runtime: abstract specials list iteration

[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, which looks at all the mcentral lists.
//
// * 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

        // centralIndex is the current unswept span class.
        // It represents an index into the mcentral span
        // sets. Accessed and updated via its load and
        // update methods. Not protected by a lock.
        //
        // Reset at mark termination.
        // Used by mheap.nextSpanForSweep.
        centralIndex sweepClass
}

// sweepClass is a spanClass and one bit to represent whether we're currently
// sweeping partial or full spans.
type sweepClass uint32

const (
        numSweepClasses            = numSpanClasses * 2
        sweepClassDone  sweepClass = sweepClass(^uint32(0))
)

func (s *sweepClass) load() sweepClass {
        return sweepClass(atomic.Load((*uint32)(s)))
}

func (s *sweepClass) update(sNew sweepClass) {
        // Only update *s if its current value is less than sNew,
        // since *s increases monotonically.
        sOld := s.load()
        for sOld < sNew && !atomic.Cas((*uint32)(s), uint32(sOld), uint32(sNew)) {
                sOld = s.load()
        }
        // TODO(mknyszek): This isn't the only place we have
        // an atomic monotonically increasing counter. It would
        // be nice to have an "atomic max" which is just implemented
        // as the above on most architectures. Some architectures
        // like RISC-V however have native support for an atomic max.
}

func (s *sweepClass) clear() {
        atomic.Store((*uint32)(s), 0)
}

// split returns the underlying span class as well as
// whether we're interested in the full or partial
// unswept lists for that class, indicated as a boolean
// (true means "full").
func (s sweepClass) split() (spc spanClass, full bool) {
        return spanClass(s >> 1), s&1 == 0
}

// nextSpanForSweep finds and pops the next span for sweeping from the
// central sweep buffers. It returns ownership of the span to the caller.
// Returns nil if no such span exists.
func (h *mheap) nextSpanForSweep() *mspan {
        sg := h.sweepgen
        for sc := sweep.centralIndex.load(); sc < numSweepClasses; sc++ {
                spc, full := sc.split()
                c := &h.central[spc].mcentral
                var s *mspan
                if full {
                        s = c.fullUnswept(sg).pop()
                } else {
                        s = c.partialUnswept(sg).pop()
                }
                if s != nil {
                        // Write down that we found something so future sweepers
                        // can start from here.
                        sweep.centralIndex.update(sc)
                        return s
                }
        }
        // Write down that we found nothing.
        sweep.centralIndex.update(sweepClassDone)
        return nil
}

// 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() {
        assertWorldStopped()

        // 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++
        }

        // Reset all the unswept buffers, which should be empty.
        // Do this in sweep termination as opposed to mark termination
        // so that we can catch unswept spans and reclaim blocks as
        // soon as possible.
        sg := mheap_.sweepgen
        for i := range mheap_.central {
                c := &mheap_.central[i].mcentral
                c.partialUnswept(sg).reset()
                c.fullUnswept(sg).reset()
        }

        // Sweeping is done, so if the scavenger isn't already awake,
        // wake it up. There's definitely work for it to do at this
        // point.
        wakeScavenger()

        nextMarkBitArenaEpoch()
}

func bgsweep() {
        sweep.g = getg()

        lockInit(&sweep.lock, lockRankSweep)
        lock(&sweep.lock)
        sweep.parked = true
        gcenable_setup <- 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_.nextSpanForSweep()
                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 {
                // Since the sweeper is done, move the scavenge gen forward (signalling
                // that there's new work to do) and wake the scavenger.
                //
                // The scavenger is signaled by the last sweeper because once
                // sweeping is done, we will definitely have useful work for
                // the scavenger to do, since the scavenger only runs over the
                // heap once per GC cyle. This update is not done during sweep
                // termination because in some cases there may be a long delay
                // between sweep done and sweep termination (e.g. not enough
                // allocations to trigger a GC) which would be nice to fill in
                // with scavenging work.
                systemstack(func() {
                        lock(&mheap_.lock)
                        mheap_.pages.scavengeStartGen()
                        unlock(&mheap_.lock)
                })
                // Since we might sweep in an allocation path, it's not possible
                // for us to wake the scavenger directly via wakeScavenger, since
                // it could allocate. Ask sysmon to do it for us instead.
                readyForScavenger()

                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

        // 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.
        hadSpecials := s.specials != nil
        siter := newSpecialsIter(s)
        for siter.valid() {
                // A finalizer can be set for an inner byte of an object, find object beginning.
                objIndex := uintptr(siter.s.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 := siter.s; 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 siter.valid() && uintptr(siter.s.offset) < endOffset {
                                // Find the exact byte for which the special was setup
                                // (as opposed to object beginning).
                                special := siter.s
                                p := s.base() + uintptr(special.offset)
                                if special.kind == _KindSpecialFinalizer || !hasFin {
                                        siter.unlinkAndNext()
                                        freeSpecial(special, unsafe.Pointer(p), size)
                                } else {
                                        // This is profile record, but the object has finalizers (so kept alive).
                                        // Keep special record.
                                        siter.next()
                                }
                        }
                } else {
                        // object is still live: keep special record
                        siter.next()
                }
        }
        if hadSpecials && s.specials == nil {
                spanHasNoSpecials(s)
        }

