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// Copyright 2009 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.
// Garbage collector: marking and scanning
package runtime
import (
"runtime/internal/atomic"
"runtime/internal/sys"
"unsafe"
)
const (
fixedRootFinalizers = iota
fixedRootFreeGStacks
fixedRootCount
// rootBlockBytes is the number of bytes to scan per data or
// BSS root.
rootBlockBytes = 256 << 10
// rootBlockSpans is the number of spans to scan per span
// root.
rootBlockSpans = 8 * 1024 // 64MB worth of spans
// maxObletBytes is the maximum bytes of an object to scan at
// once. Larger objects will be split up into "oblets" of at
// most this size. Since we can scan 1–2 MB/ms, 128 KB bounds
// scan preemption at ~100 µs.
//
// This must be > _MaxSmallSize so that the object base is the
// span base.
maxObletBytes = 128 << 10
// idleCheckThreshold specifies how many units of work to do
// between run queue checks in an idle worker. Assuming a scan
// rate of 1 MB/ms, this is ~100 µs. Lower values have higher
// overhead in the scan loop (the scheduler check may perform
// a syscall, so its overhead is nontrivial). Higher values
// make the system less responsive to incoming work.
idleCheckThreshold = 100000
)
// gcMarkRootPrepare queues root scanning jobs (stacks, globals, and
// some miscellany) and initializes scanning-related state.
//
// The caller must have call gcCopySpans().
//
// The world must be stopped.
//
//go:nowritebarrier
func gcMarkRootPrepare() {
if gcphase == _GCmarktermination {
work.nFlushCacheRoots = int(gomaxprocs)
} else {
work.nFlushCacheRoots = 0
}
work.nDataRoots = 0
// Only scan globals once per cycle; preferably concurrently.
if !work.markrootDone {
roots := gcRoots
for roots != nil {
work.nDataRoots++
roots = roots.next
}
}
if !work.markrootDone {
// On the first markroot, we need to scan span roots.
// In concurrent GC, this happens during concurrent
// mark and we depend on addfinalizer to ensure the
// above invariants for objects that get finalizers
// after concurrent mark. In STW GC, this will happen
// during mark termination.
//
// We're only interested in scanning the in-use spans,
// which will all be swept at this point. More spans
// may be added to this list during concurrent GC, but
// we only care about spans that were allocated before
// this mark phase.
work.nSpanRoots = mheap_.sweepSpans[mheap_.sweepgen/2%2].numBlocks()
// On the first markroot, we need to scan all Gs. Gs
// may be created after this point, but it's okay that
// we ignore them because they begin life without any
// roots, so there's nothing to scan, and any roots
// they create during the concurrent phase will be
// scanned during mark termination. During mark
// termination, allglen isn't changing, so we'll scan
// all Gs.
work.nStackRoots = int(atomic.Loaduintptr(&allglen))
work.nRescanRoots = 0
} else {
// We've already scanned span roots and kept the scan
// up-to-date during concurrent mark.
work.nSpanRoots = 0
// On the second pass of markroot, we're just scanning
// dirty stacks. It's safe to access rescan since the
// world is stopped.
work.nStackRoots = 0
work.nRescanRoots = len(work.rescan.list)
}
work.markrootNext = 0
work.markrootJobs = uint32(fixedRootCount + work.nFlushCacheRoots + work.nDataRoots + work.nSpanRoots + work.nStackRoots + work.nRescanRoots)
}
// gcMarkRootCheck checks that all roots have been scanned. It is
// purely for debugging.
func gcMarkRootCheck() {
if work.markrootNext < work.markrootJobs {
print(work.markrootNext, " of ", work.markrootJobs, " markroot jobs done\n")
throw("left over markroot jobs")
}
lock(&allglock)
// Check that stacks have been scanned.
var gp *g
if gcphase == _GCmarktermination && debug.gcrescanstacks > 0 {
for i := 0; i < len(allgs); i++ {
gp = allgs[i]
if !(gp.gcscandone && gp.gcscanvalid) && readgstatus(gp) != _Gdead {
goto fail
}
}
} else {
for i := 0; i < work.nStackRoots; i++ {
gp = allgs[i]
if !gp.gcscandone {
goto fail
}
}
}
unlock(&allglock)
return
fail:
println("gp", gp, "goid", gp.goid,
"status", readgstatus(gp),
"gcscandone", gp.gcscandone,
"gcscanvalid", gp.gcscanvalid)
unlock(&allglock) // Avoid self-deadlock with traceback.
throw("scan missed a g")
}
// ptrmask for an allocation containing a single pointer.
var oneptrmask = [...]uint8{1}
// markroot scans the i'th root.
//
// Preemption must be disabled (because this uses a gcWork).
//
// nowritebarrier is only advisory here.
