Exitsyscall
//go:nosplit
func syscall_exitsyscall() {
	exitsyscall()
}

func 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++
	msigsave(gp.m)
	sigblock()
}

// Called from syscall package before fork.
//go:linkname syscall_runtime_BeforeFork syscall.runtime_BeforeFork
//go:nosplit
func syscall_runtime_BeforeFork() {
	systemstack(beforefork)
}

func afterfork() {
	gp := getg().m.curg

	msigrestore(gp.m.sigmask)

	gp.m.locks--
}

// Called from syscall package after fork in parent.
//go:linkname syscall_runtime_AfterFork syscall.runtime_AfterFork
//go:nosplit
func syscall_runtime_AfterFork() {
	systemstack(afterfork)
}

// 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.
//
//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
}

// 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()
}

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

// panicgonil is used for gccgo as we need to use a compiler check for
// a nil func, in case we have to build a thunk.
//go:linkname panicgonil
func panicgonil() {
	getg().m.throwing = -1 // do not dump full stacks
	throw("go of nil func value")
}

// Create a new g running fn passing arg as the single argument.
// Put it on the queue of g's waiting to run.
// The compiler turns a go statement into a call to this.
//go:linkname newproc __go_go
func newproc(fn uintptr, arg unsafe.Pointer) *g {
	_g_ := getg()

	if fn == 0 {
		_g_.m.throwing = -1 // do not dump full stacks
		throw("go of nil func value")
	}
	acquirem() // disable preemption because it can be holding p in a local var

	_p_ := _g_.m.p.ptr()
	newg := gfget(_p_)
	var (
		sp     unsafe.Pointer
		spsize uintptr
	)
	if newg == nil {
		newg = malg(true, false, &sp, &spsize)
		casgstatus(newg, _Gidle, _Gdead)
		allgadd(newg) // publishes with a g->status of Gdead so GC scanner doesn't look at uninitialized stack.
	} else {
		resetNewG(newg, &sp, &spsize)
	}
	newg.traceback = 0

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

	// Store the C function pointer into entryfn, take the address
	// of entryfn, convert it to a Go function value, and store
	// that in entry.
	newg.entryfn = fn
	var entry func(unsafe.Pointer)
	*(*unsafe.Pointer)(unsafe.Pointer(&entry)) = unsafe.Pointer(&newg.entryfn)
	newg.entry = entry

	newg.param = arg
	newg.gopc = getcallerpc()
	newg.ancestors = saveAncestors(_g_)
	newg.startpc = fn
	if _g_.m.curg != nil {
		newg.labels = _g_.m.curg.labels
	}
	if isSystemGoroutine(newg, false) {
		atomic.Xadd(&sched.ngsys, +1)
	}
	casgstatus(newg, _Gdead, _Grunnable)

	if _p_.goidcache == _p_.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.
		_p_.goidcache = atomic.Xadd64(&sched.goidgen, _GoidCacheBatch)
		_p_.goidcache -= _GoidCacheBatch - 1
		_p_.goidcacheend = _p_.goidcache + _GoidCacheBatch
	}
	newg.goid = int64(_p_.goidcache)
	_p_.goidcache++
	if trace.enabled {
		traceGoCreate(newg, newg.startpc)
	}

	makeGContext(newg, sp, spsize)

	runqput(_p_, newg, true)

	if atomic.Load(&sched.npidle) != 0 && atomic.Load(&sched.nmspinning) == 0 && mainStarted {
		wakep()
	}
	releasem(_g_.m)
	return newg
}

// expectedSystemGoroutines counts the number of goroutines expected
// to mark themselves as system goroutines. After they mark themselves
// by calling setSystemGoroutine, this is decremented. NumGoroutines
// uses this to wait for all system goroutines to mark themselves
// before it counts them.
var expectedSystemGoroutines uint32

// expectSystemGoroutine is called when starting a goroutine that will
// call setSystemGoroutine. It increments expectedSystemGoroutines.
func expectSystemGoroutine() {
	atomic.Xadd(&expectedSystemGoroutines, +1)
}

// waitForSystemGoroutines waits for all currently expected system
// goroutines to register themselves.
func waitForSystemGoroutines() {
	for atomic.Load(&expectedSystemGoroutines) > 0 {
		Gosched()
		osyield()
	}
}

