// Copyright 2009 The Go Authors. All rights reserved. // Use of this source code is governed by a BSD-style // license that can be found in the LICENSE file. // Garbage collector: marking and scanning package runtime import ( "runtime/internal/atomic" "runtime/internal/sys" "unsafe" ) const ( fixedRootFinalizers = iota fixedRootFreeGStacks fixedRootCount // rootBlockBytes is the number of bytes to scan per data or // BSS root. rootBlockBytes = 256 << 10 // rootBlockSpans is the number of spans to scan per span // root. rootBlockSpans = 8 * 1024 // 64MB worth of spans // maxObletBytes is the maximum bytes of an object to scan at // once. Larger objects will be split up into "oblets" of at // most this size. Since we can scan 1–2 MB/ms, 128 KB bounds // scan preemption at ~100 µs. // // This must be > _MaxSmallSize so that the object base is the // span base. maxObletBytes = 128 << 10 // drainCheckThreshold specifies how many units of work to do // between self-preemption checks in gcDrain. Assuming a scan // rate of 1 MB/ms, this is ~100 µs. Lower values have higher // overhead in the scan loop (the scheduler check may perform // a syscall, so its overhead is nontrivial). Higher values // make the system less responsive to incoming work. drainCheckThreshold = 100000 ) // gcMarkRootPrepare queues root scanning jobs (stacks, globals, and // some miscellany) and initializes scanning-related state. // // The caller must have call gcCopySpans(). // // The world must be stopped. // //go:nowritebarrier func gcMarkRootPrepare() { if gcphase == _GCmarktermination { work.nFlushCacheRoots = int(gomaxprocs) } else { work.nFlushCacheRoots = 0 } // Compute how many data and BSS root blocks there are. nBlocks := func(bytes uintptr) int { return int((bytes + rootBlockBytes - 1) / rootBlockBytes) } work.nDataRoots = 0 work.nBSSRoots = 0 // Only scan globals once per cycle; preferably concurrently. if !work.markrootDone { for _, datap := range activeModules() { nDataRoots := nBlocks(datap.edata - datap.data) if nDataRoots > work.nDataRoots { work.nDataRoots = nDataRoots } } for _, datap := range activeModules() { nBSSRoots := nBlocks(datap.ebss - datap.bss) if nBSSRoots > work.nBSSRoots { work.nBSSRoots = nBSSRoots } } } if !work.markrootDone { // On the first markroot, we need to scan span roots. // In concurrent GC, this happens during concurrent // mark and we depend on addfinalizer to ensure the // above invariants for objects that get finalizers // after concurrent mark. In STW GC, this will happen // during mark termination. // // We're only interested in scanning the in-use spans, // which will all be swept at this point. More spans // may be added to this list during concurrent GC, but // we only care about spans that were allocated before // this mark phase. work.nSpanRoots = mheap_.sweepSpans[mheap_.sweepgen/2%2].numBlocks() // On the first markroot, we need to scan all Gs. Gs // may be created after this point, but it's okay that // we ignore them because they begin life without any // roots, so there's nothing to scan, and any roots // they create during the concurrent phase will be // scanned during mark termination. During mark // termination, allglen isn't changing, so we'll scan // all Gs. work.nStackRoots = int(atomic.Loaduintptr(&allglen)) } else { // We've already scanned span roots and kept the scan // up-to-date during concurrent mark. work.nSpanRoots = 0 // The hybrid barrier ensures that stacks can't // contain pointers to unmarked objects, so on the // second markroot, there's no need to scan stacks. work.nStackRoots = 0 if debug.gcrescanstacks > 0 { // Scan stacks anyway for debugging. work.nStackRoots = int(atomic.Loaduintptr(&allglen)) } } work.markrootNext = 0 work.markrootJobs = uint32(fixedRootCount + work.nFlushCacheRoots + work.nDataRoots + work.nBSSRoots + work.nSpanRoots + work.nStackRoots) } // gcMarkRootCheck checks that all roots have been scanned. It is // purely for debugging. func gcMarkRootCheck() { if work.markrootNext < work.markrootJobs { print(work.markrootNext, " of ", work.markrootJobs, " markroot jobs done\n") throw("left over markroot jobs") } lock(&allglock) // Check that stacks have been scanned. var gp *g if gcphase == _GCmarktermination && debug.gcrescanstacks > 0 { for i := 0; i < len(allgs); i++ { gp = allgs[i] if !