//
//go:nowritebarrier
func markroot(gcw *gcWork, i uint32) {
// TODO(austin): This is a bit ridiculous. Compute and store
// the bases in gcMarkRootPrepare instead of the counts.
baseFlushCache := uint32(fixedRootCount)
baseData := baseFlushCache + uint32(work.nFlushCacheRoots)
baseSpans := baseData + uint32(work.nDataRoots)
baseStacks := baseSpans + uint32(work.nSpanRoots)
baseRescan := baseStacks + uint32(work.nStackRoots)
end := baseRescan + uint32(work.nRescanRoots)
// Note: if you add a case here, please also update heapdump.go:dumproots.
switch {
case baseFlushCache <= i && i < baseData:
flushmcache(int(i - baseFlushCache))
case baseData <= i && i < baseSpans:
roots := gcRoots
c := baseData
for roots != nil {
if i == c {
markrootBlock(roots, gcw)
break
}
roots = roots.next
c++
}
case i == fixedRootFinalizers:
for fb := allfin; fb != nil; fb = fb.alllink {
cnt := uintptr(atomic.Load(&fb.cnt))
scanblock(uintptr(unsafe.Pointer(&fb.fin[0])), cnt*unsafe.Sizeof(fb.fin[0]), &finptrmask[0], gcw)
}
case i == fixedRootFreeGStacks:
// FIXME: We don't do this for gccgo.
case baseSpans <= i && i < baseStacks:
// mark MSpan.specials
markrootSpans(gcw, int(i-baseSpans))
default:
// the rest is scanning goroutine stacks
var gp *g
if baseStacks <= i && i < baseRescan {
gp = allgs[i-baseStacks]
} else if baseRescan <= i && i < end {
gp = work.rescan.list[i-baseRescan].ptr()
if gp.gcRescan != int32(i-baseRescan) {
// Looking for issue #17099.
println("runtime: gp", gp, "found at rescan index", i-baseRescan, "but should be at", gp.gcRescan)
throw("bad g rescan index")
}
} else {
throw("markroot: bad index")
}
// remember when we've first observed the G blocked
// needed only to output in traceback
status := readgstatus(gp) // We are not in a scan state
if (status == _Gwaiting || status == _Gsyscall) && gp.waitsince == 0 {
gp.waitsince = work.tstart
}
// scang must be done on the system stack in case
// we're trying to scan our own stack.
systemstack(func() {
// If this is a self-scan, put the user G in
// _Gwaiting to prevent self-deadlock. It may
// already be in _Gwaiting if this is a mark
// worker or we're in mark termination.
userG := getg().m.curg
selfScan := gp == userG && readgstatus(userG) == _Grunning
if selfScan {
casgstatus(userG, _Grunning, _Gwaiting)
userG.waitreason = "garbage collection scan"
}
// TODO: scang blocks until gp's stack has
// been scanned, which may take a while for
// running goroutines. Consider doing this in
// two phases where the first is non-blocking:
// we scan the stacks we can and ask running
// goroutines to scan themselves; and the
// second blocks.
scang(gp, gcw)
if selfScan {
casgstatus(userG, _Gwaiting, _Grunning)
}
})
}
}
// markrootBlock scans one element of the list of GC roots.
//
//go:nowritebarrier
func markrootBlock(roots *gcRootList, gcw *gcWork) {
for i := 0; i < roots.count; i++ {
r := &roots.roots[i]
scanblock(uintptr(r.decl), r.ptrdata, r.gcdata, gcw)
}
}
// markrootSpans marks roots for one shard of work.spans.
//
//go:nowritebarrier
func markrootSpans(gcw *gcWork, shard int) {
// Objects with finalizers have two GC-related invariants:
//
// 1) Everything reachable from the object must be marked.
// This ensures that when we pass the object to its finalizer,
// everything the finalizer can reach will be retained.
//
// 2) Finalizer specials (which are not in the garbage
// collected heap) are roots. In practice, this means the fn
// field must be scanned.
//
// TODO(austin): There are several ideas for making this more
// efficient in issue #11485.
if work.markrootDone {
throw("markrootSpans during second markroot")
}
sg := mheap_.sweepgen
spans := mheap_.sweepSpans[mheap_.sweepgen/2%2].block(shard)
// Note that work.spans may not include spans that were
// allocated between entering the scan phase and now. This is
// okay because any objects with finalizers in those spans
// must have been allocated and given finalizers after we
// entered the scan phase, so addfinalizer will have ensured
// the above invariants for them.
for _, s := range spans {
if s.state != mSpanInUse {
continue
}
if !useCheckmark && s.sweepgen != sg {
// sweepgen was updated (+2) during non-checkmark GC pass
print("sweep ", s.sweepgen, " ", sg, "\n")
throw("gc: unswept span")
}
// Speculatively check if there are any specials
// without acquiring the span lock. This may race with
// adding the first special to a span, but in that
// case addfinalizer will observe that the GC is
// active (which is globally synchronized) and ensure
// the above invariants. We may also ensure the
// invariants, but it's okay to scan an object twice.
if s.specials == nil {
continue
}
// Lock the specials to prevent a special from being
// removed from the list while we're traversing it.
lock(&s.speciallock)
for sp := s.specials; sp != nil; sp = sp.next {
if sp.kind != _KindSpecialFinalizer {
continue
}
// don't mark finalized object, but scan it so we
// retain everything it points to.
spf := (*specialfinalizer)(unsafe.Pointer(sp))
// A finalizer can be set for an inner byte of an object, find object beginning.
p := s.base() + uintptr(spf.special.offset)/s.elemsize*s.elemsize
// Mark everything that can be reached from
// the object (but *not* the object itself or
// we'll never collect it).
scanobject(p, gcw)
// The special itself is a root.
scanblock(uintptr(unsafe.Pointer(&spf.fn)), sys.PtrSize, &oneptrmask[0], gcw)
}
unlock(&s.speciallock)
}
}
// gcAssistAlloc performs GC work to make gp's assist debt positive.
// gp must be the calling user gorountine.