// setSystemGoroutine marks this goroutine as a "system goroutine".
// In the gc toolchain this is done by comparing startpc to a list of
// saved special PCs. In gccgo that approach does not work as startpc
// is often a thunk that invokes the real function with arguments,
// so the thunk address never matches the saved special PCs. Instead,
// since there are only a limited number of "system goroutines",
// we force each one to mark itself as special.
func setSystemGoroutine() {
	getg().isSystemGoroutine = true
	atomic.Xadd(&sched.ngsys, +1)
	atomic.Xadd(&expectedSystemGoroutines, -1)
}

// saveAncestors copies previous ancestors of the given caller g and
// includes infor 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 [_TracebackMaxFrames]uintptr
	// FIXME: This should get a traceback of callergp.
	// npcs := gcallers(callergp, 0, pcs[:])
	npcs := 0
	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(_p_ *p, gp *g) {
	if readgstatus(gp) != _Gdead {
		throw("gfput: bad status (not Gdead)")
	}

	_p_.gFree.push(gp)
	_p_.gFree.n++
	if _p_.gFree.n >= 64 {
		lock(&sched.gFree.lock)
		for _p_.gFree.n >= 32 {
			_p_.gFree.n--
			gp = _p_.gFree.pop()
			sched.gFree.list.push(gp)
			sched.gFree.n++
		}
		unlock(&sched.gFree.lock)
	}
}

// Get from gfree list.
// If local list is empty, grab a batch from global list.
func gfget(_p_ *p) *g {
retry:
	if _p_.gFree.empty() && !sched.gFree.list.empty() {
		lock(&sched.gFree.lock)
		// Move a batch of free Gs to the P.
		for _p_.gFree.n < 32 {
			gp := sched.gFree.list.pop()
			if gp == nil {
				break
			}
			sched.gFree.n--
			_p_.gFree.push(gp)
			_p_.gFree.n++
		}
		unlock(&sched.gFree.lock)
		goto retry
	}
	gp := _p_.gFree.pop()
	if gp == nil {
		return nil
	}
	_p_.gFree.n--
	return gp
}

// Purge all cached G's from gfree list to the global list.
func gfpurge(_p_ *p) {
	lock(&sched.gFree.lock)
	for !_p_.gFree.empty() {
		gp := _p_.gFree.pop()
		_p_.gFree.n--
		sched.gFree.list.push(gp)
		sched.gFree.n++
	}
	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
	}
	_g_ := getg()
	_g_.m.lockedg.set(_g_)
	_g_.lockedm.set(_g_.m)
}

//go:nosplit

// 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.
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()
	}
	_g_ := getg()
	_g_.m.lockedExt++
	if _g_.m.lockedExt == 0 {
		_g_.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
	}
	_g_ := getg()
	if _g_.m.lockedInt != 0 || _g_.m.lockedExt != 0 {
		return
	}
	_g_.m.lockedg = 0
	_g_.lockedm = 0
}

//go:nosplit

// 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.
func UnlockOSThread() {
	_g_ := getg()
	if _g_.m.lockedExt == 0 {
		return
	}
	_g_.m.lockedExt--
	dounlockOSThread()
}

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

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

func gcount() int32 {
	n := int32(allglen) - sched.gFree.n - int32(atomic.Load(&sched.ngsys))
	for _, _p_ := range allp {
		n -= _p_.gFree.n
	}

	// 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
}

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

var prof struct {
	signalLock uint32
	hz         int32
}

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

var _SystemPC = funcPC(_System)
var _ExternalCodePC = funcPC(_ExternalCode)
var _LostExternalCodePC = funcPC(_LostExternalCode)
var _GCPC = funcPC(_GC)
var _LostSIGPROFDuringAtomic64PC = funcPC(_LostSIGPROFDuringAtomic64)

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

	traceback := true

	// If SIGPROF arrived while already fetching runtime callers
	// we can have trouble on older systems because the unwind
	// library calls dl_iterate_phdr which was not reentrant in
	// the past. alreadyInCallers checks for that.
	if gp == nil || alreadyInCallers() {
		traceback = false
	}

	var stk [maxCPUProfStack]uintptr
	n := 0
	if traceback {
		var stklocs [maxCPUProfStack]location
		n = callers(0, stklocs[:])

