Adding upstream version 1.21.8.upstream/1.21.8

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 1818 insertions, 0 deletions

diff --git a/src/runtime/mgc.go b/src/runtime/mgc.go
new file mode 100644
index 0000000..a12dbfe
--- /dev/null
+++ b/src/runtime/mgc.go
@@ -0,0 +1,1818 @@
+// 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 (GC).
+//
+// The GC runs concurrently with mutator threads, is type accurate (aka precise), allows multiple
+// GC thread to run in parallel. It is a concurrent mark and sweep that uses a write barrier. It is
+// non-generational and non-compacting. Allocation is done using size segregated per P allocation
+// areas to minimize fragmentation while eliminating locks in the common case.
+//
+// The algorithm decomposes into several steps.
+// This is a high level description of the algorithm being used. For an overview of GC a good
+// place to start is Richard Jones' gchandbook.org.
+//
+// The algorithm's intellectual heritage includes Dijkstra's on-the-fly algorithm, see
+// Edsger W. Dijkstra, Leslie Lamport, A. J. Martin, C. S. Scholten, and E. F. M. Steffens. 1978.
+// On-the-fly garbage collection: an exercise in cooperation. Commun. ACM 21, 11 (November 1978),
+// 966-975.
+// For journal quality proofs that these steps are complete, correct, and terminate see
+// Hudson, R., and Moss, J.E.B. Copying Garbage Collection without stopping the world.
+// Concurrency and Computation: Practice and Experience 15(3-5), 2003.
+//
+// 1. GC performs sweep termination.
+//
+//    a. Stop the world. This causes all Ps to reach a GC safe-point.
+//
+//    b. Sweep any unswept spans. There will only be unswept spans if
+//    this GC cycle was forced before the expected time.
+//
+// 2. GC performs the mark phase.
+//
+//    a. Prepare for the mark phase by setting gcphase to _GCmark
+//    (from _GCoff), enabling the write barrier, enabling mutator
+//    assists, and enqueueing root mark jobs. No objects may be
+//    scanned until all Ps have enabled the write barrier, which is
+//    accomplished using STW.
+//
+//    b. Start the world. From this point, GC work is done by mark
+//    workers started by the scheduler and by assists performed as
+//    part of allocation. The write barrier shades both the
+//    overwritten pointer and the new pointer value for any pointer
+//    writes (see mbarrier.go for details). Newly allocated objects
+//    are immediately marked black.
+//
+//    c. GC performs root marking jobs. This includes scanning all
+//    stacks, shading all globals, and shading any heap pointers in
+//    off-heap runtime data structures. Scanning a stack stops a
+//    goroutine, shades any pointers found on its stack, and then
+//    resumes the goroutine.
+//
+//    d. GC drains the work queue of grey objects, scanning each grey
+//    object to black and shading all pointers found in the object
+//    (which in turn may add those pointers to the work queue).
+//
+//    e. Because GC work is spread across local caches, GC uses a
+//    distributed termination algorithm to detect when there are no
+//    more root marking jobs or grey objects (see gcMarkDone). At this
+//    point, GC transitions to mark termination.
+//
+// 3. GC performs mark termination.
+//
+//    a. Stop the world.
+//
+//    b. Set gcphase to _GCmarktermination, and disable workers and
+//    assists.
+//
+//    c. Perform housekeeping like flushing mcaches.
+//
+// 4. GC performs the sweep phase.
+//
+//    a. Prepare for the sweep phase by setting gcphase to _GCoff,
+//    setting up sweep state and disabling the write barrier.
+//
+//    b. Start the world. From this point on, newly allocated objects
+//    are white, and allocating sweeps spans before use if necessary.
+//
+//    c. GC does concurrent sweeping in the background and in response
+//    to allocation. See description below.
+//
+// 5. When sufficient allocation has taken place, replay the sequence
+// starting with 1 above. See discussion of GC rate below.
+
+// Concurrent sweep.
+//
+// The sweep phase proceeds concurrently with normal program execution.
+// The heap is swept span-by-span both lazily (when a goroutine needs another span)
+// and concurrently in a background goroutine (this helps programs that are not CPU bound).
+// At the end of STW mark termination all spans are marked as "needs sweeping".
+//
+// The background sweeper goroutine simply sweeps spans one-by-one.
+//
+// To avoid requesting more OS memory while there are unswept spans, when a
+// goroutine needs another span, it first attempts to reclaim that much memory
+// by sweeping. When a goroutine needs to allocate a new small-object span, it
+// sweeps small-object spans for the same object size until it frees at least
+// one object. When a goroutine needs to allocate large-object span from heap,
+// it sweeps spans until it frees at least that many pages into heap. There is
+// one case where this may not suffice: if a goroutine sweeps and frees two
+// nonadjacent one-page spans to the heap, it will allocate a new two-page
+// span, but there can still be other one-page unswept spans which could be
+// combined into a two-page span.
+//
+// It's critical to ensure that no operations proceed on unswept spans (that would corrupt
+// mark bits in GC bitmap). During GC all mcaches are flushed into the central cache,
+// so they are empty. When a goroutine grabs a new span into mcache, it sweeps it.
+// When a goroutine explicitly frees an object or sets a finalizer, it ensures that
+// the span is swept (either by sweeping it, or by waiting for the concurrent sweep to finish).
+// The finalizer goroutine is kicked off only when all spans are swept.
+// When the next GC starts, it sweeps all not-yet-swept spans (if any).
+
+// GC rate.
+// Next GC is after we've allocated an extra amount of memory proportional to
+// the amount already in use. The proportion is controlled by GOGC environment variable
+// (100 by default). If GOGC=100 and we're using 4M, we'll GC again when we get to 8M
+// (this mark is computed by the gcController.heapGoal method). This keeps the GC cost in
+// linear proportion to the allocation cost. Adjusting GOGC just changes the linear constant
+// (and also the amount of extra memory used).
+
+// Oblets
+//
+// In order to prevent long pauses while scanning large objects and to
+// improve parallelism, the garbage collector breaks up scan jobs for
+// objects larger than maxObletBytes into "oblets" of at most
+// maxObletBytes. When scanning encounters the beginning of a large
+// object, it scans only the first oblet and enqueues the remaining
+// oblets as new scan jobs.
+
+package runtime
+
+import (
+	"internal/cpu"
+	"runtime/internal/atomic"
+	"unsafe"
+)
+
+const (
+	_DebugGC         = 0
+	_ConcurrentSweep = true
+	_FinBlockSize    = 4 * 1024
+
+	// debugScanConservative enables debug logging for stack
+	// frames that are scanned conservatively.
+	debugScanConservative = false
+
+	// sweepMinHeapDistance is a lower bound on the heap distance
+	// (in bytes) reserved for concurrent sweeping between GC
+	// cycles.
+	sweepMinHeapDistance = 1024 * 1024
+)
+
+// heapObjectsCanMove always returns false in the current garbage collector.
+// It exists for go4.org/unsafe/assume-no-moving-gc, which is an
+// unfortunate idea that had an even more unfortunate implementation.
+// Every time a new Go release happened, the package stopped building,
+// and the authors had to add a new file with a new //go:build line, and
+// then the entire ecosystem of packages with that as a dependency had to
+// explicitly update to the new version. Many packages depend on
+// assume-no-moving-gc transitively, through paths like
+// inet.af/netaddr -> go4.org/intern -> assume-no-moving-gc.
+// This was causing a significant amount of friction around each new
+// release, so we added this bool for the package to //go:linkname
+// instead. The bool is still unfortunate, but it's not as bad as
+// breaking the ecosystem on every new release.
+//
+// If the Go garbage collector ever does move heap objects, we can set
+// this to true to break all the programs using assume-no-moving-gc.
+//
+//go:linkname heapObjectsCanMove
+func heapObjectsCanMove() bool {
+	return false
+}
+
+func gcinit() {
+	if unsafe.Sizeof(workbuf{}) != _WorkbufSize {
+		throw("size of Workbuf is suboptimal")
+	}
+	// No sweep on the first cycle.
+	sweep.active.state.Store(sweepDrainedMask)
+
+	// Initialize GC pacer state.
+	// Use the environment variable GOGC for the initial gcPercent value.
+	// Use the environment variable GOMEMLIMIT for the initial memoryLimit value.