        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()
                }
        }

        // Check for zombie objects.
        if s.freeindex < s.nelems {
                // Everything < freeindex is allocated and hence
                // cannot be zombies.
                //
                // Check the first bitmap byte, where we have to be
                // careful with freeindex.
                obj := s.freeindex
                if (*s.gcmarkBits.bytep(obj / 8)&^*s.allocBits.bytep(obj / 8))>>(obj%8) != 0 {
                        s.reportZombies()
                }
                // Check remaining bytes.
                for i := obj/8 + 1; i < divRoundUp(s.nelems, 8); i++ {
                        if *s.gcmarkBits.bytep(i)&^*s.allocBits.bytep(i) != 0 {
                                s.reportZombies()
                        }
                }
        }

        // Count the number of free objects in this span.
        nalloc := uint16(s.countAlloc())
        nfreed := s.allocCount - nalloc
        if nalloc > s.allocCount {
                // The zombie check above should have caught this in
                // more detail.
                print("runtime: nelems=", s.nelems, " nalloc=", nalloc, " previous allocCount=", s.allocCount, " nfreed=", nfreed, "\n")
                throw("sweep increased allocation count")
        }

        s.allocCount = nalloc
        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)

        // 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")
        }
        if s.sweepgen == sweepgen+1 || s.sweepgen == sweepgen+3 {
                throw("swept cached span")
        }

        // 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.
        //
        // 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 spc.sizeclass() != 0 {
                // Handle spans for small objects.
                if nfreed > 0 {
                        // Only mark the span as needing zeroing if we've freed any
                        // objects, because a fresh span that had been allocated into,
                        // wasn't totally filled, but then swept, still has all of its
                        // free slots zeroed.
                        s.needzero = 1
                        stats := memstats.heapStats.acquire()
                        atomic.Xadduintptr(&stats.smallFreeCount[spc.sizeclass()], uintptr(nfreed))
                        memstats.heapStats.release()
                }
                if !preserve {
                        // The caller may not have removed this span from whatever
                        // unswept set its on but taken ownership of the span for
                        // sweeping by updating sweepgen. If this span still is in
                        // an unswept set, then the mcentral will pop it off the
                        // set, check its sweepgen, and ignore it.
                        if nalloc == 0 {
                                // Free totally free span directly back to the heap.
                                mheap_.freeSpan(s)
                                return true
                        }
                        // Return span back to the right mcentral list.
                        if uintptr(nalloc) == s.nelems {
                                mheap_.central[spc].mcentral.fullSwept(sweepgen).push(s)
                        } else {
                                mheap_.central[spc].mcentral.partialSwept(sweepgen).push(s)
                        }
                }
        } else if !preserve {
                // Handle spans for large objects.
                if nfreed != 0 {
                        // Free large object 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)
                        }
                        stats := memstats.heapStats.acquire()
                        atomic.Xadduintptr(&stats.largeFreeCount, 1)
                        atomic.Xadduintptr(&stats.largeFree, size)
                        memstats.heapStats.release()
                        return true
                }

                // Add a large span directly onto the full+swept list.
                mheap_.central[spc].mcentral.fullSwept(sweepgen).push(s)
        }
        return false
}

// reportZombies reports any marked but free objects in s and throws.
//
// This generally means one of the following:
//
// 1. User code converted a pointer to a uintptr and then back
// unsafely, and a GC ran while the uintptr was the only reference to
// an object.
//
// 2. User code (or a compiler bug) constructed a bad pointer that
// points to a free slot, often a past-the-end pointer.
//
// 3. The GC two cycles ago missed a pointer and freed a live object,
// but it was still live in the last cycle, so this GC cycle found a
// pointer to that object and marked it.
func (s *mspan) reportZombies() {
        printlock()
        print("runtime: marked free object in span ", s, ", elemsize=", s.elemsize, " freeindex=", s.freeindex, " (bad use of unsafe.Pointer? try -d=checkptr)\n")
        mbits := s.markBitsForBase()
        abits := s.allocBitsForIndex(0)
        for i := uintptr(0); i < s.nelems; i++ {
                addr := s.base() + i*s.elemsize
                print(hex(addr))
                alloc := i < s.freeindex || abits.isMarked()
                if alloc {
                        print(" alloc")
                } else {
                        print(" free ")
                }
                if mbits.isMarked() {
                        print(" marked  ")
                } else {
                        print(" unmarked")
                }
                zombie := mbits.isMarked() && !alloc
                if zombie {
                        print(" zombie")
                }
                print("\n")
                if zombie {
                        length := s.elemsize
                        if length > 1024 {
                                length = 1024
                        }
                        hexdumpWords(addr, addr+length, nil)
                }
                mbits.advance()
                abits.advance()
        }
        throw("found pointer to free object")
}

// 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
        }
}