		// Issue 26595: the stack trace we've just collected is going
		// to include frames that we don't want to report in the CPU
		// profile, including signal handler frames. Here is what we
		// might typically see at the point of "callers" above for a
		// signal delivered to the application routine "interesting"
		// called by "main".
		//
		//  0: runtime.sigprof
		//  1: runtime.sighandler
		//  2: runtime.sigtrampgo
		//  3: runtime.sigtramp
		//  4: <signal handler called>
		//  5: main.interesting_routine
		//  6: main.main
		//
		// To ensure a sane profile, walk through the frames in
		// "stklocs" until we find the "runtime.sigtramp" frame, then
		// report only those frames below the frame one down from
		// that. On systems that don't split stack, "sigtramp" can
		// do a sibling call to "sigtrampgo", so use "sigtrampgo"
		// if we don't find "sigtramp". If for some reason
		// neither "runtime.sigtramp" nor "runtime.sigtrampgo" is
		// present, don't make any changes.
		framesToDiscard := 0
		for i := 0; i < n; i++ {
			if stklocs[i].function == "runtime.sigtrampgo" && i+2 < n {
				framesToDiscard = i + 2
			}
			if stklocs[i].function == "runtime.sigtramp" && i+2 < n {
				framesToDiscard = i + 2
				break
			}
		}
		n -= framesToDiscard
		for i := 0; i < n; i++ {
			stk[i] = stklocs[i+framesToDiscard].pc
		}
	}

	if n <= 0 {
		// Normal traceback is impossible or has failed.
		// Account it against abstract "System" or "GC".
		n = 2
		stk[0] = pc
		if mp.preemptoff != "" {
			stk[1] = _GCPC + sys.PCQuantum
		} else {
			stk[1] = _SystemPC + sys.PCQuantum
		}
	}

	if prof.hz != 0 {
		cpuprof.add(gp, stk[:n])
	}
	getg().m.mallocing--
}

// Use global arrays rather than using up lots of stack space in the
// signal handler. This is safe since while we are executing a SIGPROF
// signal other SIGPROF signals are blocked.
var nonprofGoStklocs [maxCPUProfStack]location
var nonprofGoStk [maxCPUProfStack]uintptr

// sigprofNonGo is called if we receive a SIGPROF signal on a non-Go thread,
// and the signal handler collected a stack trace in sigprofCallers.
// When this is called, sigprofCallersUse will be non-zero.
// g is nil, and what we can do is very limited.
//go:nosplit
//go:nowritebarrierrec
func sigprofNonGo(pc uintptr) {
	if prof.hz != 0 {
		n := callers(0, nonprofGoStklocs[:])

		for i := 0; i < n; i++ {
			nonprofGoStk[i] = nonprofGoStklocs[i].pc
		}

		if n <= 0 {
			n = 2
			nonprofGoStk[0] = pc
			nonprofGoStk[1] = _ExternalCodePC + sys.PCQuantum
		}

		cpuprof.addNonGo(nonprofGoStk[:n])
	}
}

// sigprofNonGoPC is called when a profiling signal arrived on a
// non-Go thread and we have a single PC value, not a stack trace.
// g is nil, and what we can do is very limited.
//go:nosplit
//go:nowritebarrierrec
func sigprofNonGoPC(pc uintptr) {
	if prof.hz != 0 {
		stk := []uintptr{
			pc,
			_ExternalCodePC + sys.PCQuantum,
		}
		cpuprof.addNonGo(stk)
	}
}

// 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.
	_g_ := getg()
	_g_.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 !atomic.Cas(&prof.signalLock, 0, 1) {
		osyield()
	}
	if prof.hz != hz {
		setProcessCPUProfiler(hz)
		prof.hz = hz
	}
	atomic.Store(&prof.signalLock, 0)

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

	if hz != 0 {
		setThreadCPUProfiler(hz)
	}

	_g_.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.status = _Pgcstop
	pp.sudogcache = pp.sudogbuf[:0]
	pp.deferpool = pp.deferpoolbuf[:0]
	pp.wbBuf.reset()
	if pp.mcache == nil {
		if id == 0 {
			if getg().m.mcache == nil {
				throw("missing mcache?")
			}
			pp.mcache = getg().m.mcache // bootstrap
		} else {
			pp.mcache = allocmcache()
		}
	}
	if raceenabled && pp.raceprocctx == 0 {
		if id == 0 {
			pp.raceprocctx = raceprocctx0
			raceprocctx0 = 0 // bootstrap
		} else {
			pp.raceprocctx = raceproccreate()
		}
	}
}