//
// This must be called with preemption enabled.
func gcAssistAlloc(gp *g) {
// Don't assist in non-preemptible contexts. These are
// generally fragile and won't allow the assist to block.
if getg() == gp.m.g0 {
return
}
if mp := getg().m; mp.locks > 0 || mp.preemptoff != "" {
return
}
retry:
// Compute the amount of scan work we need to do to make the
// balance positive. When the required amount of work is low,
// we over-assist to build up credit for future allocations
// and amortize the cost of assisting.
debtBytes := -gp.gcAssistBytes
scanWork := int64(gcController.assistWorkPerByte * float64(debtBytes))
if scanWork < gcOverAssistWork {
scanWork = gcOverAssistWork
debtBytes = int64(gcController.assistBytesPerWork * float64(scanWork))
}
// Steal as much credit as we can from the background GC's
// scan credit. This is racy and may drop the background
// credit below 0 if two mutators steal at the same time. This
// will just cause steals to fail until credit is accumulated
// again, so in the long run it doesn't really matter, but we
// do have to handle the negative credit case.
bgScanCredit := atomic.Loadint64(&gcController.bgScanCredit)
stolen := int64(0)
if bgScanCredit > 0 {
if bgScanCredit < scanWork {
stolen = bgScanCredit
gp.gcAssistBytes += 1 + int64(gcController.assistBytesPerWork*float64(stolen))
} else {
stolen = scanWork
gp.gcAssistBytes += debtBytes
}
atomic.Xaddint64(&gcController.bgScanCredit, -stolen)
scanWork -= stolen
if scanWork == 0 {
// We were able to steal all of the credit we
// needed.
return
}
}
// Perform assist work
systemstack(func() {
gcAssistAlloc1(gp, scanWork)
// The user stack may have moved, so this can't touch
// anything on it until it returns from systemstack.
})
completed := gp.param != nil
gp.param = nil
if completed {
gcMarkDone()
}
if gp.gcAssistBytes < 0 {
// We were unable steal enough credit or perform
// enough work to pay off the assist debt. We need to
// do one of these before letting the mutator allocate
// more to prevent over-allocation.
//
// If this is because we were preempted, reschedule
// and try some more.
if gp.preempt {
Gosched()
goto retry
}
// Add this G to an assist queue and park. When the GC
// has more background credit, it will satisfy queued
// assists before flushing to the global credit pool.
//
// Note that this does *not* get woken up when more
// work is added to the work list. The theory is that
// there wasn't enough work to do anyway, so we might
// as well let background marking take care of the
// work that is available.
if !gcParkAssist() {
goto retry
}
// At this point either background GC has satisfied
// this G's assist debt, or the GC cycle is over.
}
}
// gcAssistAlloc1 is the part of gcAssistAlloc that runs on the system
// stack. This is a separate function to make it easier to see that
// we're not capturing anything from the user stack, since the user
// stack may move while we're in this function.
//
// gcAssistAlloc1 indicates whether this assist completed the mark
// phase by setting gp.param to non-nil. This can't be communicated on
// the stack since it may move.
//
//go:systemstack
func gcAssistAlloc1(gp *g, scanWork int64) {
// Clear the flag indicating that this assist completed the
// mark phase.
gp.param = nil
if atomic.Load(&gcBlackenEnabled) == 0 {
// The gcBlackenEnabled check in malloc races with the
// store that clears it but an atomic check in every malloc
// would be a performance hit.
// Instead we recheck it here on the non-preemptable system
// stack to determine if we should preform an assist.
// GC is done, so ignore any remaining debt.
gp.gcAssistBytes = 0
return
}
// Track time spent in this assist. Since we're on the
// system stack, this is non-preemptible, so we can
// just measure start and end time.
startTime := nanotime()
decnwait := atomic.Xadd(&work.nwait, -1)
if decnwait == work.nproc {
println("runtime: work.nwait =", decnwait, "work.nproc=", work.nproc)
throw("nwait > work.nprocs")
}
// gcDrainN requires the caller to be preemptible.
casgstatus(gp, _Grunning, _Gwaiting)
gp.waitreason = "GC assist marking"
// drain own cached work first in the hopes that it
// will be more cache friendly.
gcw := &getg().m.p.ptr().gcw
workDone := gcDrainN(gcw, scanWork)
// If we are near the end of the mark phase
// dispose of the gcw.
if gcBlackenPromptly {
gcw.dispose()
}
casgstatus(gp, _Gwaiting, _Grunning)
// Record that we did this much scan work.
//
// Back out the number of bytes of assist credit that
// this scan work counts for. The "1+" is a poor man's
// round-up, to ensure this adds credit even if
// assistBytesPerWork is very low.
gp.gcAssistBytes += 1 + int64(gcController.assistBytesPerWork*float64(workDone))
// If this is the last worker and we ran out of work,
// signal a completion point.
incnwait := atomic.Xadd(&work.nwait, +1)
if incnwait > work.nproc {
println("runtime: work.nwait=", incnwait,
"work.nproc=", work.nproc,
"gcBlackenPromptly=", gcBlackenPromptly)
throw("work.nwait > work.nproc")
}
if incnwait == work.nproc && !gcMarkWorkAvailable(nil) {
// This has reached a background completion point. Set
// gp.param to a non-nil value to indicate this. It
// doesn't matter what we set it to (it just has to be
// a valid pointer).
gp.param = unsafe.Pointer(gp)
}
duration := nanotime() - startTime
_p_ := gp.m.p.ptr()
_p_.gcAssistTime += duration
if _p_.gcAssistTime > gcAssistTimeSlack {
atomic.Xaddint64(&gcController.assistTime, _p_.gcAssistTime)
_p_.gcAssistTime = 0
}
}
// gcWakeAllAssists wakes all currently blocked assists. This is used
// at the end of a GC cycle. gcBlackenEnabled must be false to prevent
// new assists from going to sleep after this point.
func gcWakeAllAssists() {
lock(&work.assistQueue.lock)
injectglist(work.assistQueue.head.ptr())
work.assistQueue.head.set(nil)
work.assistQueue.tail.set(nil)
unlock(&work.assistQueue.lock)
}
// gcParkAssist puts the current goroutine on the assist queue and parks.