+	gcController.init(readGOGC(), readGOMEMLIMIT())
+
+	work.startSema = 1
+	work.markDoneSema = 1
+	lockInit(&work.sweepWaiters.lock, lockRankSweepWaiters)
+	lockInit(&work.assistQueue.lock, lockRankAssistQueue)
+	lockInit(&work.wbufSpans.lock, lockRankWbufSpans)
+}
+
+// gcenable is called after the bulk of the runtime initialization,
+// just before we're about to start letting user code run.
+// It kicks off the background sweeper goroutine, the background
+// scavenger goroutine, and enables GC.
+func gcenable() {
+	// Kick off sweeping and scavenging.
+	c := make(chan int, 2)
+	go bgsweep(c)
+	go bgscavenge(c)
+	<-c
+	<-c
+	memstats.enablegc = true // now that runtime is initialized, GC is okay
+}
+
+// Garbage collector phase.
+// Indicates to write barrier and synchronization task to perform.
+var gcphase uint32
+
+// The compiler knows about this variable.
+// If you change it, you must change builtin/runtime.go, too.
+// If you change the first four bytes, you must also change the write
+// barrier insertion code.
+var writeBarrier struct {
+	enabled bool    // compiler emits a check of this before calling write barrier
+	pad     [3]byte // compiler uses 32-bit load for "enabled" field
+	needed  bool    // identical to enabled, for now (TODO: dedup)
+	alignme uint64  // guarantee alignment so that compiler can use a 32 or 64-bit load
+}
+
+// gcBlackenEnabled is 1 if mutator assists and background mark
+// workers are allowed to blacken objects. This must only be set when
+// gcphase == _GCmark.
+var gcBlackenEnabled uint32
+
+const (
+	_GCoff             = iota // GC not running; sweeping in background, write barrier disabled
+	_GCmark                   // GC marking roots and workbufs: allocate black, write barrier ENABLED
+	_GCmarktermination        // GC mark termination: allocate black, P's help GC, write barrier ENABLED
+)
+
+//go:nosplit
+func setGCPhase(x uint32) {
+	atomic.Store(&gcphase, x)
+	writeBarrier.needed = gcphase == _GCmark || gcphase == _GCmarktermination
+	writeBarrier.enabled = writeBarrier.needed
+}
+
+// gcMarkWorkerMode represents the mode that a concurrent mark worker
+// should operate in.
+//
+// Concurrent marking happens through four different mechanisms. One
+// is mutator assists, which happen in response to allocations and are
+// not scheduled. The other three are variations in the per-P mark
+// workers and are distinguished by gcMarkWorkerMode.
+type gcMarkWorkerMode int
+
+const (
+	// gcMarkWorkerNotWorker indicates that the next scheduled G is not
+	// starting work and the mode should be ignored.
+	gcMarkWorkerNotWorker gcMarkWorkerMode = iota
+
+	// gcMarkWorkerDedicatedMode indicates that the P of a mark
+	// worker is dedicated to running that mark worker. The mark
+	// worker should run without preemption.
+	gcMarkWorkerDedicatedMode
+
+	// gcMarkWorkerFractionalMode indicates that a P is currently
+	// running the "fractional" mark worker. The fractional worker
+	// is necessary when GOMAXPROCS*gcBackgroundUtilization is not
+	// an integer and using only dedicated workers would result in
+	// utilization too far from the target of gcBackgroundUtilization.
+	// The fractional worker should run until it is preempted and
+	// will be scheduled to pick up the fractional part of
+	// GOMAXPROCS*gcBackgroundUtilization.
+	gcMarkWorkerFractionalMode
+
+	// gcMarkWorkerIdleMode indicates that a P is running the mark
+	// worker because it has nothing else to do. The idle worker
+	// should run until it is preempted and account its time
+	// against gcController.idleMarkTime.
+	gcMarkWorkerIdleMode
+)
+
+// gcMarkWorkerModeStrings are the strings labels of gcMarkWorkerModes
+// to use in execution traces.
+var gcMarkWorkerModeStrings = [...]string{
+	"Not worker",
+	"GC (dedicated)",
+	"GC (fractional)",
+	"GC (idle)",
+}
+
+// pollFractionalWorkerExit reports whether a fractional mark worker
+// should self-preempt. It assumes it is called from the fractional
+// worker.
+func pollFractionalWorkerExit() bool {
+	// This should be kept in sync with the fractional worker
+	// scheduler logic in findRunnableGCWorker.
+	now := nanotime()
+	delta := now - gcController.markStartTime
+	if delta <= 0 {
+		return true
+	}
+	p := getg().m.p.ptr()
+	selfTime := p.gcFractionalMarkTime + (now - p.gcMarkWorkerStartTime)
+	// Add some slack to the utilization goal so that the
+	// fractional worker isn't behind again the instant it exits.
+	return float64(selfTime)/float64(delta) > 1.2*gcController.fractionalUtilizationGoal
+}
+
+var work workType
+
+type workType struct {
+	full  lfstack          // lock-free list of full blocks workbuf
+	_     cpu.CacheLinePad // prevents false-sharing between full and empty
+	empty lfstack          // lock-free list of empty blocks workbuf
+	_     cpu.CacheLinePad // prevents false-sharing between empty and nproc/nwait
+
+	wbufSpans struct {
+		lock mutex
+		// free is a list of spans dedicated to workbufs, but
+		// that don't currently contain any workbufs.
+		free mSpanList
+		// busy is a list of all spans containing workbufs on
+		// one of the workbuf lists.
+		busy mSpanList
+	}
+
+	// Restore 64-bit alignment on 32-bit.
+	_ uint32
+
+	// bytesMarked is the number of bytes marked this cycle. This
+	// includes bytes blackened in scanned objects, noscan objects
+	// that go straight to black, and permagrey objects scanned by
+	// markroot during the concurrent scan phase. This is updated
+	// atomically during the cycle. Updates may be batched
+	// arbitrarily, since the value is only read at the end of the
+	// cycle.
+	//
+	// Because of benign races during marking, this number may not
+	// be the exact number of marked bytes, but it should be very
+	// close.
+	//
+	// Put this field here because it needs 64-bit atomic access
+	// (and thus 8-byte alignment even on 32-bit architectures).
+	bytesMarked uint64
+
+	markrootNext uint32 // next markroot job
+	markrootJobs uint32 // number of markroot jobs
+
+	nproc  uint32
+	tstart int64
+	nwait  uint32
+
+	// Number of roots of various root types. Set by gcMarkRootPrepare.
+	//
+	// nStackRoots == len(stackRoots), but we have nStackRoots for
+	// consistency.
+	nDataRoots, nBSSRoots, nSpanRoots, nStackRoots int
+
+	// Base indexes of each root type. Set by gcMarkRootPrepare.
+	baseData, baseBSS, baseSpans, baseStacks, baseEnd uint32
+
+	// stackRoots is a snapshot of all of the Gs that existed
+	// before the beginning of concurrent marking. The backing
+	// store of this must not be modified because it might be
+	// shared with allgs.
+	stackRoots []*g
+
+	// Each type of GC state transition is protected by a lock.
+	// Since multiple threads can simultaneously detect the state
+	// transition condition, any thread that detects a transition
+	// condition must acquire the appropriate transition lock,
+	// re-check the transition condition and return if it no
+	// longer holds or perform the transition if it does.
+	// Likewise, any transition must invalidate the transition
+	// condition before releasing the lock. This ensures that each
+	// transition is performed by exactly one thread and threads
+	// that need the transition to happen block until it has
+	// happened.
+	//
+	// startSema protects the transition from "off" to mark or
+	// mark termination.
+	startSema uint32
+	// markDoneSema protects transitions from mark to mark termination.
+	markDoneSema uint32
+
+	bgMarkReady note   // signal background mark worker has started
+	bgMarkDone  uint32 // cas to 1 when at a background mark completion point
+	// Background mark completion signaling
+
+	// mode is the concurrency mode of the current GC cycle.
+	mode gcMode
+
+	// userForced indicates the current GC cycle was forced by an
+	// explicit user call.
+	userForced bool
+
+	// initialHeapLive is the value of gcController.heapLive at the
+	// beginning of this GC cycle.
+	initialHeapLive uint64
+
+	// assistQueue is a queue of assists that are blocked because
+	// there was neither enough credit to steal or enough work to
+	// do.
+	assistQueue struct {
+		lock mutex
+		q    gQueue
+	}
+
+	// sweepWaiters is a list of blocked goroutines to wake when
+	// we transition from mark termination to sweep.
+	sweepWaiters struct {
+		lock mutex
+		list gList
+	}
+
+	// cycles is the number of completed GC cycles, where a GC
+	// cycle is sweep termination, mark, mark termination, and
+	// sweep. This differs from memstats.numgc, which is
+	// incremented at mark termination.