// 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() {
	// 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
	}
	if len(pp.timers) > 0 {
		plocal := getg().m.p.ptr()
		// The world is stopped, but we acquire timersLock to
		// protect against sysmon calling timeSleepUntil.
		// This is the only case where we hold the timersLock of
		// more than one P, so there are no deadlock concerns.
		lock(&plocal.timersLock)
		lock(&pp.timersLock)
		moveTimers(plocal, pp.timers)
		pp.timers = nil
		pp.numTimers = 0
		pp.adjustTimers = 0
		pp.deletedTimers = 0
		atomic.Store64(&pp.timer0When, 0)
		unlock(&pp.timersLock)
		unlock(&plocal.timersLock)
	}
	// If there's a background worker, make it runnable and put
	// it on the global queue so it can clean itself up.
	if gp := pp.gcBgMarkWorker.ptr(); gp != nil {
		casgstatus(gp, _Gwaiting, _Grunnable)
		if trace.enabled {
			traceGoUnpark(gp, 0)
		}
		globrunqput(gp)
		// This assignment doesn't race because the
		// world is stopped.
		pp.gcBgMarkWorker.set(nil)
	}
	// Flush p's write barrier buffer.
	if gcphase != _GCoff {
		wbBufFlush1(pp)
		pp.gcw.dispose()
	}
	for i := range pp.sudogbuf {
		pp.sudogbuf[i] = nil
	}
	pp.sudogcache = pp.sudogbuf[:0]
	for i := range pp.deferpoolbuf {
		pp.deferpoolbuf[i] = nil
	}
	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
		pp.pcache.flush(&mheap_.pages)
	})
	freemcache(pp.mcache)
	pp.mcache = nil
	gfpurge(pp)
	traceProcFree(pp)
	pp.gcAssistTime = 0
	pp.status = _Pdead
}

// Change number of processors. The world is stopped, sched is locked.
// gcworkbufs are not being modified by either the GC or
// the write barrier code.
// Returns list of Ps with local work, they need to be scheduled by the caller.
func procresize(nprocs int32) *p {
	old := gomaxprocs
	if old < 0 || nprocs <= 0 {
		throw("procresize: invalid arg")
	}
	if trace.enabled {
		traceGomaxprocs(nprocs)
	}

	// 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
		}
		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))
	}

	_g_ := getg()
	if _g_.m.p != 0 && _g_.m.p.ptr().id < nprocs {
		// continue to use the current P
		_g_.m.p.ptr().status = _Prunning
		_g_.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 _g_.m.p != 0 {
			if trace.enabled {
				// Pretend that we were descheduled
				// and then scheduled again to keep
				// the trace sane.
				traceGoSched()
				traceProcStop(_g_.m.p.ptr())
			}
			_g_.m.p.ptr().m = 0
		}
		_g_.m.p = 0
		_g_.m.mcache = nil
		p := allp[0]
		p.m = 0
		p.status = _Pidle
		acquirep(p)
		if trace.enabled {
			traceGoStart()
		}
	}

	// release resources from unused P's
	for i := nprocs; i < old; i++ {
		p := allp[i]
		p.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]
		unlock(&allpLock)
	}

	var runnablePs *p
	for i := nprocs - 1; i >= 0; i-- {
		p := allp[i]
		if _g_.m.p.ptr() == p {
			continue
		}
		p.status = _Pidle
		if runqempty(p) {
			pidleput(p)
		} else {
			p.m.set(mget())
			p.link.set(runnablePs)
			runnablePs = p
		}
	}
	stealOrder.reset(uint32(nprocs))
	var int32p *int32 = &gomaxprocs // make compiler check that gomaxprocs is an int32
	atomic.Store((*uint32)(unsafe.Pointer(int32p)), uint32(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 _p_.
//
//go:yeswritebarrierrec
func acquirep(_p_ *p) {
	// Do the part that isn't allowed to have write barriers.
	wirep(_p_)

	// Have p; write barriers now allowed.

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

	if trace.enabled {
		traceProcStart()
	}
}

// wirep is the first step of acquirep, which actually associates the
// current M to _p_. 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(_p_ *p) {
	_g_ := getg()

	if _g_.m.p != 0 || _g_.m.mcache != nil {
		throw("wirep: already in go")
	}
	if _p_.m != 0 || _p_.status != _Pidle {
		id := int64(0)
		if _p_.m != 0 {
			id = _p_.m.ptr().id
		}
		print("wirep: p->m=", _p_.m, "(", id, ") p->status=", _p_.status, "\n")
		throw("wirep: invalid p state")
	}
	_g_.m.mcache = _p_.mcache
	_g_.m.p.set(_p_)
	_p_.m.set(_g_.m)
	_p_.status = _Prunning
}