//
// gcParkAssist returns whether the assist is now satisfied. If it
// returns false, the caller must retry the assist.
//
//go:nowritebarrier
func gcParkAssist() bool {
lock(&work.assistQueue.lock)
// If the GC cycle finished while we were getting the lock,
// exit the assist. The cycle can't finish while we hold the
// lock.
if atomic.Load(&gcBlackenEnabled) == 0 {
unlock(&work.assistQueue.lock)
return true
}
gp := getg()
oldHead, oldTail := work.assistQueue.head, work.assistQueue.tail
if oldHead == 0 {
work.assistQueue.head.set(gp)
} else {
oldTail.ptr().schedlink.set(gp)
}
work.assistQueue.tail.set(gp)
gp.schedlink.set(nil)
// Recheck for background credit now that this G is in
// the queue, but can still back out. This avoids a
// race in case background marking has flushed more
// credit since we checked above.
if atomic.Loadint64(&gcController.bgScanCredit) > 0 {
work.assistQueue.head = oldHead
work.assistQueue.tail = oldTail
if oldTail != 0 {
oldTail.ptr().schedlink.set(nil)
}
unlock(&work.assistQueue.lock)
return false
}
// Park.
goparkunlock(&work.assistQueue.lock, "GC assist wait", traceEvGoBlockGC, 2)
return true
}
// gcFlushBgCredit flushes scanWork units of background scan work
// credit. This first satisfies blocked assists on the
// work.assistQueue and then flushes any remaining credit to
// gcController.bgScanCredit.
//
// Write barriers are disallowed because this is used by gcDrain after
// it has ensured that all work is drained and this must preserve that
// condition.
//
//go:nowritebarrierrec
func gcFlushBgCredit(scanWork int64) {
if work.assistQueue.head == 0 {
// Fast path; there are no blocked assists. There's a
// small window here where an assist may add itself to
// the blocked queue and park. If that happens, we'll
// just get it on the next flush.
atomic.Xaddint64(&gcController.bgScanCredit, scanWork)
return
}
scanBytes := int64(float64(scanWork) * gcController.assistBytesPerWork)
lock(&work.assistQueue.lock)
gp := work.assistQueue.head.ptr()
for gp != nil && scanBytes > 0 {
// Note that gp.gcAssistBytes is negative because gp
// is in debt. Think carefully about the signs below.
if scanBytes+gp.gcAssistBytes >= 0 {
// Satisfy this entire assist debt.
scanBytes += gp.gcAssistBytes
gp.gcAssistBytes = 0
xgp := gp
gp = gp.schedlink.ptr()
// It's important that we *not* put xgp in
// runnext. Otherwise, it's possible for user
// code to exploit the GC worker's high
// scheduler priority to get itself always run
// before other goroutines and always in the
// fresh quantum started by GC.
ready(xgp, 0, false)
} else {
// Partially satisfy this assist.
gp.gcAssistBytes += scanBytes
scanBytes = 0
// As a heuristic, we move this assist to the
// back of the queue so that large assists
// can't clog up the assist queue and
// substantially delay small assists.
xgp := gp
gp = gp.schedlink.ptr()
if gp == nil {
// gp is the only assist in the queue.
gp = xgp
} else {
xgp.schedlink = 0
work.assistQueue.tail.ptr().schedlink.set(xgp)
work.assistQueue.tail.set(xgp)
}
break
}
}
work.assistQueue.head.set(gp)
if gp == nil {
work.assistQueue.tail.set(nil)
}
if scanBytes > 0 {
// Convert from scan bytes back to work.
scanWork = int64(float64(scanBytes) * gcController.assistWorkPerByte)
atomic.Xaddint64(&gcController.bgScanCredit, scanWork)
}
unlock(&work.assistQueue.lock)
}
// We use a C function to find the stack.
func doscanstack(*g, *gcWork)
// scanstack scans gp's stack, greying all pointers found on the stack.
//
// During mark phase, it also installs stack barriers while traversing
// gp's stack. During mark termination, it stops scanning when it
// reaches an unhit stack barrier.
//
// scanstack is marked go:systemstack because it must not be preempted
// while using a workbuf.