+	cycles atomic.Uint32
+
+	// Timing/utilization stats for this cycle.
+	stwprocs, maxprocs                 int32
+	tSweepTerm, tMark, tMarkTerm, tEnd int64 // nanotime() of phase start
+
+	pauseNS    int64 // total STW time this cycle
+	pauseStart int64 // nanotime() of last STW
+
+	// debug.gctrace heap sizes for this cycle.
+	heap0, heap1, heap2 uint64
+
+	// Cumulative estimated CPU usage.
+	cpuStats
+}
+
+// GC runs a garbage collection and blocks the caller until the
+// garbage collection is complete. It may also block the entire
+// program.
+func GC() {
+	// We consider a cycle to be: sweep termination, mark, mark
+	// termination, and sweep. This function shouldn't return
+	// until a full cycle has been completed, from beginning to
+	// end. Hence, we always want to finish up the current cycle
+	// and start a new one. That means:
+	//
+	// 1. In sweep termination, mark, or mark termination of cycle
+	// N, wait until mark termination N completes and transitions
+	// to sweep N.
+	//
+	// 2. In sweep N, help with sweep N.
+	//
+	// At this point we can begin a full cycle N+1.
+	//
+	// 3. Trigger cycle N+1 by starting sweep termination N+1.
+	//
+	// 4. Wait for mark termination N+1 to complete.
+	//
+	// 5. Help with sweep N+1 until it's done.
+	//
+	// This all has to be written to deal with the fact that the
+	// GC may move ahead on its own. For example, when we block
+	// until mark termination N, we may wake up in cycle N+2.
+
+	// Wait until the current sweep termination, mark, and mark
+	// termination complete.
+	n := work.cycles.Load()
+	gcWaitOnMark(n)
+
+	// We're now in sweep N or later. Trigger GC cycle N+1, which
+	// will first finish sweep N if necessary and then enter sweep
+	// termination N+1.
+	gcStart(gcTrigger{kind: gcTriggerCycle, n: n + 1})
+
+	// Wait for mark termination N+1 to complete.
+	gcWaitOnMark(n + 1)
+
+	// Finish sweep N+1 before returning. We do this both to
+	// complete the cycle and because runtime.GC() is often used
+	// as part of tests and benchmarks to get the system into a
+	// relatively stable and isolated state.
+	for work.cycles.Load() == n+1 && sweepone() != ^uintptr(0) {
+		sweep.nbgsweep++
+		Gosched()
+	}
+
+	// Callers may assume that the heap profile reflects the
+	// just-completed cycle when this returns (historically this
+	// happened because this was a STW GC), but right now the
+	// profile still reflects mark termination N, not N+1.
+	//
+	// As soon as all of the sweep frees from cycle N+1 are done,
+	// we can go ahead and publish the heap profile.
+	//
+	// First, wait for sweeping to finish. (We know there are no
+	// more spans on the sweep queue, but we may be concurrently
+	// sweeping spans, so we have to wait.)
+	for work.cycles.Load() == n+1 && !isSweepDone() {
+		Gosched()
+	}
+
+	// Now we're really done with sweeping, so we can publish the
+	// stable heap profile. Only do this if we haven't already hit
+	// another mark termination.
+	mp := acquirem()
+	cycle := work.cycles.Load()
+	if cycle == n+1 || (gcphase == _GCmark && cycle == n+2) {
+		mProf_PostSweep()
+	}
+	releasem(mp)
+}
+
+// gcWaitOnMark blocks until GC finishes the Nth mark phase. If GC has
+// already completed this mark phase, it returns immediately.
+func gcWaitOnMark(n uint32) {
+	for {
+		// Disable phase transitions.
+		lock(&work.sweepWaiters.lock)
+		nMarks := work.cycles.Load()
+		if gcphase != _GCmark {
+			// We've already completed this cycle's mark.
+			nMarks++
+		}
+		if nMarks > n {
+			// We're done.
+			unlock(&work.sweepWaiters.lock)
+			return
+		}
+
+		// Wait until sweep termination, mark, and mark
+		// termination of cycle N complete.
+		work.sweepWaiters.list.push(getg())
+		goparkunlock(&work.sweepWaiters.lock, waitReasonWaitForGCCycle, traceBlockUntilGCEnds, 1)
+	}
+}
+
+// gcMode indicates how concurrent a GC cycle should be.
+type gcMode int
+
+const (
+	gcBackgroundMode gcMode = iota // concurrent GC and sweep
+	gcForceMode                    // stop-the-world GC now, concurrent sweep
+	gcForceBlockMode               // stop-the-world GC now and STW sweep (forced by user)
+)
+
+// A gcTrigger is a predicate for starting a GC cycle. Specifically,
+// it is an exit condition for the _GCoff phase.
+type gcTrigger struct {
+	kind gcTriggerKind
+	now  int64  // gcTriggerTime: current time
+	n    uint32 // gcTriggerCycle: cycle number to start
+}
+
+type gcTriggerKind int
+
+const (
+	// gcTriggerHeap indicates that a cycle should be started when
+	// the heap size reaches the trigger heap size computed by the
+	// controller.
+	gcTriggerHeap gcTriggerKind = iota
+
+	// gcTriggerTime indicates that a cycle should be started when
+	// it's been more than forcegcperiod nanoseconds since the
+	// previous GC cycle.
+	gcTriggerTime
+
+	// gcTriggerCycle indicates that a cycle should be started if
+	// we have not yet started cycle number gcTrigger.n (relative
+	// to work.cycles).
+	gcTriggerCycle
+)
+
+// test reports whether the trigger condition is satisfied, meaning
+// that the exit condition for the _GCoff phase has been met. The exit
+// condition should be tested when allocating.
+func (t gcTrigger) test() bool {
+	if !memstats.enablegc || panicking.Load() != 0 || gcphase != _GCoff {
+		return false
+	}
+	switch t.kind {
+	case gcTriggerHeap:
+		// Non-atomic access to gcController.heapLive for performance. If
+		// we are going to trigger on this, this thread just
+		// atomically wrote gcController.heapLive anyway and we'll see our
+		// own write.
+		trigger, _ := gcController.trigger()
+		return gcController.heapLive.Load() >= trigger
+	case gcTriggerTime:
+		if gcController.gcPercent.Load() < 0 {
+			return false
+		}
+		lastgc := int64(atomic.Load64(&memstats.last_gc_nanotime))
+		return lastgc != 0 && t.now-lastgc > forcegcperiod
+	case gcTriggerCycle:
+		// t.n > work.cycles, but accounting for wraparound.
+		return int32(t.n-work.cycles.Load()) > 0
+	}
+	return true
+}
+
+// gcStart starts the GC. It transitions from _GCoff to _GCmark (if
+// debug.gcstoptheworld == 0) or performs all of GC (if
+// debug.gcstoptheworld != 0).
+//
+// This may return without performing this transition in some cases,
+// such as when called on a system stack or with locks held.
+func gcStart(trigger gcTrigger) {
+	// Since this is called from malloc and malloc is called in
+	// the guts of a number of libraries that might be holding
+	// locks, don't attempt to start GC in non-preemptible or
+	// potentially unstable situations.
+	mp := acquirem()
+	if gp := getg(); gp == mp.g0 || mp.locks > 1 || mp.preemptoff != "" {
+		releasem(mp)
+		return
+	}
+	releasem(mp)
+	mp = nil
+
+	// Pick up the remaining unswept/not being swept spans concurrently
+	//
+	// This shouldn't happen if we're being invoked in background
+	// mode since proportional sweep should have just finished
+	// sweeping everything, but rounding errors, etc, may leave a
+	// few spans unswept. In forced mode, this is necessary since
+	// GC can be forced at any point in the sweeping cycle.
+	//
+	// We check the transition condition continuously here in case
+	// this G gets delayed in to the next GC cycle.
+	for trigger.test() && sweepone() != ^uintptr(0) {
+		sweep.nbgsweep++
+	}
+
+	// Perform GC initialization and the sweep termination
+	// transition.
+	semacquire(&work.startSema)
+	// Re-check transition condition under transition lock.
+	if !trigger.test() {
+		semrelease(&work.startSema)
+		return
+	}
+
+	// In gcstoptheworld debug mode, upgrade the mode accordingly.
+	// We do this after re-checking the transition condition so
+	// that multiple goroutines that detect the heap trigger don't
+	// start multiple STW GCs.