// Disassociate p and the current m.
func releasep() *p {
	_g_ := getg()

	if _g_.m.p == 0 || _g_.m.mcache == nil {
		throw("releasep: invalid arg")
	}
	_p_ := _g_.m.p.ptr()
	if _p_.m.ptr() != _g_.m || _p_.mcache != _g_.m.mcache || _p_.status != _Prunning {
		print("releasep: m=", _g_.m, " m->p=", _g_.m.p.ptr(), " p->m=", hex(_p_.m), " m->mcache=", _g_.m.mcache, " p->mcache=", _p_.mcache, " p->status=", _p_.status, "\n")
		throw("releasep: invalid p state")
	}
	if trace.enabled {
		traceProcStop(_g_.m.p.ptr())
	}
	_g_.m.p = 0
	_g_.m.mcache = nil
	_p_.m = 0
	_p_.status = _Pidle
	return _p_
}

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() {
	// 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.
	if islibrary || isarchive {
		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 > 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 {
		mp := lockextra(true)
		haveExtraM := extraMCount > 0
		unlockextra(mp)
		if haveExtraM {
			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")
		throw("checkdead: inconsistent counts")
	}

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

	// Maybe jump time forward for playground.
	if faketime != 0 {
		when, _p_ := timeSleepUntil()
		if _p_ != nil {
			faketime = when
			for pp := &sched.pidle; *pp != 0; pp = &(*pp).ptr().link {
				if (*pp).ptr() == _p_ {
					*pp = _p_.link
					break
				}
			}
			mp := mget()
			if mp == nil {
				// There should always be a free M since
				// nothing is running.
				throw("checkdead: no m for timer")
			}
			mp.nextp.set(_p_)
			notewakeup(&mp.park)
			return
		}
	}

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

	getg().m.throwing = -1 // do not dump full stacks
	unlock(&sched.lock)    // unlock so that GODEBUG=scheddetail=1 doesn't hang
	throw("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

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

	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)
		now := nanotime()
		next, _ := timeSleepUntil()
		if debug.schedtrace <= 0 && (sched.gcwaiting != 0 || atomic.Load(&sched.npidle) == uint32(gomaxprocs)) {
			lock(&sched.lock)
			if atomic.Load(&sched.gcwaiting) != 0 || atomic.Load(&sched.npidle) == uint32(gomaxprocs) {
				if next > now {
					atomic.Store(&sched.sysmonwait, 1)
					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)
					}
					notetsleep(&sched.sysmonnote, sleep)
					if shouldRelax {
						osRelax(false)
					}
					now = nanotime()
					next, _ = timeSleepUntil()
					lock(&sched.lock)
					atomic.Store(&sched.sysmonwait, 0)
					noteclear(&sched.sysmonnote)
				}
				idle = 0
				delay = 20
			}
			unlock(&sched.lock)
		}
		// trigger libc interceptors if needed
		if *cgo_yield != nil {
			asmcgocall(*cgo_yield, nil)
		}
		// poll network if not polled for more than 10ms
		lastpoll := int64(atomic.Load64(&sched.lastpoll))
		if netpollinited() && lastpoll != 0 && lastpoll+10*1000*1000 < now {
			atomic.Cas64(&sched.lastpoll, uint64(lastpoll), uint64(now))
			list := 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)
			}
		}
		if next < now {
			// There are timers that should have already run,
			// perhaps because there is an unpreemptible P.
			// Try to start an M to run them.
			startm(nil, false)
		}
		// 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() && atomic.Load(&forcegc.idle) != 0 {
			lock(&forcegc.lock)
			forcegc.idle = 0
			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)
		}
	}
}

type sysmontick struct {
	schedtick   uint32
	schedwhen   int64
	syscalltick uint32
	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++ {
		_p_ := allp[i]
		if _p_ == nil {
			// This can happen if procresize has grown
			// allp but not yet created new Ps.
			continue
		}
		pd := &_p_.sysmontick
		s := _p_.status
		sysretake := false
		if s == _Prunning || s == _Psyscall {
			// Preempt G if it's running for too long.
			t := int64(_p_.schedtick)
			if int64(pd.schedtick) != t {
				pd.schedtick = uint32(t)
				pd.schedwhen = now
			} else if pd.schedwhen+forcePreemptNS <= now {
				preemptone(_p_)
				// In case of syscall, preemptone() doesn't
				// work, because there is no M wired to P.
				sysretake = true
			}
		}
		if s == _Psyscall {
			// Retake P from syscall if it's there for more than 1 sysmon tick (at least 20us).
			t := int64(_p_.syscalltick)
			if !sysretake && int64(pd.syscalltick) != t {
				pd.syscalltick = uint32(t)
				pd.syscallwhen = now
				continue
			}
			// 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(_p_) && atomic.Load(&sched.nmspinning)+atomic.Load(&sched.npidle) > 0 && pd.syscallwhen+10*1000*1000 > now {
				continue
			}
			// 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 the CAS.
			// Otherwise the M from which we retake can exit the syscall,
			// increment nmidle and report deadlock.
			incidlelocked(-1)
			if atomic.Cas(&_p_.status, s, _Pidle) {
				if trace.enabled {
					traceGoSysBlock(_p_)
					traceProcStop(_p_)
				}
				n++
				_p_.syscalltick++
				handoffp(_p_)
			}
			incidlelocked(1)
			lock(&allpLock)
		}
	}
	unlock(&allpLock)
	return uint32(n)
}

