[dev.link] all: merge branch 'master' into dev.link

[gostls13.git] / src / runtime / mgcsweep.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: sweeping

// The sweeper consists of two different algorithms:
//
// * The object reclaimer finds and frees unmarked slots in spans. It
//   can free a whole span if none of the objects are marked, but that
//   isn't its goal. This can be driven either synchronously by
//   mcentral.cacheSpan for mcentral spans, or asynchronously by
//   sweepone from the list of all in-use spans in mheap_.sweepSpans.
//
// * The span reclaimer looks for spans that contain no marked objects
//   and frees whole spans. This is a separate algorithm because
//   freeing whole spans is the hardest task for the object reclaimer,
//   but is critical when allocating new spans. The entry point for
//   this is mheap_.reclaim and it's driven by a sequential scan of
//   the page marks bitmap in the heap arenas.
//
// Both algorithms ultimately call mspan.sweep, which sweeps a single
// heap span.

package runtime

import (
        "runtime/internal/atomic"
        "unsafe"
)

var sweep sweepdata

// State of background sweep.
type sweepdata struct {
        lock    mutex
        g       *g
        parked  bool
        started bool

        nbgsweep    uint32
        npausesweep uint32
}

// finishsweep_m ensures that all spans are swept.
//
// The world must be stopped. This ensures there are no sweeps in
// progress.
//
//go:nowritebarrier
func finishsweep_m() {
        // Sweeping must be complete before marking commences, so
        // sweep any unswept spans. If this is a concurrent GC, there
        // shouldn't be any spans left to sweep, so this should finish
        // instantly. If GC was forced before the concurrent sweep
        // finished, there may be spans to sweep.
        for sweepone() != ^uintptr(0) {
                sweep.npausesweep++
        }

        nextMarkBitArenaEpoch()
}

func bgsweep(c chan int) {
        sweep.g = getg()

        lock(&sweep.lock)
        sweep.parked = true
        c <- 1
        goparkunlock(&sweep.lock, waitReasonGCSweepWait, traceEvGoBlock, 1)

        for {
                for sweepone() != ^uintptr(0) {
                        sweep.nbgsweep++
                        Gosched()
                }
                for freeSomeWbufs(true) {
                        Gosched()
                }
                lock(&sweep.lock)
                if !isSweepDone() {
                        // This can happen if a GC runs between
                        // gosweepone returning ^0 above
                        // and the lock being acquired.
                        unlock(&sweep.lock)
                        continue
                }
                sweep.parked = true
                goparkunlock(&sweep.lock, waitReasonGCSweepWait, traceEvGoBlock, 1)
        }
}

// sweepone sweeps some unswept heap span and returns the number of pages returned
// to the heap, or ^uintptr(0) if there was nothing to sweep.
func sweepone() uintptr {
        _g_ := getg()
        sweepRatio := mheap_.sweepPagesPerByte // For debugging

        // increment locks to ensure that the goroutine is not preempted
        // in the middle of sweep thus leaving the span in an inconsistent state for next GC
        _g_.m.locks++
        if atomic.Load(&mheap_.sweepdone) != 0 {
                _g_.m.locks--
                return ^uintptr(0)
        }
        atomic.Xadd(&mheap_.sweepers, +1)

        // Find a span to sweep.
        var s *mspan
        sg := mheap_.sweepgen
        for {
                s = mheap_.sweepSpans[1-sg/2%2].pop()
                if s == nil {
                        atomic.Store(&mheap_.sweepdone, 1)
                        break
                }
                if state := s.state.get(); state != mSpanInUse {
                        // This can happen if direct sweeping already
                        // swept this span, but in that case the sweep
                        // generation should always be up-to-date.
                        if !(s.sweepgen == sg || s.sweepgen == sg+3) {
                                print("runtime: bad span s.state=", state, " s.sweepgen=", s.sweepgen, " sweepgen=", sg, "\n")
                                throw("non in-use span in unswept list")
                        }
                        continue
                }
                if s.sweepgen == sg-2 && atomic.Cas(&s.sweepgen, sg-2, sg-1) {
                        break
                }
        }