//
//go:nowritebarrier
//go:systemstack
func scanstack(gp *g, gcw *gcWork) {
if gp.gcscanvalid {
return
}
if readgstatus(gp)&_Gscan == 0 {
print("runtime:scanstack: gp=", gp, ", goid=", gp.goid, ", gp->atomicstatus=", hex(readgstatus(gp)), "\n")
throw("scanstack - bad status")
}
switch readgstatus(gp) &^ _Gscan {
default:
print("runtime: gp=", gp, ", goid=", gp.goid, ", gp->atomicstatus=", readgstatus(gp), "\n")
throw("mark - bad status")
case _Gdead:
return
case _Grunning:
// ok for gccgo, though not for gc.
case _Grunnable, _Gsyscall, _Gwaiting:
// ok
}
mp := gp.m
if mp != nil && mp.helpgc != 0 {
throw("can't scan gchelper stack")
}
// Scan the stack.
doscanstack(gp, gcw)
// Conservatively scan the saved register values.
scanstackblock(uintptr(unsafe.Pointer(&gp.gcregs)), unsafe.Sizeof(gp.gcregs), gcw)
scanstackblock(uintptr(unsafe.Pointer(&gp.context)), unsafe.Sizeof(gp.context), gcw)
if gcphase == _GCmark {
// gp may have added itself to the rescan list between
// when GC started and now. It's clean now, so remove
// it. This isn't safe during mark termination because
// mark termination is consuming this list, but it's
// also not necessary.
dequeueRescan(gp)
}
gp.gcscanvalid = true
}
// queueRescan adds gp to the stack rescan list and clears
// gp.gcscanvalid. The caller must own gp and ensure that gp isn't
// already on the rescan list.
func queueRescan(gp *g) {
if debug.gcrescanstacks == 0 {
// Clear gcscanvalid to keep assertions happy.
//
// TODO: Remove gcscanvalid entirely when we remove
// stack rescanning.
gp.gcscanvalid = false
return
}
if gcphase == _GCoff {
gp.gcscanvalid = false
return
}
if gp.gcRescan != -1 {
throw("g already on rescan list")
}
lock(&work.rescan.lock)
gp.gcscanvalid = false
// Recheck gcphase under the lock in case there was a phase change.
if gcphase == _GCoff {
unlock(&work.rescan.lock)
return
}
if len(work.rescan.list) == cap(work.rescan.list) {
throw("rescan list overflow")
}
n := len(work.rescan.list)
gp.gcRescan = int32(n)
work.rescan.list = work.rescan.list[:n+1]
work.rescan.list[n].set(gp)
unlock(&work.rescan.lock)
}
// dequeueRescan removes gp from the stack rescan list, if gp is on
// the rescan list. The caller must own gp.
func dequeueRescan(gp *g) {
if debug.gcrescanstacks == 0 {
return
}
if gp.gcRescan == -1 {
return
}
if gcphase == _GCoff {
gp.gcRescan = -1
return
}
lock(&work.rescan.lock)
if work.rescan.list[gp.gcRescan].ptr() != gp {
throw("bad dequeueRescan")
}
// Careful: gp may itself be the last G on the list.
last := work.rescan.list[len(work.rescan.list)-1]
work.rescan.list[gp.gcRescan] = last
last.ptr().gcRescan = gp.gcRescan
gp.gcRescan = -1
work.rescan.list = work.rescan.list[:len(work.rescan.list)-1]
unlock(&work.rescan.lock)
}
type gcDrainFlags int
const (
gcDrainUntilPreempt gcDrainFlags = 1 << iota
gcDrainNoBlock
gcDrainFlushBgCredit
gcDrainIdle
// gcDrainBlock means neither gcDrainUntilPreempt or
// gcDrainNoBlock. It is the default, but callers should use
// the constant for documentation purposes.
gcDrainBlock gcDrainFlags = 0
)
// gcDrain scans roots and objects in work buffers, blackening grey
// objects until all roots and work buffers have been drained.
//
// If flags&gcDrainUntilPreempt != 0, gcDrain returns when g.preempt
// is set. This implies gcDrainNoBlock.
//
// If flags&gcDrainIdle != 0, gcDrain returns when there is other work
// to do. This implies gcDrainNoBlock.
//
// If flags&gcDrainNoBlock != 0, gcDrain returns as soon as it is
// unable to get more work. Otherwise, it will block until all
// blocking calls are blocked in gcDrain.
//
// If flags&gcDrainFlushBgCredit != 0, gcDrain flushes scan work
// credit to gcController.bgScanCredit every gcCreditSlack units of
// scan work.
//
//go:nowritebarrier
func gcDrain(gcw *gcWork, flags gcDrainFlags) {
if !writeBarrier.needed {
throw("gcDrain phase incorrect")
}
gp := getg().m.curg
preemptible := flags&gcDrainUntilPreempt != 0
blocking := flags&(gcDrainUntilPreempt|gcDrainIdle|gcDrainNoBlock) == 0
flushBgCredit := flags&gcDrainFlushBgCredit != 0
idle := flags&gcDrainIdle != 0
initScanWork := gcw.scanWork
// idleCheck is the scan work at which to perform the next
// idle check with the scheduler.
idleCheck := initScanWork + idleCheckThreshold
// Drain root marking jobs.
if work.markrootNext < work.markrootJobs {
for !(preemptible && gp.preempt) {
job := atomic.Xadd(&work.markrootNext, +1) - 1
if job >= work.markrootJobs {
break
}
markroot(gcw, job)
if idle && pollWork() {
goto done
}
}
}
// Drain heap marking jobs.
for !(preemptible && gp.preempt) {
// Try to keep work available on the global queue. We used to
// check if there were waiting workers, but it's better to
// just keep work available than to make workers wait. In the
// worst case, we'll do O(log(_WorkbufSize)) unnecessary
// balances.
if work.full == 0 {
gcw.balance()
}
var b uintptr
if blocking {
b = gcw.get()
} else {
b = gcw.tryGetFast()
if b == 0 {
b = gcw.tryGet()
}
}
if b == 0 {
// work barrier reached or tryGet failed.