+	mode := gcBackgroundMode
+	if debug.gcstoptheworld == 1 {
+		mode = gcForceMode
+	} else if debug.gcstoptheworld == 2 {
+		mode = gcForceBlockMode
+	}
+
+	// Ok, we're doing it! Stop everybody else
+	semacquire(&gcsema)
+	semacquire(&worldsema)
+
+	// For stats, check if this GC was forced by the user.
+	// Update it under gcsema to avoid gctrace getting wrong values.
+	work.userForced = trigger.kind == gcTriggerCycle
+
+	if traceEnabled() {
+		traceGCStart()
+	}
+
+	// Check that all Ps have finished deferred mcache flushes.
+	for _, p := range allp {
+		if fg := p.mcache.flushGen.Load(); fg != mheap_.sweepgen {
+			println("runtime: p", p.id, "flushGen", fg, "!= sweepgen", mheap_.sweepgen)
+			throw("p mcache not flushed")
+		}
+	}
+
+	gcBgMarkStartWorkers()
+
+	systemstack(gcResetMarkState)
+
+	work.stwprocs, work.maxprocs = gomaxprocs, gomaxprocs
+	if work.stwprocs > ncpu {
+		// This is used to compute CPU time of the STW phases,
+		// so it can't be more than ncpu, even if GOMAXPROCS is.
+		work.stwprocs = ncpu
+	}
+	work.heap0 = gcController.heapLive.Load()
+	work.pauseNS = 0
+	work.mode = mode
+
+	now := nanotime()
+	work.tSweepTerm = now
+	work.pauseStart = now
+	systemstack(func() { stopTheWorldWithSema(stwGCSweepTerm) })
+	// Finish sweep before we start concurrent scan.
+	systemstack(func() {
+		finishsweep_m()
+	})
+
+	// clearpools before we start the GC. If we wait they memory will not be
+	// reclaimed until the next GC cycle.
+	clearpools()
+
+	work.cycles.Add(1)
+
+	// Assists and workers can start the moment we start
+	// the world.
+	gcController.startCycle(now, int(gomaxprocs), trigger)
+
+	// Notify the CPU limiter that assists may begin.
+	gcCPULimiter.startGCTransition(true, now)
+
+	// In STW mode, disable scheduling of user Gs. This may also
+	// disable scheduling of this goroutine, so it may block as
+	// soon as we start the world again.
+	if mode != gcBackgroundMode {
+		schedEnableUser(false)
+	}
+
+	// Enter concurrent mark phase and enable
+	// write barriers.
+	//
+	// Because the world is stopped, all Ps will
+	// observe that write barriers are enabled by
+	// the time we start the world and begin
+	// scanning.
+	//
+	// Write barriers must be enabled before assists are
+	// enabled because they must be enabled before
+	// any non-leaf heap objects are marked. Since
+	// allocations are blocked until assists can
+	// happen, we want enable assists as early as
+	// possible.
+	setGCPhase(_GCmark)
+
+	gcBgMarkPrepare() // Must happen before assist enable.
+	gcMarkRootPrepare()
+
+	// Mark all active tinyalloc blocks. Since we're
+	// allocating from these, they need to be black like
+	// other allocations. The alternative is to blacken
+	// the tiny block on every allocation from it, which
+	// would slow down the tiny allocator.
+	gcMarkTinyAllocs()
+
+	// At this point all Ps have enabled the write
+	// barrier, thus maintaining the no white to
+	// black invariant. Enable mutator assists to
+	// put back-pressure on fast allocating
+	// mutators.
+	atomic.Store(&gcBlackenEnabled, 1)
+
+	// In STW mode, we could block the instant systemstack
+	// returns, so make sure we're not preemptible.
+	mp = acquirem()
+
+	// Concurrent mark.
+	systemstack(func() {
+		now = startTheWorldWithSema()
+		work.pauseNS += now - work.pauseStart
+		work.tMark = now
+		memstats.gcPauseDist.record(now - work.pauseStart)
+
+		sweepTermCpu := int64(work.stwprocs) * (work.tMark - work.tSweepTerm)
+		work.cpuStats.gcPauseTime += sweepTermCpu
+		work.cpuStats.gcTotalTime += sweepTermCpu
+
+		// Release the CPU limiter.
+		gcCPULimiter.finishGCTransition(now)
+	})
+
+	// Release the world sema before Gosched() in STW mode
+	// because we will need to reacquire it later but before
+	// this goroutine becomes runnable again, and we could
+	// self-deadlock otherwise.
+	semrelease(&worldsema)
+	releasem(mp)
+
+	// Make sure we block instead of returning to user code
+	// in STW mode.
+	if mode != gcBackgroundMode {
+		Gosched()
+	}
+
+	semrelease(&work.startSema)
+}
+
+// gcMarkDoneFlushed counts the number of P's with flushed work.
+//
+// Ideally this would be a captured local in gcMarkDone, but forEachP
+// escapes its callback closure, so it can't capture anything.
+//
+// This is protected by markDoneSema.
+var gcMarkDoneFlushed uint32
+
+// gcMarkDone transitions the GC from mark to mark termination if all
+// reachable objects have been marked (that is, there are no grey
+// objects and can be no more in the future). Otherwise, it flushes
+// all local work to the global queues where it can be discovered by
+// other workers.
+//
+// This should be called when all local mark work has been drained and
+// there are no remaining workers. Specifically, when
+//
+//	work.nwait == work.nproc && !gcMarkWorkAvailable(p)
+//
+// The calling context must be preemptible.
+//
+// Flushing local work is important because idle Ps may have local
+// work queued. This is the only way to make that work visible and
+// drive GC to completion.
+//
+// It is explicitly okay to have write barriers in this function. If
+// it does transition to mark termination, then all reachable objects
+// have been marked, so the write barrier cannot shade any more
+// objects.
+func gcMarkDone() {
+	// Ensure only one thread is running the ragged barrier at a
+	// time.
+	semacquire(&work.markDoneSema)
+
+top:
+	// Re-check transition condition under transition lock.
+	//
+	// It's critical that this checks the global work queues are
+	// empty before performing the ragged barrier. Otherwise,
+	// there could be global work that a P could take after the P
+	// has passed the ragged barrier.
+	if !(gcphase == _GCmark && work.nwait == work.nproc && !gcMarkWorkAvailable(nil)) {
+		semrelease(&work.markDoneSema)
+		return
+	}
+
+	// forEachP needs worldsema to execute, and we'll need it to
+	// stop the world later, so acquire worldsema now.
+	semacquire(&worldsema)
+
+	// Flush all local buffers and collect flushedWork flags.
+	gcMarkDoneFlushed = 0
+	systemstack(func() {
+		gp := getg().m.curg
+		// Mark the user stack as preemptible so that it may be scanned.
+		// Otherwise, our attempt to force all P's to a safepoint could
+		// result in a deadlock as we attempt to preempt a worker that's
+		// trying to preempt us (e.g. for a stack scan).
+		casGToWaiting(gp, _Grunning, waitReasonGCMarkTermination)
+		forEachP(func(pp *p) {
+			// Flush the write barrier buffer, since this may add
+			// work to the gcWork.
+			wbBufFlush1(pp)
+
+			// Flush the gcWork, since this may create global work
+			// and set the flushedWork flag.
+			//
+			// TODO(austin): Break up these workbufs to
+			// better distribute work.
+			pp.gcw.dispose()
+			// Collect the flushedWork flag.
+			if pp.gcw.flushedWork {
+				atomic.Xadd(&gcMarkDoneFlushed, 1)
+				pp.gcw.flushedWork = false
+			}
+		})
+		casgstatus(gp, _Gwaiting, _Grunning)
+	})
+
+	if gcMarkDoneFlushed != 0 {
+		// More grey objects were discovered since the
+		// previous termination check, so there may be more
+		// work to do. Keep going. It's possible the
+		// transition condition became true again during the
+		// ragged barrier, so re-check it.
+		semrelease(&worldsema)
+		goto top
+	}
+
+	// There was no global work, no local work, and no Ps
+	// communicated work since we took markDoneSema. Therefore
+	// there are no grey objects and no more objects can be
+	// shaded. Transition to mark termination.
+	now := nanotime()
+	work.tMarkTerm = now
+	work.pauseStart = now
+	getg().m.preemptoff = "gcing"
+	systemstack(func() { stopTheWorldWithSema(stwGCMarkTerm) })
+	// The gcphase is _GCmark, it will transition to _GCmarktermination
+	// below. The important thing is that the wb remains active until
+	// all marking is complete. This includes writes made by the GC.