        // Sweep the span we found.
        npages := ^uintptr(0)
        if s != nil {
                npages = s.npages
                if s.sweep(false) {
                        // Whole span was freed. Count it toward the
                        // page reclaimer credit since these pages can
                        // now be used for span allocation.
                        atomic.Xadduintptr(&mheap_.reclaimCredit, npages)
                } else {
                        // Span is still in-use, so this returned no
                        // pages to the heap and the span needs to
                        // move to the swept in-use list.
                        npages = 0
                }
        }

        // Decrement the number of active sweepers and if this is the
        // last one print trace information.
        if atomic.Xadd(&mheap_.sweepers, -1) == 0 && atomic.Load(&mheap_.sweepdone) != 0 {
                if debug.gcpacertrace > 0 {
                        print("pacer: sweep done at heap size ", memstats.heap_live>>20, "MB; allocated ", (memstats.heap_live-mheap_.sweepHeapLiveBasis)>>20, "MB during sweep; swept ", mheap_.pagesSwept, " pages at ", sweepRatio, " pages/byte\n")
                }
        }
        _g_.m.locks--
        return npages
}

// isSweepDone reports whether all spans are swept or currently being swept.
//
// Note that this condition may transition from false to true at any
// time as the sweeper runs. It may transition from true to false if a
// GC runs; to prevent that the caller must be non-preemptible or must
// somehow block GC progress.
func isSweepDone() bool {
        return mheap_.sweepdone != 0
}

// Returns only when span s has been swept.
//go:nowritebarrier
func (s *mspan) ensureSwept() {
        // Caller must disable preemption.
        // Otherwise when this function returns the span can become unswept again
        // (if GC is triggered on another goroutine).
        _g_ := getg()
        if _g_.m.locks == 0 && _g_.m.mallocing == 0 && _g_ != _g_.m.g0 {
                throw("mspan.ensureSwept: m is not locked")
        }

        sg := mheap_.sweepgen
        spangen := atomic.Load(&s.sweepgen)
        if spangen == sg || spangen == sg+3 {
                return
        }
        // The caller must be sure that the span is a mSpanInUse span.
        if atomic.Cas(&s.sweepgen, sg-2, sg-1) {
                s.sweep(false)
                return
        }
        // unfortunate condition, and we don't have efficient means to wait
        for {
                spangen := atomic.Load(&s.sweepgen)
                if spangen == sg || spangen == sg+3 {
                        break
                }
                osyield()
        }
}

// Sweep frees or collects finalizers for blocks not marked in the mark phase.
// It clears the mark bits in preparation for the next GC round.
// Returns true if the span was returned to heap.
// If preserve=true, don't return it to heap nor relink in mcentral lists;
// caller takes care of it.
func (s *mspan) sweep(preserve bool) bool {
        // It's critical that we enter this function with preemption disabled,
        // GC must not start while we are in the middle of this function.
        _g_ := getg()
        if _g_.m.locks == 0 && _g_.m.mallocing == 0 && _g_ != _g_.m.g0 {
                throw("mspan.sweep: m is not locked")
        }
        sweepgen := mheap_.sweepgen
        if state := s.state.get(); state != mSpanInUse || s.sweepgen != sweepgen-1 {
                print("mspan.sweep: state=", state, " sweepgen=", s.sweepgen, " mheap.sweepgen=", sweepgen, "\n")
                throw("mspan.sweep: bad span state")
        }

        if trace.enabled {
                traceGCSweepSpan(s.npages * _PageSize)
        }

        atomic.Xadd64(&mheap_.pagesSwept, int64(s.npages))

        spc := s.spanclass
        size := s.elemsize
        res := false

        c := _g_.m.mcache
        freeToHeap := false

        // The allocBits indicate which unmarked objects don't need to be
        // processed since they were free at the end of the last GC cycle
        // and were not allocated since then.
        // If the allocBits index is >= s.freeindex and the bit
        // is not marked then the object remains unallocated
        // since the last GC.
        // This situation is analogous to being on a freelist.