break
}
scanobject(b, gcw)
// Flush background scan work credit to the global
// account if we've accumulated enough locally so
// mutator assists can draw on it.
if gcw.scanWork >= gcCreditSlack {
atomic.Xaddint64(&gcController.scanWork, gcw.scanWork)
if flushBgCredit {
gcFlushBgCredit(gcw.scanWork - initScanWork)
initScanWork = 0
}
idleCheck -= gcw.scanWork
gcw.scanWork = 0
if idle && idleCheck <= 0 {
idleCheck += idleCheckThreshold
if pollWork() {
break
}
}
}
}
// In blocking mode, write barriers are not allowed after this
// point because we must preserve the condition that the work
// buffers are empty.
done:
// Flush remaining scan work credit.
if gcw.scanWork > 0 {
atomic.Xaddint64(&gcController.scanWork, gcw.scanWork)
if flushBgCredit {
gcFlushBgCredit(gcw.scanWork - initScanWork)
}
gcw.scanWork = 0
}
}
// gcDrainN blackens grey objects until it has performed roughly
// scanWork units of scan work or the G is preempted. This is
// best-effort, so it may perform less work if it fails to get a work
// buffer. Otherwise, it will perform at least n units of work, but
// may perform more because scanning is always done in whole object
// increments. It returns the amount of scan work performed.
//
// The caller goroutine must be in a preemptible state (e.g.,
// _Gwaiting) to prevent deadlocks during stack scanning. As a
// consequence, this must be called on the system stack.
//
//go:nowritebarrier
//go:systemstack
func gcDrainN(gcw *gcWork, scanWork int64) int64 {
if !writeBarrier.needed {
throw("gcDrainN phase incorrect")
}
// There may already be scan work on the gcw, which we don't
// want to claim was done by this call.
workFlushed := -gcw.scanWork
gp := getg().m.curg
for !gp.preempt && workFlushed+gcw.scanWork < scanWork {
// See gcDrain comment.
if work.full == 0 {
gcw.balance()
}
// This might be a good place to add prefetch code...
// if(wbuf.nobj > 4) {
// PREFETCH(wbuf->obj[wbuf.nobj - 3];
// }
//
b := gcw.tryGetFast()
if b == 0 {
b = gcw.tryGet()
}
if b == 0 {
// Try to do a root job.
//
// TODO: Assists should get credit for this
// work.
if work.markrootNext < work.markrootJobs {
job := atomic.Xadd(&work.markrootNext, +1) - 1
if job < work.markrootJobs {
markroot(gcw, job)
continue
}
}
// No heap or root jobs.
break
}
scanobject(b, gcw)
// Flush background scan work credit.
if gcw.scanWork >= gcCreditSlack {
atomic.Xaddint64(&gcController.scanWork, gcw.scanWork)
workFlushed += gcw.scanWork
gcw.scanWork = 0
}
}
// Unlike gcDrain, there's no need to flush remaining work
// here because this never flushes to bgScanCredit and
// gcw.dispose will flush any remaining work to scanWork.
return workFlushed + gcw.scanWork
}
// scanblock scans b as scanobject would, but using an explicit
// pointer bitmap instead of the heap bitmap.
//
// This is used to scan non-heap roots, so it does not update
// gcw.bytesMarked or gcw.scanWork.
//
//go:nowritebarrier
func scanblock(b0, n0 uintptr, ptrmask *uint8, gcw *gcWork) {
// Use local copies of original parameters, so that a stack trace
// due to one of the throws below shows the original block
// base and extent.
b := b0
n := n0
arena_start := mheap_.arena_start
arena_used := mheap_.arena_used
for i := uintptr(0); i < n; {
// Find bits for the next word.
bits := uint32(*addb(ptrmask, i/(sys.PtrSize*8)))
if bits == 0 {
i += sys.PtrSize * 8
continue
}
for j := 0; j < 8 && i < n; j++ {
if bits&1 != 0 {
// Same work as in scanobject; see comments there.
obj := *(*uintptr)(unsafe.Pointer(b + i))
if obj != 0 && arena_start <= obj && obj < arena_used {
if obj, hbits, span, objIndex := heapBitsForObject(obj, b, i, false); obj != 0 {
greyobject(obj, b, i, hbits, span, gcw, objIndex, false)
}
}
}
bits >>= 1
i += sys.PtrSize
}
}
}
// scanobject scans the object starting at b, adding pointers to gcw.
// b must point to the beginning of a heap object or an oblet.
// scanobject consults the GC bitmap for the pointer mask and the
// spans for the size of the object.
//
//go:nowritebarrier
func scanobject(b uintptr, gcw *gcWork) {
// Note that arena_used may change concurrently during
// scanobject and hence scanobject may encounter a pointer to
// a newly allocated heap object that is *not* in
// [start,used). It will not mark this object; however, we
// know that it was just installed by a mutator, which means
// that mutator will execute a write barrier and take care of
// marking it. This is even more pronounced on relaxed memory
// architectures since we access arena_used without barriers
// or synchronization, but the same logic applies.
arena_start := mheap_.arena_start
arena_used := mheap_.arena_used
// Find the bits for b and the size of the object at b.