+
+	// There is sometimes work left over when we enter mark termination due
+	// to write barriers performed after the completion barrier above.
+	// Detect this and resume concurrent mark. This is obviously
+	// unfortunate.
+	//
+	// See issue #27993 for details.
+	//
+	// Switch to the system stack to call wbBufFlush1, though in this case
+	// it doesn't matter because we're non-preemptible anyway.
+	restart := false
+	systemstack(func() {
+		for _, p := range allp {
+			wbBufFlush1(p)
+			if !p.gcw.empty() {
+				restart = true
+				break
+			}
+		}
+	})
+	if restart {
+		getg().m.preemptoff = ""
+		systemstack(func() {
+			now := startTheWorldWithSema()
+			work.pauseNS += now - work.pauseStart
+			memstats.gcPauseDist.record(now - work.pauseStart)
+		})
+		semrelease(&worldsema)
+		goto top
+	}
+
+	gcComputeStartingStackSize()
+
+	// Disable assists and background workers. We must do
+	// this before waking blocked assists.
+	atomic.Store(&gcBlackenEnabled, 0)
+
+	// Notify the CPU limiter that GC assists will now cease.
+	gcCPULimiter.startGCTransition(false, now)
+
+	// Wake all blocked assists. These will run when we
+	// start the world again.
+	gcWakeAllAssists()
+
+	// Likewise, release the transition lock. Blocked
+	// workers and assists will run when we start the
+	// world again.
+	semrelease(&work.markDoneSema)
+
+	// In STW mode, re-enable user goroutines. These will be
+	// queued to run after we start the world.
+	schedEnableUser(true)
+
+	// endCycle depends on all gcWork cache stats being flushed.
+	// The termination algorithm above ensured that up to
+	// allocations since the ragged barrier.
+	gcController.endCycle(now, int(gomaxprocs), work.userForced)
+
+	// Perform mark termination. This will restart the world.
+	gcMarkTermination()
+}
+
+// World must be stopped and mark assists and background workers must be
+// disabled.
+func gcMarkTermination() {
+	// Start marktermination (write barrier remains enabled for now).
+	setGCPhase(_GCmarktermination)
+
+	work.heap1 = gcController.heapLive.Load()
+	startTime := nanotime()
+
+	mp := acquirem()
+	mp.preemptoff = "gcing"
+	mp.traceback = 2
+	curgp := mp.curg
+	casGToWaiting(curgp, _Grunning, waitReasonGarbageCollection)
+
+	// Run gc on the g0 stack. We do this so that the g stack
+	// we're currently running on will no longer change. Cuts
+	// the root set down a bit (g0 stacks are not scanned, and
+	// we don't need to scan gc's internal state).  We also
+	// need to switch to g0 so we can shrink the stack.
+	systemstack(func() {
+		gcMark(startTime)
+		// Must return immediately.
+		// The outer function's stack may have moved
+		// during gcMark (it shrinks stacks, including the
+		// outer function's stack), so we must not refer
+		// to any of its variables. Return back to the
+		// non-system stack to pick up the new addresses
+		// before continuing.
+	})
+
+	systemstack(func() {
+		work.heap2 = work.bytesMarked
+		if debug.gccheckmark > 0 {
+			// Run a full non-parallel, stop-the-world
+			// mark using checkmark bits, to check that we
+			// didn't forget to mark anything during the
+			// concurrent mark process.
+			startCheckmarks()
+			gcResetMarkState()
+			gcw := &getg().m.p.ptr().gcw
+			gcDrain(gcw, 0)
+			wbBufFlush1(getg().m.p.ptr())
+			gcw.dispose()
+			endCheckmarks()
+		}
+
+		// marking is complete so we can turn the write barrier off
+		setGCPhase(_GCoff)
+		gcSweep(work.mode)
+	})
+
+	mp.traceback = 0
+	casgstatus(curgp, _Gwaiting, _Grunning)
+
+	if traceEnabled() {
+		traceGCDone()
+	}
+
+	// all done
+	mp.preemptoff = ""
+
+	if gcphase != _GCoff {
+		throw("gc done but gcphase != _GCoff")
+	}
+
+	// Record heapInUse for scavenger.
+	memstats.lastHeapInUse = gcController.heapInUse.load()
+
+	// Update GC trigger and pacing, as well as downstream consumers
+	// of this pacing information, for the next cycle.
+	systemstack(gcControllerCommit)
+
+	// Update timing memstats
+	now := nanotime()
+	sec, nsec, _ := time_now()
+	unixNow := sec*1e9 + int64(nsec)
+	work.pauseNS += now - work.pauseStart
+	work.tEnd = now
+	memstats.gcPauseDist.record(now - work.pauseStart)
+	atomic.Store64(&memstats.last_gc_unix, uint64(unixNow)) // must be Unix time to make sense to user
+	atomic.Store64(&memstats.last_gc_nanotime, uint64(now)) // monotonic time for us
+	memstats.pause_ns[memstats.numgc%uint32(len(memstats.pause_ns))] = uint64(work.pauseNS)
+	memstats.pause_end[memstats.numgc%uint32(len(memstats.pause_end))] = uint64(unixNow)
+	memstats.pause_total_ns += uint64(work.pauseNS)
+
+	markTermCpu := int64(work.stwprocs) * (work.tEnd - work.tMarkTerm)
+	work.cpuStats.gcPauseTime += markTermCpu
+	work.cpuStats.gcTotalTime += markTermCpu
+
+	// Accumulate CPU stats.
+	//
+	// Pass gcMarkPhase=true so we can get all the latest GC CPU stats in there too.
+	work.cpuStats.accumulate(now, true)
+
+	// Compute overall GC CPU utilization.
+	// Omit idle marking time from the overall utilization here since it's "free".
+	memstats.gc_cpu_fraction = float64(work.cpuStats.gcTotalTime-work.cpuStats.gcIdleTime) / float64(work.cpuStats.totalTime)
+
+	// Reset assist time and background time stats.
+	//
+	// Do this now, instead of at the start of the next GC cycle, because
+	// these two may keep accumulating even if the GC is not active.
+	scavenge.assistTime.Store(0)
+	scavenge.backgroundTime.Store(0)
+
+	// Reset idle time stat.
+	sched.idleTime.Store(0)
+
+	// Reset sweep state.
+	sweep.nbgsweep = 0
+	sweep.npausesweep = 0
+
+	if work.userForced {
+		memstats.numforcedgc++
+	}
+
+	// Bump GC cycle count and wake goroutines waiting on sweep.
+	lock(&work.sweepWaiters.lock)
+	memstats.numgc++
+	injectglist(&work.sweepWaiters.list)
+	unlock(&work.sweepWaiters.lock)
+
+	// Increment the scavenge generation now.
+	//
+	// This moment represents peak heap in use because we're
+	// about to start sweeping.
+	mheap_.pages.scav.index.nextGen()
+
+	// Release the CPU limiter.
+	gcCPULimiter.finishGCTransition(now)
+
+	// Finish the current heap profiling cycle and start a new
+	// heap profiling cycle. We do this before starting the world
+	// so events don't leak into the wrong cycle.
+	mProf_NextCycle()
+
+	// There may be stale spans in mcaches that need to be swept.
+	// Those aren't tracked in any sweep lists, so we need to
+	// count them against sweep completion until we ensure all
+	// those spans have been forced out.
+	sl := sweep.active.begin()
+	if !sl.valid {
+		throw("failed to set sweep barrier")
+	}
+
+	systemstack(func() { startTheWorldWithSema() })
+
+	// Flush the heap profile so we can start a new cycle next GC.
+	// This is relatively expensive, so we don't do it with the
+	// world stopped.
+	mProf_Flush()
+
+	// Prepare workbufs for freeing by the sweeper. We do this
+	// asynchronously because it can take non-trivial time.
+	prepareFreeWorkbufs()
+
+	// Free stack spans. This must be done between GC cycles.
+	systemstack(freeStackSpans)
+
+	// Ensure all mcaches are flushed. Each P will flush its own
+	// mcache before allocating, but idle Ps may not. Since this
+	// is necessary to sweep all spans, we need to ensure all
+	// mcaches are flushed before we start the next GC cycle.
+	//
+	// While we're here, flush the page cache for idle Ps to avoid
+	// having pages get stuck on them. These pages are hidden from
+	// the scavenger, so in small idle heaps a significant amount
+	// of additional memory might be held onto.
+	//
+	// Also, flush the pinner cache, to avoid leaking that memory
+	// indefinitely.