        // Unlink & free special records for any objects we're about to free.
        // Two complications here:
        // 1. An object can have both finalizer and profile special records.
        //    In such case we need to queue finalizer for execution,
        //    mark the object as live and preserve the profile special.
        // 2. A tiny object can have several finalizers setup for different offsets.
        //    If such object is not marked, we need to queue all finalizers at once.
        // Both 1 and 2 are possible at the same time.
        specialp := &s.specials
        special := *specialp
        for special != nil {
                // A finalizer can be set for an inner byte of an object, find object beginning.
                objIndex := uintptr(special.offset) / size
                p := s.base() + objIndex*size
                mbits := s.markBitsForIndex(objIndex)
                if !mbits.isMarked() {
                        // This object is not marked and has at least one special record.
                        // Pass 1: see if it has at least one finalizer.
                        hasFin := false
                        endOffset := p - s.base() + size
                        for tmp := special; tmp != nil && uintptr(tmp.offset) < endOffset; tmp = tmp.next {
                                if tmp.kind == _KindSpecialFinalizer {
                                        // Stop freeing of object if it has a finalizer.
                                        mbits.setMarkedNonAtomic()
                                        hasFin = true
                                        break
                                }
                        }
                        // Pass 2: queue all finalizers _or_ handle profile record.
                        for special != nil && uintptr(special.offset) < endOffset {
                                // Find the exact byte for which the special was setup
                                // (as opposed to object beginning).
                                p := s.base() + uintptr(special.offset)
                                if special.kind == _KindSpecialFinalizer || !hasFin {
                                        // Splice out special record.
                                        y := special
                                        special = special.next
                                        *specialp = special
                                        freespecial(y, unsafe.Pointer(p), size)
                                } else {
                                        // This is profile record, but the object has finalizers (so kept alive).
                                        // Keep special record.
                                        specialp = &special.next
                                        special = *specialp
                                }
                        }
                } else {
                        // object is still live: keep special record
                        specialp = &special.next
                        special = *specialp
                }
        }

        if debug.allocfreetrace != 0 || debug.clobberfree != 0 || raceenabled || msanenabled {
                // Find all newly freed objects. This doesn't have to
                // efficient; allocfreetrace has massive overhead.
                mbits := s.markBitsForBase()
                abits := s.allocBitsForIndex(0)
                for i := uintptr(0); i < s.nelems; i++ {
                        if !mbits.isMarked() && (abits.index < s.freeindex || abits.isMarked()) {
                                x := s.base() + i*s.elemsize
                                if debug.allocfreetrace != 0 {
                                        tracefree(unsafe.Pointer(x), size)
                                }
                                if debug.clobberfree != 0 {
                                        clobberfree(unsafe.Pointer(x), size)
                                }
                                if raceenabled {
                                        racefree(unsafe.Pointer(x), size)
                                }
                                if msanenabled {
                                        msanfree(unsafe.Pointer(x), size)
                                }
                        }
                        mbits.advance()
                        abits.advance()
                }
        }

        // Count the number of free objects in this span.
        nalloc := uint16(s.countAlloc())
        if spc.sizeclass() == 0 && nalloc == 0 {
                s.needzero = 1
                freeToHeap = true
        }
        nfreed := s.allocCount - nalloc
        if nalloc > s.allocCount {
                print("runtime: nelems=", s.nelems, " nalloc=", nalloc, " previous allocCount=", s.allocCount, " nfreed=", nfreed, "\n")
                throw("sweep increased allocation count")
        }

        s.allocCount = nalloc
        wasempty := s.nextFreeIndex() == s.nelems
        s.freeindex = 0 // reset allocation index to start of span.
        if trace.enabled {
                getg().m.p.ptr().traceReclaimed += uintptr(nfreed) * s.elemsize
        }

        // gcmarkBits becomes the allocBits.
        // get a fresh cleared gcmarkBits in preparation for next GC
        s.allocBits = s.gcmarkBits
        s.gcmarkBits = newMarkBits(s.nelems)

        // Initialize alloc bits cache.
        s.refillAllocCache(0)