//
// b is either the beginning of an object, in which case this
// is the size of the object to scan, or it points to an
// oblet, in which case we compute the size to scan below.
hbits := heapBitsForAddr(b)
s := spanOfUnchecked(b)
n := s.elemsize
if n == 0 {
throw("scanobject n == 0")
}
if n > maxObletBytes {
// Large object. Break into oblets for better
// parallelism and lower latency.
if b == s.base() {
// It's possible this is a noscan object (not
// from greyobject, but from other code
// paths), in which case we must *not* enqueue
// oblets since their bitmaps will be
// uninitialized.
if !hbits.hasPointers(n) {
// Bypass the whole scan.
gcw.bytesMarked += uint64(n)
return
}
// Enqueue the other oblets to scan later.
// Some oblets may be in b's scalar tail, but
// these will be marked as "no more pointers",
// so we'll drop out immediately when we go to
// scan those.
for oblet := b + maxObletBytes; oblet < s.base()+s.elemsize; oblet += maxObletBytes {
if !gcw.putFast(oblet) {
gcw.put(oblet)
}
}
}
// Compute the size of the oblet. Since this object
// must be a large object, s.base() is the beginning
// of the object.
n = s.base() + s.elemsize - b
if n > maxObletBytes {
n = maxObletBytes
}
}
var i uintptr
for i = 0; i < n; i += sys.PtrSize {
// Find bits for this word.
if i != 0 {
// Avoid needless hbits.next() on last iteration.
hbits = hbits.next()
}
// Load bits once. See CL 22712 and issue 16973 for discussion.
bits := hbits.bits()
// During checkmarking, 1-word objects store the checkmark
// in the type bit for the one word. The only one-word objects
// are pointers, or else they'd be merged with other non-pointer
// data into larger allocations.
if i != 1*sys.PtrSize && bits&bitScan == 0 {
break // no more pointers in this object
}
if bits&bitPointer == 0 {
continue // not a pointer
}
// Work here is duplicated in scanblock and above.
// If you make changes here, make changes there too.
obj := *(*uintptr)(unsafe.Pointer(b + i))
// At this point we have extracted the next potential pointer.
// Check if it points into heap and not back at the current object.
if obj != 0 && arena_start <= obj && obj < arena_used && obj-b >= n {
// Mark the object.
if obj, hbits, span, objIndex := heapBitsForObject(obj, b, i, false); obj != 0 {
greyobject(obj, b, i, hbits, span, gcw, objIndex, false)
}
}
}
gcw.bytesMarked += uint64(n)
gcw.scanWork += int64(i)
}
//go:linkname scanstackblock runtime.scanstackblock
// scanstackblock is called by the stack scanning code in C to
// actually find and mark pointers in the stack block. This is like
// scanblock, but we scan the stack conservatively, so there is no
// bitmask of pointers.
func scanstackblock(b, n uintptr, gcw *gcWork) {
arena_start := mheap_.arena_start
arena_used := mheap_.arena_used
for i := uintptr(0); i < n; i += sys.PtrSize {
// Same work as in scanobject; see comments there.
obj := *(*uintptr)(unsafe.Pointer(b + i))
if obj != 0 && arena_start <= obj && obj < arena_used {
if obj, hbits, span, objIndex := heapBitsForObject(obj, b, i, true); obj != 0 {
greyobject(obj, b, i, hbits, span, gcw, objIndex, true)
}
}
}
}
// Shade the object if it isn't already.
// The object is not nil and known to be in the heap.
// Preemption must be disabled.
//go:nowritebarrier
func shade(b uintptr) {
// shade can be called to shade a pointer found on the stack,
// so pass forStack as true to heapBitsForObject and greyobject.
if obj, hbits, span, objIndex := heapBitsForObject(b, 0, 0, true); obj != 0 {
gcw := &getg().m.p.ptr().gcw
greyobject(obj, 0, 0, hbits, span, gcw, objIndex, true)
if gcphase == _GCmarktermination || gcBlackenPromptly {
// Ps aren't allowed to cache work during mark
// termination.
gcw.dispose()
}
}
}
// obj is the start of an object with mark mbits.
// If it isn't already marked, mark it and enqueue into gcw.
// base and off are for debugging only and could be removed.
//go:nowritebarrierrec
func greyobject(obj, base, off uintptr, hbits heapBits, span *mspan, gcw *gcWork, objIndex uintptr, forStack bool) {
// obj should be start of allocation, and so must be at least pointer-aligned.
if obj&(sys.PtrSize-1) != 0 {
throw("greyobject: obj not pointer-aligned")
}
mbits := span.markBitsForIndex(objIndex)
if useCheckmark {
if !mbits.isMarked() {
// Stack scanning is conservative, so we can
// see a reference to an object not previously
// found. Assume the object was correctly not
// marked and ignore the pointer.
if forStack {
return
}
printlock()
print("runtime:greyobject: checkmarks finds unexpected unmarked object obj=", hex(obj), "\n")
print("runtime: found obj at *(", hex(base), "+", hex(off), ")\n")
// Dump the source (base) object
gcDumpObject("base", base, off)
// Dump the object
gcDumpObject("obj", obj, ^uintptr(0))
throw("checkmark found unmarked object")
}
if hbits.isCheckmarked(span.elemsize) {
return
}
hbits.setCheckmarked(span.elemsize)
if !hbits.isCheckmarked(span.elemsize) {
throw("setCheckmarked and isCheckmarked disagree")
}
} else {
// Stack scanning is conservative, so we can see a
// pointer to a free object. Assume the object was
// correctly freed and we must ignore the pointer.
if forStack && span.isFree(objIndex) {
return
}
if debug.gccheckmark > 0 && span.isFree(objIndex) {
print("runtime: marking free object ", hex(obj), " found at *(", hex(base), "+", hex(off), ")\n")
gcDumpObject("base", base, off)
gcDumpObject("obj", obj, ^uintptr(0))
throw("marking free object")
}
// If marked we have nothing to do.
if mbits.isMarked() {
return
}
// mbits.setMarked() // Avoid extra call overhead with manual inlining.
atomic.Or8(mbits.bytep, mbits.mask)
// If this is a noscan object, fast-track it to black
// instead of greying it.
if !hbits.hasPointers(span.elemsize) {
gcw.bytesMarked += uint64(span.elemsize)
return
}
}
// Queue the obj for scanning. The PREFETCH(obj) logic has been removed but
// seems like a nice optimization that can be added back in.