+	systemstack(func() {
+		forEachP(func(pp *p) {
+			pp.mcache.prepareForSweep()
+			if pp.status == _Pidle {
+				systemstack(func() {
+					lock(&mheap_.lock)
+					pp.pcache.flush(&mheap_.pages)
+					unlock(&mheap_.lock)
+				})
+			}
+			pp.pinnerCache = nil
+		})
+	})
+	// Now that we've swept stale spans in mcaches, they don't
+	// count against unswept spans.
+	sweep.active.end(sl)
+
+	// Print gctrace before dropping worldsema. As soon as we drop
+	// worldsema another cycle could start and smash the stats
+	// we're trying to print.
+	if debug.gctrace > 0 {
+		util := int(memstats.gc_cpu_fraction * 100)
+
+		var sbuf [24]byte
+		printlock()
+		print("gc ", memstats.numgc,
+			" @", string(itoaDiv(sbuf[:], uint64(work.tSweepTerm-runtimeInitTime)/1e6, 3)), "s ",
+			util, "%: ")
+		prev := work.tSweepTerm
+		for i, ns := range []int64{work.tMark, work.tMarkTerm, work.tEnd} {
+			if i != 0 {
+				print("+")
+			}
+			print(string(fmtNSAsMS(sbuf[:], uint64(ns-prev))))
+			prev = ns
+		}
+		print(" ms clock, ")
+		for i, ns := range []int64{
+			int64(work.stwprocs) * (work.tMark - work.tSweepTerm),
+			gcController.assistTime.Load(),
+			gcController.dedicatedMarkTime.Load() + gcController.fractionalMarkTime.Load(),
+			gcController.idleMarkTime.Load(),
+			markTermCpu,
+		} {
+			if i == 2 || i == 3 {
+				// Separate mark time components with /.
+				print("/")
+			} else if i != 0 {
+				print("+")
+			}
+			print(string(fmtNSAsMS(sbuf[:], uint64(ns))))
+		}
+		print(" ms cpu, ",
+			work.heap0>>20, "->", work.heap1>>20, "->", work.heap2>>20, " MB, ",
+			gcController.lastHeapGoal>>20, " MB goal, ",
+			gcController.lastStackScan.Load()>>20, " MB stacks, ",
+			gcController.globalsScan.Load()>>20, " MB globals, ",
+			work.maxprocs, " P")
+		if work.userForced {
+			print(" (forced)")
+		}
+		print("\n")
+		printunlock()
+	}
+
+	// Set any arena chunks that were deferred to fault.
+	lock(&userArenaState.lock)
+	faultList := userArenaState.fault
+	userArenaState.fault = nil
+	unlock(&userArenaState.lock)
+	for _, lc := range faultList {
+		lc.mspan.setUserArenaChunkToFault()
+	}
+
+	// Enable huge pages on some metadata if we cross a heap threshold.
+	if gcController.heapGoal() > minHeapForMetadataHugePages {
+		systemstack(func() {
+			mheap_.enableMetadataHugePages()
+		})
+	}
+
+	semrelease(&worldsema)
+	semrelease(&gcsema)
+	// Careful: another GC cycle may start now.
+
+	releasem(mp)
+	mp = nil
+
+	// now that gc is done, kick off finalizer thread if needed
+	if !concurrentSweep {
+		// give the queued finalizers, if any, a chance to run
+		Gosched()
+	}
+}
+
+// gcBgMarkStartWorkers prepares background mark worker goroutines. These
+// goroutines will not run until the mark phase, but they must be started while
+// the work is not stopped and from a regular G stack. The caller must hold
+// worldsema.
+func gcBgMarkStartWorkers() {
+	// Background marking is performed by per-P G's. Ensure that each P has
+	// a background GC G.
+	//
+	// Worker Gs don't exit if gomaxprocs is reduced. If it is raised
+	// again, we can reuse the old workers; no need to create new workers.
+	for gcBgMarkWorkerCount < gomaxprocs {
+		go gcBgMarkWorker()
+
+		notetsleepg(&work.bgMarkReady, -1)
+		noteclear(&work.bgMarkReady)
+		// The worker is now guaranteed to be added to the pool before
+		// its P's next findRunnableGCWorker.
+
+		gcBgMarkWorkerCount++
+	}
+}
+
+// gcBgMarkPrepare sets up state for background marking.
+// Mutator assists must not yet be enabled.
+func gcBgMarkPrepare() {
+	// Background marking will stop when the work queues are empty
+	// and there are no more workers (note that, since this is
+	// concurrent, this may be a transient state, but mark
+	// termination will clean it up). Between background workers
+	// and assists, we don't really know how many workers there
+	// will be, so we pretend to have an arbitrarily large number
+	// of workers, almost all of which are "waiting". While a
+	// worker is working it decrements nwait. If nproc == nwait,
+	// there are no workers.
+	work.nproc = ^uint32(0)
+	work.nwait = ^uint32(0)
+}
+
+// gcBgMarkWorkerNode is an entry in the gcBgMarkWorkerPool. It points to a single
+// gcBgMarkWorker goroutine.
+type gcBgMarkWorkerNode struct {
+	// Unused workers are managed in a lock-free stack. This field must be first.
+	node lfnode
+
+	// The g of this worker.
+	gp guintptr
+
+	// Release this m on park. This is used to communicate with the unlock
+	// function, which cannot access the G's stack. It is unused outside of
+	// gcBgMarkWorker().
+	m muintptr
+}
+
+func gcBgMarkWorker() {
+	gp := getg()
+
+	// We pass node to a gopark unlock function, so it can't be on
+	// the stack (see gopark). Prevent deadlock from recursively
+	// starting GC by disabling preemption.
+	gp.m.preemptoff = "GC worker init"
+	node := new(gcBgMarkWorkerNode)
+	gp.m.preemptoff = ""
+
+	node.gp.set(gp)
+
+	node.m.set(acquirem())
+	notewakeup(&work.bgMarkReady)
+	// After this point, the background mark worker is generally scheduled
+	// cooperatively by gcController.findRunnableGCWorker. While performing
+	// work on the P, preemption is disabled because we are working on
+	// P-local work buffers. When the preempt flag is set, this puts itself
+	// into _Gwaiting to be woken up by gcController.findRunnableGCWorker
+	// at the appropriate time.
+	//
+	// When preemption is enabled (e.g., while in gcMarkDone), this worker
+	// may be preempted and schedule as a _Grunnable G from a runq. That is
+	// fine; it will eventually gopark again for further scheduling via
+	// findRunnableGCWorker.
+	//
+	// Since we disable preemption before notifying bgMarkReady, we
+	// guarantee that this G will be in the worker pool for the next
+	// findRunnableGCWorker. This isn't strictly necessary, but it reduces
+	// latency between _GCmark starting and the workers starting.
+
+	for {
+		// Go to sleep until woken by
+		// gcController.findRunnableGCWorker.
+		gopark(func(g *g, nodep unsafe.Pointer) bool {
+			node := (*gcBgMarkWorkerNode)(nodep)
+
+			if mp := node.m.ptr(); mp != nil {
+				// The worker G is no longer running; release
+				// the M.
+				//
+				// N.B. it is _safe_ to release the M as soon
+				// as we are no longer performing P-local mark
+				// work.
+				//
+				// However, since we cooperatively stop work
+				// when gp.preempt is set, if we releasem in
+				// the loop then the following call to gopark
+				// would immediately preempt the G. This is
+				// also safe, but inefficient: the G must
+				// schedule again only to enter gopark and park
+				// again. Thus, we defer the release until
+				// after parking the G.
+				releasem(mp)
+			}
+
+			// Release this G to the pool.
+			gcBgMarkWorkerPool.push(&node.node)
+			// Note that at this point, the G may immediately be
+			// rescheduled and may be running.
+			return true
+		}, unsafe.Pointer(node), waitReasonGCWorkerIdle, traceBlockSystemGoroutine, 0)
+
+		// Preemption must not occur here, or another G might see
+		// p.gcMarkWorkerMode.
+
+		// Disable preemption so we can use the gcw. If the
+		// scheduler wants to preempt us, we'll stop draining,
+		// dispose the gcw, and then preempt.
+		node.m.set(acquirem())
+		pp := gp.m.p.ptr() // P can't change with preemption disabled.
