[gostls13.git] / src / runtime / mgc.go

// 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
        _FinBlockSize = 4 * 1024

        // concurrentSweep is a debug flag. Disabling this flag
        // ensures all spans are swept while the world is stopped.
        concurrentSweep = true

        // 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
        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.enabled = gcphase == _GCmark || gcphase == _GCmarktermination
}

// 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) {
                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:
                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) {
        }

        // 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 the 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 to enable assists as early as
        // possible.
        setGCPhase(_GCmark)

        gcBgMarkPrepare() // Must happen before assists are enabled.
        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.
        })

        var stwSwept bool
        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)
                stwSwept = 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)

        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.
        //
        // If gcSweep fully swept the heap (for example if the sweep
        // is not concurrent due to a GODEBUG setting), then we expect
        // the sweepLocker to be invalid, since sweeping is done.
        //
        // N.B. Below we might duplicate some work from gcSweep; this is
        // fine as all that work is idempotent within a GC cycle, and
        // we're still holding worldsema so a new cycle can't start.
        sl := sweep.active.begin()
        if !stwSwept && !sl.valid {
                throw("failed to set sweep barrier")
        } else if stwSwept && sl.valid {
                throw("non-concurrent sweep failed to drain all sweep queues")
        }

        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
                })
        })
        if sl.valid {
                // Now that we've swept stale spans in mcaches, they don't
                // count against unswept spans.
                //
                // Note: this sweepLocker may not be valid if sweeping had
                // already completed during the STW. See the corresponding
                // begin() call that produced sl.
                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 {
                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:
                                gcDrainMarkWorkerDedicated(&pp.gcw, true)
                                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.
                                gcDrainMarkWorkerDedicated(&pp.gcw, false)
                        case gcMarkWorkerFractionalMode:
                                gcDrainMarkWorkerFractional(&pp.gcw)
                        case gcMarkWorkerIdleMode:
                                gcDrainMarkWorkerIdle(&pp.gcw)
                        }
                        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.
//
// Returns true if the heap was fully swept by this function.
//
// The world must be stopped.
//
//go:systemstack
func gcSweep(mode gcMode) bool {
        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)
                // Flush all mcaches.
                for _, pp := range allp {
                        pp.mcache.prepareForSweep()
                }
                // Sweep all spans eagerly.
                for sweepone() != ^uintptr(0) {
                }
                // 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 true
        }

        // Background sweep.
        lock(&sweep.lock)
        if sweep.parked {
                sweep.parked = false
                ready(sweep.g, 0, true)
        }
        unlock(&sweep.lock)
        return false
}

// 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"
}