// There needs to be time between the PREFETCH and the use.
// Previously we put the obj in an 8 element buffer that is drained at a rate
// to give the PREFETCH time to do its work.
// Use of PREFETCHNTA might be more appropriate than PREFETCH
if !gcw.putFast(obj) {
gcw.put(obj)
}
}
// gcDumpObject dumps the contents of obj for debugging and marks the
// field at byte offset off in obj.
func gcDumpObject(label string, obj, off uintptr) {
if obj < mheap_.arena_start || obj >= mheap_.arena_used {
print(label, "=", hex(obj), " is not in the Go heap\n")
return
}
k := obj >> _PageShift
x := k
x -= mheap_.arena_start >> _PageShift
s := mheap_.spans[x]
print(label, "=", hex(obj), " k=", hex(k))
if s == nil {
print(" s=nil\n")
return
}
print(" s.base()=", hex(s.base()), " s.limit=", hex(s.limit), " s.sizeclass=", s.sizeclass, " s.elemsize=", s.elemsize, " s.state=")
if 0 <= s.state && int(s.state) < len(mSpanStateNames) {
print(mSpanStateNames[s.state], "\n")
} else {
print("unknown(", s.state, ")\n")
}
skipped := false
size := s.elemsize
if s.state == _MSpanStack && size == 0 {
// We're printing something from a stack frame. We
// don't know how big it is, so just show up to an
// including off.
size = off + sys.PtrSize
}
for i := uintptr(0); i < size; i += sys.PtrSize {
// For big objects, just print the beginning (because
// that usually hints at the object's type) and the
// fields around off.
if !(i < 128*sys.PtrSize || off-16*sys.PtrSize < i && i < off+16*sys.PtrSize) {
skipped = true
continue
}
if skipped {
print(" ...\n")
skipped = false
}
print(" *(", label, "+", i, ") = ", hex(*(*uintptr)(unsafe.Pointer(obj + i))))
if i == off {
print(" <==")
}
print("\n")
}
if skipped {
print(" ...\n")
}
}
// gcmarknewobject marks a newly allocated object black. obj must
// not contain any non-nil pointers.
//
// This is nosplit so it can manipulate a gcWork without preemption.
//
//go:nowritebarrier
//go:nosplit
func gcmarknewobject(obj, size, scanSize uintptr) {
if useCheckmark && !gcBlackenPromptly { // The world should be stopped so this should not happen.
throw("gcmarknewobject called while doing checkmark")
}
markBitsForAddr(obj).setMarked()
gcw := &getg().m.p.ptr().gcw
gcw.bytesMarked += uint64(size)
gcw.scanWork += int64(scanSize)
if gcBlackenPromptly {
// There shouldn't be anything in the work queue, but
// we still need to flush stats.
gcw.dispose()
}
}
// gcMarkTinyAllocs greys all active tiny alloc blocks.
//
// The world must be stopped.
func gcMarkTinyAllocs() {
for _, p := range &allp {
if p == nil || p.status == _Pdead {
break
}
c := p.mcache
if c == nil || c.tiny == 0 {
continue
}
_, hbits, span, objIndex := heapBitsForObject(c.tiny, 0, 0, false)
gcw := &p.gcw
greyobject(c.tiny, 0, 0, hbits, span, gcw, objIndex, false)
if gcBlackenPromptly {
gcw.dispose()
}
}
}
// Checkmarking
// To help debug the concurrent GC we remark with the world
// stopped ensuring that any object encountered has their normal
// mark bit set. To do this we use an orthogonal bit
// pattern to indicate the object is marked. The following pattern
// uses the upper two bits in the object's boundary nibble.
// 01: scalar not marked
// 10: pointer not marked
// 11: pointer marked
// 00: scalar marked
// Xoring with 01 will flip the pattern from marked to unmarked and vica versa.
// The higher bit is 1 for pointers and 0 for scalars, whether the object
// is marked or not.
// The first nibble no longer holds the typeDead pattern indicating that the
// there are no more pointers in the object. This information is held
// in the second nibble.
// If useCheckmark is true, marking of an object uses the
// checkmark bits (encoding above) instead of the standard
// mark bits.
var useCheckmark = false
//go:nowritebarrier
func initCheckmarks() {
useCheckmark = true
for _, s := range mheap_.allspans {
if s.state == _MSpanInUse {
heapBitsForSpan(s.base()).initCheckmarkSpan(s.layout())
}
}
}
func clearCheckmarks() {
useCheckmark = false
for _, s := range mheap_.allspans {
if s.state == _MSpanInUse {
heapBitsForSpan(s.base()).clearCheckmarkSpan(s.layout())
}
}
}