+
+		if gcBlackenEnabled == 0 {
+			println("worker mode", pp.gcMarkWorkerMode)
+			throw("gcBgMarkWorker: blackening not enabled")
+		}
+
+		if pp.gcMarkWorkerMode == gcMarkWorkerNotWorker {
+			throw("gcBgMarkWorker: mode not set")
+		}
+
+		startTime := nanotime()
+		pp.gcMarkWorkerStartTime = startTime
+		var trackLimiterEvent bool
+		if pp.gcMarkWorkerMode == gcMarkWorkerIdleMode {
+			trackLimiterEvent = pp.limiterEvent.start(limiterEventIdleMarkWork, startTime)
+		}
+
+		decnwait := atomic.Xadd(&work.nwait, -1)
+		if decnwait == work.nproc {
+			println("runtime: work.nwait=", decnwait, "work.nproc=", work.nproc)
+			throw("work.nwait was > work.nproc")
+		}
+
+		systemstack(func() {
+			// Mark our goroutine preemptible so its stack
+			// can be scanned. This lets two mark workers
+			// scan each other (otherwise, they would
+			// deadlock). We must not modify anything on
+			// the G stack. However, stack shrinking is
+			// disabled for mark workers, so it is safe to
+			// read from the G stack.
+			casGToWaiting(gp, _Grunning, waitReasonGCWorkerActive)
+			switch pp.gcMarkWorkerMode {
+			default:
+				throw("gcBgMarkWorker: unexpected gcMarkWorkerMode")
+			case gcMarkWorkerDedicatedMode:
+				gcDrain(&pp.gcw, gcDrainUntilPreempt|gcDrainFlushBgCredit)
+				if gp.preempt {
+					// We were preempted. This is
+					// a useful signal to kick
+					// everything out of the run
+					// queue so it can run
+					// somewhere else.
+					if drainQ, n := runqdrain(pp); n > 0 {
+						lock(&sched.lock)
+						globrunqputbatch(&drainQ, int32(n))
+						unlock(&sched.lock)
+					}
+				}
+				// Go back to draining, this time
+				// without preemption.
+				gcDrain(&pp.gcw, gcDrainFlushBgCredit)
+			case gcMarkWorkerFractionalMode:
+				gcDrain(&pp.gcw, gcDrainFractional|gcDrainUntilPreempt|gcDrainFlushBgCredit)
+			case gcMarkWorkerIdleMode:
+				gcDrain(&pp.gcw, gcDrainIdle|gcDrainUntilPreempt|gcDrainFlushBgCredit)
+			}
+			casgstatus(gp, _Gwaiting, _Grunning)
+		})
+
+		// Account for time and mark us as stopped.
+		now := nanotime()
+		duration := now - startTime
+		gcController.markWorkerStop(pp.gcMarkWorkerMode, duration)
+		if trackLimiterEvent {
+			pp.limiterEvent.stop(limiterEventIdleMarkWork, now)
+		}
+		if pp.gcMarkWorkerMode == gcMarkWorkerFractionalMode {
+			atomic.Xaddint64(&pp.gcFractionalMarkTime, duration)
+		}
+
+		// Was this the last worker and did we run out
+		// of work?
+		incnwait := atomic.Xadd(&work.nwait, +1)
+		if incnwait > work.nproc {
+			println("runtime: p.gcMarkWorkerMode=", pp.gcMarkWorkerMode,
+				"work.nwait=", incnwait, "work.nproc=", work.nproc)
+			throw("work.nwait > work.nproc")
+		}
+
+		// We'll releasem after this point and thus this P may run
+		// something else. We must clear the worker mode to avoid
+		// attributing the mode to a different (non-worker) G in
+		// traceGoStart.
+		pp.gcMarkWorkerMode = gcMarkWorkerNotWorker
+
+		// If this worker reached a background mark completion
+		// point, signal the main GC goroutine.
+		if incnwait == work.nproc && !gcMarkWorkAvailable(nil) {
+			// We don't need the P-local buffers here, allow
+			// preemption because we may schedule like a regular
+			// goroutine in gcMarkDone (block on locks, etc).
+			releasem(node.m.ptr())
+			node.m.set(nil)
+
+			gcMarkDone()
+		}
+	}
+}
+
+// gcMarkWorkAvailable reports whether executing a mark worker
+// on p is potentially useful. p may be nil, in which case it only
+// checks the global sources of work.
+func gcMarkWorkAvailable(p *p) bool {
+	if p != nil && !p.gcw.empty() {
+		return true
+	}
+	if !work.full.empty() {
+		return true // global work available
+	}
+	if work.markrootNext < work.markrootJobs {
+		return true // root scan work available
+	}
+	return false
+}
+
+// gcMark runs the mark (or, for concurrent GC, mark termination)
+// All gcWork caches must be empty.
+// STW is in effect at this point.
+func gcMark(startTime int64) {
+	if debug.allocfreetrace > 0 {
+		tracegc()
+	}
+
+	if gcphase != _GCmarktermination {
+		throw("in gcMark expecting to see gcphase as _GCmarktermination")
+	}
+	work.tstart = startTime
+
+	// Check that there's no marking work remaining.
+	if work.full != 0 || work.markrootNext < work.markrootJobs {
+		print("runtime: full=", hex(work.full), " next=", work.markrootNext, " jobs=", work.markrootJobs, " nDataRoots=", work.nDataRoots, " nBSSRoots=", work.nBSSRoots, " nSpanRoots=", work.nSpanRoots, " nStackRoots=", work.nStackRoots, "\n")
+		panic("non-empty mark queue after concurrent mark")
+	}
+
+	if debug.gccheckmark > 0 {
+		// This is expensive when there's a large number of
+		// Gs, so only do it if checkmark is also enabled.
+		gcMarkRootCheck()
+	}
+
+	// Drop allg snapshot. allgs may have grown, in which case
+	// this is the only reference to the old backing store and
+	// there's no need to keep it around.
+	work.stackRoots = nil
+
+	// Clear out buffers and double-check that all gcWork caches
+	// are empty. This should be ensured by gcMarkDone before we
+	// enter mark termination.
+	//
+	// TODO: We could clear out buffers just before mark if this
+	// has a non-negligible impact on STW time.
+	for _, p := range allp {
+		// The write barrier may have buffered pointers since
+		// the gcMarkDone barrier. However, since the barrier
+		// ensured all reachable objects were marked, all of
+		// these must be pointers to black objects. Hence we
+		// can just discard the write barrier buffer.
+		if debug.gccheckmark > 0 {
+			// For debugging, flush the buffer and make
+			// sure it really was all marked.
+			wbBufFlush1(p)
+		} else {
+			p.wbBuf.reset()
+		}
+
+		gcw := &p.gcw
+		if !gcw.empty() {
+			printlock()
+			print("runtime: P ", p.id, " flushedWork ", gcw.flushedWork)
+			if gcw.wbuf1 == nil {
+				print(" wbuf1=<nil>")
+			} else {
+				print(" wbuf1.n=", gcw.wbuf1.nobj)
+			}
+			if gcw.wbuf2 == nil {
+				print(" wbuf2=<nil>")
+			} else {
+				print(" wbuf2.n=", gcw.wbuf2.nobj)
+			}
+			print("\n")
+			throw("P has cached GC work at end of mark termination")
+		}
+		// There may still be cached empty buffers, which we
+		// need to flush since we're going to free them. Also,
+		// there may be non-zero stats because we allocated
+		// black after the gcMarkDone barrier.
+		gcw.dispose()
+	}
+
+	// Flush scanAlloc from each mcache since we're about to modify
+	// heapScan directly. If we were to flush this later, then scanAlloc
+	// might have incorrect information.
+	//
+	// Note that it's not important to retain this information; we know
+	// exactly what heapScan is at this point via scanWork.
+	for _, p := range allp {
+		c := p.mcache
+		if c == nil {
+			continue
+		}
+		c.scanAlloc = 0
+	}
+
+	// Reset controller state.
+	gcController.resetLive(work.bytesMarked)
+}
+
+// gcSweep must be called on the system stack because it acquires the heap
+// lock. See mheap for details.
+//
+// The world must be stopped.
+//
+//go:systemstack
+func gcSweep(mode gcMode) {
+	assertWorldStopped()
+
+	if gcphase != _GCoff {
+		throw("gcSweep being done but phase is not GCoff")
+	}
+
+	lock(&mheap_.lock)
+	mheap_.sweepgen += 2
+	sweep.active.reset()
+	mheap_.pagesSwept.Store(0)
+	mheap_.sweepArenas = mheap_.allArenas
+	mheap_.reclaimIndex.Store(0)
+	mheap_.reclaimCredit.Store(0)
+	unlock(&mheap_.lock)
+
+	sweep.centralIndex.clear()
+
+	if !_ConcurrentSweep || mode == gcForceBlockMode {
+		// Special case synchronous sweep.