        // We need to set s.sweepgen = h.sweepgen only when all blocks are swept,
        // because of the potential for a concurrent free/SetFinalizer.
        // But we need to set it before we make the span available for allocation
        // (return it to heap or mcentral), because allocation code assumes that a
        // span is already swept if available for allocation.
        if freeToHeap || nfreed == 0 {
                // The span must be in our exclusive ownership until we update sweepgen,
                // check for potential races.
                if state := s.state.get(); state != mSpanInUse || s.sweepgen != sweepgen-1 {
                        print("mspan.sweep: state=", state, " sweepgen=", s.sweepgen, " mheap.sweepgen=", sweepgen, "\n")
                        throw("mspan.sweep: bad span state after sweep")
                }
                // Serialization point.
                // At this point the mark bits are cleared and allocation ready
                // to go so release the span.
                atomic.Store(&s.sweepgen, sweepgen)
        }

        if nfreed > 0 && spc.sizeclass() != 0 {
                c.local_nsmallfree[spc.sizeclass()] += uintptr(nfreed)
                res = mheap_.central[spc].mcentral.freeSpan(s, preserve, wasempty)
                // mcentral.freeSpan updates sweepgen
        } else if freeToHeap {
                // Free large span to heap

                // NOTE(rsc,dvyukov): The original implementation of efence
                // in CL 22060046 used sysFree instead of sysFault, so that
                // the operating system would eventually give the memory
                // back to us again, so that an efence program could run
                // longer without running out of memory. Unfortunately,
                // calling sysFree here without any kind of adjustment of the
                // heap data structures means that when the memory does
                // come back to us, we have the wrong metadata for it, either in
                // the mspan structures or in the garbage collection bitmap.
                // Using sysFault here means that the program will run out of
                // memory fairly quickly in efence mode, but at least it won't
                // have mysterious crashes due to confused memory reuse.
                // It should be possible to switch back to sysFree if we also
                // implement and then call some kind of mheap.deleteSpan.
                if debug.efence > 0 {
                        s.limit = 0 // prevent mlookup from finding this span
                        sysFault(unsafe.Pointer(s.base()), size)
                } else {
                        mheap_.freeSpan(s)
                }
                c.local_nlargefree++
                c.local_largefree += size
                res = true
        }
        if !res {
                // The span has been swept and is still in-use, so put
                // it on the swept in-use list.
                mheap_.sweepSpans[sweepgen/2%2].push(s)
        }
        return res
}

// deductSweepCredit deducts sweep credit for allocating a span of
// size spanBytes. This must be performed *before* the span is
// allocated to ensure the system has enough credit. If necessary, it
// performs sweeping to prevent going in to debt. If the caller will
// also sweep pages (e.g., for a large allocation), it can pass a
// non-zero callerSweepPages to leave that many pages unswept.
//
// deductSweepCredit makes a worst-case assumption that all spanBytes
// bytes of the ultimately allocated span will be available for object
// allocation.
//
// deductSweepCredit is the core of the "proportional sweep" system.
// It uses statistics gathered by the garbage collector to perform
// enough sweeping so that all pages are swept during the concurrent
// sweep phase between GC cycles.
//
// mheap_ must NOT be locked.
func deductSweepCredit(spanBytes uintptr, callerSweepPages uintptr) {
        if mheap_.sweepPagesPerByte == 0 {
                // Proportional sweep is done or disabled.
                return
        }

        if trace.enabled {
                traceGCSweepStart()
        }

retry:
        sweptBasis := atomic.Load64(&mheap_.pagesSweptBasis)

        // Fix debt if necessary.
        newHeapLive := uintptr(atomic.Load64(&memstats.heap_live)-mheap_.sweepHeapLiveBasis) + spanBytes
        pagesTarget := int64(mheap_.sweepPagesPerByte*float64(newHeapLive)) - int64(callerSweepPages)
        for pagesTarget > int64(atomic.Load64(&mheap_.pagesSwept)-sweptBasis) {
                if sweepone() == ^uintptr(0) {
                        mheap_.sweepPagesPerByte = 0
                        break
                }
                if atomic.Load64(&mheap_.pagesSweptBasis) != sweptBasis {
                        // Sweep pacing changed. Recompute debt.
                        goto retry
                }
        }

        if trace.enabled {
                traceGCSweepDone()
        }
}

// clobberfree sets the memory content at x to bad content, for debugging
// purposes.
func clobberfree(x unsafe.Pointer, size uintptr) {
        // size (span.elemsize) is always a multiple of 4.
        for i := uintptr(0); i < size; i += 4 {
                *(*uint32)(add(x, i)) = 0xdeadbeef
        }
}