// 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 _, _p_ := range allp {
		if _p_.status != _Prunning {
			continue
		}
		if preemptone(_p_) {
			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 send 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(_p_ *p) bool {
	mp := _p_.m.ptr()
	if mp == nil || mp == getg().m {
		return false
	}
	gp := mp.curg
	if gp == nil || gp == mp.g0 {
		return false
	}

	gp.preempt = true

	// At this point the gc implementation sets gp.stackguard0 to
	// a value that causes the goroutine to suspend itself.
	// gccgo has no support for this, and it's hard to support.
	// The split stack code reads a value from its TCB.
	// We have no way to set a value in the TCB of a different thread.
	// And, of course, not all systems support split stack anyhow.
	// Checking the field in the g is expensive, since it requires
	// loading the g from TLS.  The best mechanism is likely to be
	// setting a global variable and figuring out a way to efficiently
	// check that global variable.
	//
	// For now we check gp.preempt in schedule, mallocgc, selectgo,
	// and a few other places, which is at least better than doing
	// nothing at all.

	// Request an async preemption of this P.
	if preemptMSupported && debug.asyncpreemptoff == 0 {
		_p_.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, " threads=", mcount(), " spinningthreads=", sched.nmspinning, " idlethreads=", sched.nmidle, " runqueue=", sched.runqsize)
	if detailed {
		print(" gcwaiting=", sched.gcwaiting, " nmidlelocked=", sched.nmidlelocked, " stopwait=", sched.stopwait, " sysmonwait=", sched.sysmonwait, "\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, _p_ := range allp {
		mp := _p_.m.ptr()
		h := atomic.Load(&_p_.runqhead)
		t := atomic.Load(&_p_.runqtail)
		if detailed {
			id := int64(-1)
			if mp != nil {
				id = mp.id
			}
			print("  P", i, ": status=", _p_.status, " schedtick=", _p_.schedtick, " syscalltick=", _p_.syscalltick, " m=", id, " runqsize=", t-h, " gfreecnt=", _p_.gFree.n, " timerslen=", len(_p_.timers), "\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("]\n")
			}
		}
	}

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

	for mp := allm; mp != nil; mp = mp.alllink {
		_p_ := mp.p.ptr()
		gp := mp.curg
		lockedg := mp.lockedg.ptr()
		id1 := int32(-1)
		if _p_ != nil {
			id1 = _p_.id
		}
		id2 := int64(-1)
		if gp != nil {
			id2 = gp.goid
		}
		id3 := int64(-1)
		if lockedg != nil {
			id3 = lockedg.goid
		}
		print("  M", mp.id, ": p=", id1, " curg=", id2, " mallocing=", mp.mallocing, " throwing=", mp.throwing, " preemptoff=", mp.preemptoff, ""+" locks=", mp.locks, " dying=", mp.dying, " spinning=", mp.spinning, " blocked=", mp.blocked, " lockedg=", id3, "\n")
	}

	lock(&allglock)
	for gi := 0; gi < len(allgs); gi++ {
		gp := allgs[gi]
		mp := gp.m
		lockedm := gp.lockedm.ptr()
		id1 := int64(-1)
		if mp != nil {
			id1 = mp.id
		}
		id2 := int64(-1)
		if lockedm != nil {
			id2 = lockedm.id
		}
		print("  G", gp.goid, ": status=", readgstatus(gp), "(", gp.waitreason.String(), ") m=", id1, " lockedm=", id2, "\n")
	}
	unlock(&allglock)
	unlock(&sched.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.n
		sched.disable.n = 0
		globrunqputbatch(&sched.disable.runnable, n)
		unlock(&sched.lock)
		for ; n != 0 && sched.npidle != 0; n-- {
			startm(nil, false)
		}
	} else {
		unlock(&sched.lock)
	}
}