+		// Record that no proportional sweeping has to happen.
+		lock(&mheap_.lock)
+		mheap_.sweepPagesPerByte = 0
+		unlock(&mheap_.lock)
+		// Sweep all spans eagerly.
+		for sweepone() != ^uintptr(0) {
+			sweep.npausesweep++
+		}
+		// Free workbufs eagerly.
+		prepareFreeWorkbufs()
+		for freeSomeWbufs(false) {
+		}
+		// All "free" events for this mark/sweep cycle have
+		// now happened, so we can make this profile cycle
+		// available immediately.
+		mProf_NextCycle()
+		mProf_Flush()
+		return
+	}
+
+	// Background sweep.
+	lock(&sweep.lock)
+	if sweep.parked {
+		sweep.parked = false
+		ready(sweep.g, 0, true)
+	}
+	unlock(&sweep.lock)
+}
+
+// gcResetMarkState resets global state prior to marking (concurrent
+// or STW) and resets the stack scan state of all Gs.
+//
+// This is safe to do without the world stopped because any Gs created
+// during or after this will start out in the reset state.
+//
+// gcResetMarkState must be called on the system stack because it acquires
+// the heap lock. See mheap for details.
+//
+//go:systemstack
+func gcResetMarkState() {
+	// This may be called during a concurrent phase, so lock to make sure
+	// allgs doesn't change.
+	forEachG(func(gp *g) {
+		gp.gcscandone = false // set to true in gcphasework
+		gp.gcAssistBytes = 0
+	})
+
+	// Clear page marks. This is just 1MB per 64GB of heap, so the
+	// time here is pretty trivial.
+	lock(&mheap_.lock)
+	arenas := mheap_.allArenas
+	unlock(&mheap_.lock)
+	for _, ai := range arenas {
+		ha := mheap_.arenas[ai.l1()][ai.l2()]
+		for i := range ha.pageMarks {
+			ha.pageMarks[i] = 0
+		}
+	}
+
+	work.bytesMarked = 0
+	work.initialHeapLive = gcController.heapLive.Load()
+}
+
+// Hooks for other packages
+
+var poolcleanup func()
+var boringCaches []unsafe.Pointer // for crypto/internal/boring
+
+//go:linkname sync_runtime_registerPoolCleanup sync.runtime_registerPoolCleanup
+func sync_runtime_registerPoolCleanup(f func()) {
+	poolcleanup = f
+}
+
+//go:linkname boring_registerCache crypto/internal/boring/bcache.registerCache
+func boring_registerCache(p unsafe.Pointer) {
+	boringCaches = append(boringCaches, p)
+}
+
+func clearpools() {
+	// clear sync.Pools
+	if poolcleanup != nil {
+		poolcleanup()
+	}
+
+	// clear boringcrypto caches
+	for _, p := range boringCaches {
+		atomicstorep(p, nil)
+	}
+
+	// Clear central sudog cache.
+	// Leave per-P caches alone, they have strictly bounded size.
+	// Disconnect cached list before dropping it on the floor,
+	// so that a dangling ref to one entry does not pin all of them.
+	lock(&sched.sudoglock)
+	var sg, sgnext *sudog
+	for sg = sched.sudogcache; sg != nil; sg = sgnext {
+		sgnext = sg.next
+		sg.next = nil
+	}
+	sched.sudogcache = nil
+	unlock(&sched.sudoglock)
+
+	// Clear central defer pool.
+	// Leave per-P pools alone, they have strictly bounded size.
+	lock(&sched.deferlock)
+	// disconnect cached list before dropping it on the floor,
+	// so that a dangling ref to one entry does not pin all of them.
+	var d, dlink *_defer
+	for d = sched.deferpool; d != nil; d = dlink {
+		dlink = d.link
+		d.link = nil
+	}
+	sched.deferpool = nil
+	unlock(&sched.deferlock)
+}
+
+// Timing
+
+// itoaDiv formats val/(10**dec) into buf.
+func itoaDiv(buf []byte, val uint64, dec int) []byte {
+	i := len(buf) - 1
+	idec := i - dec
+	for val >= 10 || i >= idec {
+		buf[i] = byte(val%10 + '0')
+		i--
+		if i == idec {
+			buf[i] = '.'
+			i--
+		}
+		val /= 10
+	}
+	buf[i] = byte(val + '0')
+	return buf[i:]
+}
+
+// fmtNSAsMS nicely formats ns nanoseconds as milliseconds.
+func fmtNSAsMS(buf []byte, ns uint64) []byte {
+	if ns >= 10e6 {
+		// Format as whole milliseconds.
+		return itoaDiv(buf, ns/1e6, 0)
+	}
+	// Format two digits of precision, with at most three decimal places.
+	x := ns / 1e3
+	if x == 0 {
+		buf[0] = '0'
+		return buf[:1]
+	}
+	dec := 3
+	for x >= 100 {
+		x /= 10
+		dec--
+	}
+	return itoaDiv(buf, x, dec)
+}
+
+// Helpers for testing GC.
+
+// gcTestMoveStackOnNextCall causes the stack to be moved on a call
+// immediately following the call to this. It may not work correctly
+// if any other work appears after this call (such as returning).
+// Typically the following call should be marked go:noinline so it
+// performs a stack check.
+//
+// In rare cases this may not cause the stack to move, specifically if
+// there's a preemption between this call and the next.
+func gcTestMoveStackOnNextCall() {
+	gp := getg()
+	gp.stackguard0 = stackForceMove
+}
+
+// gcTestIsReachable performs a GC and returns a bit set where bit i
+// is set if ptrs[i] is reachable.
+func gcTestIsReachable(ptrs ...unsafe.Pointer) (mask uint64) {
+	// This takes the pointers as unsafe.Pointers in order to keep
+	// them live long enough for us to attach specials. After
+	// that, we drop our references to them.
+
+	if len(ptrs) > 64 {
+		panic("too many pointers for uint64 mask")
+	}
+
+	// Block GC while we attach specials and drop our references
+	// to ptrs. Otherwise, if a GC is in progress, it could mark
+	// them reachable via this function before we have a chance to
+	// drop them.
+	semacquire(&gcsema)
+
+	// Create reachability specials for ptrs.
+	specials := make([]*specialReachable, len(ptrs))
+	for i, p := range ptrs {
+		lock(&mheap_.speciallock)
+		s := (*specialReachable)(mheap_.specialReachableAlloc.alloc())
+		unlock(&mheap_.speciallock)
+		s.special.kind = _KindSpecialReachable
+		if !addspecial(p, &s.special) {
+			throw("already have a reachable special (duplicate pointer?)")
+		}
+		specials[i] = s
+		// Make sure we don't retain ptrs.
+		ptrs[i] = nil
+	}
+
+	semrelease(&gcsema)
+
+	// Force a full GC and sweep.
+	GC()
+
+	// Process specials.
+	for i, s := range specials {
+		if !s.done {
+			printlock()
+			println("runtime: object", i, "was not swept")
+			throw("IsReachable failed")
+		}
+		if s.reachable {
+			mask |= 1 << i
+		}
+		lock(&mheap_.speciallock)
+		mheap_.specialReachableAlloc.free(unsafe.Pointer(s))
+		unlock(&mheap_.speciallock)
+	}
+
+	return mask
+}
+
+// gcTestPointerClass returns the category of what p points to, one of:
+// "heap", "stack", "data", "bss", "other". This is useful for checking
+// that a test is doing what it's intended to do.
+//
+// This is nosplit simply to avoid extra pointer shuffling that may
+// complicate a test.
+//
+//go:nosplit
+func gcTestPointerClass(p unsafe.Pointer) string {
+	p2 := uintptr(noescape(p))
+	gp := getg()
+	if gp.stack.lo <= p2 && p2 < gp.stack.hi {
+		return "stack"
+	}
+	if base, _, _ := findObject(p2, 0, 0); base != 0 {
+		return "heap"
+	}
+	for _, datap := range activeModules() {
+		if datap.data <= p2 && p2 < datap.edata || datap.noptrdata <= p2 && p2 < datap.enoptrdata {
+			return "data"
+		}
+		if datap.bss <= p2 && p2 < datap.ebss || datap.noptrbss <= p2 && p2 <= datap.enoptrbss {
+			return "bss"
+		}
+	}
+	KeepAlive(p)
+	return "other"
+}