// schedEnabled reports whether gp should be scheduled. It returns
// false is scheduling of gp is disabled.
func schedEnabled(gp *g) bool {
	if sched.disable.user {
		return isSystemGoroutine(gp, true)
	}
	return true
}

// Put mp on midle list.
// Sched must be locked.
// May run during STW, so write barriers are not allowed.
//go:nowritebarrierrec
func mput(mp *m) {
	mp.schedlink = sched.midle
	sched.midle.set(mp)
	sched.nmidle++
	checkdead()
}

// Try to get an m from midle list.
// Sched must be locked.
// May run during STW, so write barriers are not allowed.
//go:nowritebarrierrec
func mget() *m {
	mp := sched.midle.ptr()
	if mp != nil {
		sched.midle = mp.schedlink
		sched.nmidle--
	}
	return mp
}

// Put gp on the global runnable queue.
// Sched must be locked.
// May run during STW, so write barriers are not allowed.
//go:nowritebarrierrec
func globrunqput(gp *g) {
	sched.runq.pushBack(gp)
	sched.runqsize++
}

// Put gp at the head of the global runnable queue.
// Sched must be locked.
// May run during STW, so write barriers are not allowed.
//go:nowritebarrierrec
func globrunqputhead(gp *g) {
	sched.runq.push(gp)
	sched.runqsize++
}

// Put a batch of runnable goroutines on the global runnable queue.
// This clears *batch.
// Sched must be locked.
func globrunqputbatch(batch *gQueue, n int32) {
	sched.runq.pushBackAll(*batch)
	sched.runqsize += n
	*batch = gQueue{}
}

// Try get a batch of G's from the global runnable queue.
// Sched must be locked.
func globrunqget(_p_ *p, max int32) *g {
	if sched.runqsize == 0 {
		return nil
	}

	n := sched.runqsize/gomaxprocs + 1
	if n > sched.runqsize {
		n = sched.runqsize
	}
	if max > 0 && n > max {
		n = max
	}
	if n > int32(len(_p_.runq))/2 {
		n = int32(len(_p_.runq)) / 2
	}

	sched.runqsize -= n

	gp := sched.runq.pop()
	n--
	for ; n > 0; n-- {
		gp1 := sched.runq.pop()
		runqput(_p_, gp1, false)
	}
	return gp
}

// Put p to on _Pidle list.
// Sched must be locked.
// May run during STW, so write barriers are not allowed.
//go:nowritebarrierrec
func pidleput(_p_ *p) {
	if !runqempty(_p_) {
		throw("pidleput: P has non-empty run queue")
	}
	_p_.link = sched.pidle
	sched.pidle.set(_p_)
	atomic.Xadd(&sched.npidle, 1) // TODO: fast atomic
}

// Try get a p from _Pidle list.
// Sched must be locked.
// May run during STW, so write barriers are not allowed.
//go:nowritebarrierrec
func pidleget() *p {
	_p_ := sched.pidle.ptr()
	if _p_ != nil {
		sched.pidle = _p_.link
		atomic.Xadd(&sched.npidle, -1) // TODO: fast atomic
	}
	return _p_
}

// runqempty reports whether _p_ has no Gs on its local run queue.
// It never returns true spuriously.
func runqempty(_p_ *p) bool {
	// Defend against a race where 1) _p_ has G1 in runqnext but runqhead == runqtail,
	// 2) runqput on _p_ kicks G1 to the runq, 3) runqget on _p_ empties runqnext.
	// Simply observing that runqhead == runqtail and then observing that runqnext == nil
	// does not mean the queue is empty.
	for {
		head := atomic.Load(&_p_.runqhead)
		tail := atomic.Load(&_p_.runqtail)
		runnext := atomic.Loaduintptr((*uintptr)(unsafe.Pointer(&_p_.runnext)))
		if tail == atomic.Load(&_p_.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 _p_.runnext slot.
// If the run queue is full, runnext puts g on the global queue.
// Executed only by the owner P.
func runqput(_p_ *p, gp *g, next bool) {
	if randomizeScheduler && next && fastrand()%2 == 0 {
		next = false
	}

	if next {
	retryNext:
		oldnext := _p_.runnext
		if !_p_.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(&_p_.runqhead) // load-acquire, synchronize with consumers
	t := _p_.runqtail
	if t-h < uint32(len(_p_.runq)) {
		_p_.runq[t%uint32(len(_p_.runq))].set(gp)
		atomic.StoreRel(&_p_.runqtail, t+1) // store-release, makes the item available for consumption
		return
	}
	if runqputslow(_p_, 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(_p_ *p, gp *g, h, t uint32) bool {
	var batch [len(_p_.runq)/2 + 1]*g

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

	if randomizeScheduler {
		for i := uint32(1); i <= n; i++ {
			j := fastrandn(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])
	}
	var q gQueue
	q.head.set(batch[0])
	q.tail.set(batch[n])

	// Now put the batch on global queue.
	lock(&sched.lock)
	globrunqputbatch(&q, int32(n+1))
	unlock(&sched.lock)
	return true
}

// 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(_p_ *p) (gp *g, inheritTime bool) {
	// If there's a runnext, it's the next G to run.
	for {
		next := _p_.runnext
		if next == 0 {
			break
		}
		if _p_.runnext.cas(next, 0) {
			return next.ptr(), true
		}
	}

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

// Grabs a batch of goroutines from _p_'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(_p_ *p, batch *[256]guintptr, batchHead uint32, stealRunNextG bool) uint32 {
	for {
		h := atomic.LoadAcq(&_p_.runqhead) // load-acquire, synchronize with other consumers
		t := atomic.LoadAcq(&_p_.runqtail) // load-acquire, synchronize with the producer
		n := t - h
		n = n - n/2
		if n == 0 {
			if stealRunNextG {
				// Try to steal from _p_.runnext.
				if next := _p_.runnext; next != 0 {
					if _p_.status == _Prunning {
						// Sleep to ensure that _p_ isn't about to run the g
						// we are about to steal.
						// The important use case here is when the g running
						// on _p_ ready()s another g and then almost
						// immediately blocks. Instead of stealing runnext
						// in this window, back off to give _p_ 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 GOOS != "windows" {
							usleep(3)
						} else {
							// On windows system timer granularity is
							// 1-15ms, which is way too much for this
							// optimization. So just yield.
							osyield()
						}
					}
					if !_p_.runnext.cas(next, 0) {
						continue
					}
					batch[batchHead%uint32(len(batch))] = next
					return 1
				}
			}
			return 0
		}
		if n > uint32(len(_p_.runq)/2) { // read inconsistent h and t
			continue
		}
		for i := uint32(0); i < n; i++ {
			g := _p_.runq[(h+i)%uint32(len(_p_.runq))]
			batch[(batchHead+i)%uint32(len(batch))] = g
		}
		if atomic.CasRel(&_p_.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(_p_, p2 *p, stealRunNextG bool) *g {
	t := _p_.runqtail
	n := runqgrab(p2, &_p_.runq, t, stealRunNextG)
	if n == 0 {
		return nil
	}
	n--
	gp := _p_.runq[(t+n)%uint32(len(_p_.runq))].ptr()
	if n == 0 {
		return gp
	}
	h := atomic.LoadAcq(&_p_.runqhead) // load-acquire, synchronize with consumers
	if t-h+n >= uint32(len(_p_.runq)) {
		throw("runqsteal: runq overflow")
	}
	atomic.StoreRel(&_p_.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
}

// 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)
	}
}

// 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)
}

// pushBackAll adds all Gs in l2 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
}

// 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
		}
	}
	return gp
}

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

// 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)
}

// pushAll prepends all Gs in q to l.
func (l *gList) pushAll(q gQueue) {
	if !q.empty() {
		q.tail.ptr().schedlink = l.head
		l.head = q.head
	}
}

// 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
	}
	return gp
}

//go:linkname setMaxThreads runtime..z2fdebug.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
}

func haveexperiment(name string) bool {
	// The gofrontend does not support experiments.
	return false
}

//go:nosplit
func procPin() int {
	_g_ := getg()
	mp := _g_.m

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

//go:nosplit
func procUnpin() {
	_g_ := getg()
	_g_.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..z2fatomic.runtime_procPin
//go:nosplit
func sync_atomic_runtime_procPin() int {
	return procPin()
}

//go:linkname sync_atomic_runtime_procUnpin sync..z2fatomic.runtime_procUnpin
//go:nosplit
func sync_atomic_runtime_procUnpin() {
	procUnpin()
}

// Active spinning for sync.Mutex.
//go:linkname sync_runtime_canSpin sync.runtime_canSpin
//go:nosplit
func 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 || ncpu <= 1 || gomaxprocs <= int32(sched.npidle+sched.nmspinning)+1 {
		return false
	}
	if p := getg().m.p.ptr(); !runqempty(p) {
		return false
	}
	return true
}

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

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%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
}