(gp.gcscandone && gp.gcscanvalid) && readgstatus(gp) != _Gdead { goto fail } } } else { for i := 0; i < work.nStackRoots; i++ { gp = allgs[i] if !gp.gcscandone { goto fail } } } unlock(&allglock) return fail: println("gp", gp, "goid", gp.goid, "status", readgstatus(gp), "gcscandone", gp.gcscandone, "gcscanvalid", gp.gcscanvalid) unlock(&allglock) // Avoid self-deadlock with traceback. throw("scan missed a g") } // ptrmask for an allocation containing a single pointer. var oneptrmask = [...]uint8{1} // markroot scans the i'th root. // // Preemption must be disabled (because this uses a gcWork). // // nowritebarrier is only advisory here. // //go:nowritebarrier func markroot(gcw *gcWork, i uint32) { // TODO(austin): This is a bit ridiculous. Compute and store // the bases in gcMarkRootPrepare instead of the counts. baseFlushCache := uint32(fixedRootCount) baseData := baseFlushCache + uint32(work.nFlushCacheRoots) baseBSS := baseData + uint32(work.nDataRoots) baseSpans := baseBSS + uint32(work.nBSSRoots) baseStacks := baseSpans + uint32(work.nSpanRoots) end := baseStacks + uint32(work.nStackRoots) // Note: if you add a case here, please also update heapdump.go:dumproots. switch { case baseFlushCache <= i && i < baseData: flushmcache(int(i - baseFlushCache)) case baseData <= i && i < baseBSS: for _, datap := range activeModules() { markrootBlock(datap.data, datap.edata-datap.data, datap.gcdatamask.bytedata, gcw, int(i-baseData)) } case baseBSS <= i && i < baseSpans: for _, datap := range activeModules() { markrootBlock(datap.bss, datap.ebss-datap.bss, datap.gcbssmask.bytedata, gcw, int(i-baseBSS)) } case i == fixedRootFinalizers: // Only do this once per GC cycle since we don't call // queuefinalizer during marking. if work.markrootDone { break } for fb := allfin; fb != nil; fb = fb.alllink { cnt := uintptr(atomic.Load(&fb.cnt)) scanblock(uintptr(unsafe.Pointer(&fb.fin[0])), cnt*unsafe.Sizeof(fb.fin[0]), &finptrmask[0], gcw) } case i == fixedRootFreeGStacks: // Only do this once per GC cycle; preferably // concurrently. if !work.markrootDone { // Switch to the system stack so we can call // stackfree. systemstack(markrootFreeGStacks) } case baseSpans <= i && i < baseStacks: // mark MSpan.specials markrootSpans(gcw, int(i-baseSpans)) default: // the rest is scanning goroutine stacks var gp *g if baseStacks <= i && i < end { gp = allgs[i-baseStacks] } else { throw("markroot: bad index") } // remember when we've first observed the G blocked // needed only to output in traceback status := readgstatus(gp) // We are not in a scan state if (status == _Gwaiting || status == _Gsyscall) && gp.waitsince == 0 { gp.waitsince = work.tstart } // scang must be done on the system stack in case // we're trying to scan our own stack. systemstack(func() { // If this is a self-scan, put the user G in // _Gwaiting to prevent self-deadlock. It may // already be in _Gwaiting if this is a mark // worker or we're in mark termination. userG := getg().m.curg selfScan := gp == userG && readgstatus(userG) == _Grunning if selfScan { casgstatus(userG, _Grunning, _Gwaiting) userG.waitreason = waitReasonGarbageCollectionScan } // TODO: scang blocks until gp's stack has // been scanned, which may take a while for // running goroutines. Consider doing this in // two phases where the first is non-blocking: // we scan the stacks we can and ask running // goroutines to scan themselves; and the // second blocks. scang(gp, gcw) if selfScan { casgstatus(userG, _Gwaiting, _Grunning) } }) } } // markrootBlock scans the shard'th shard of the block of memory [b0, // b0+n0), with the given pointer mask. // //go:nowritebarrier func markrootBlock(b0, n0 uintptr, ptrmask0 *uint8, gcw *gcWork, shard int) { if rootBlockBytes%(8*sys.PtrSize) != 0 { // This is necessary to pick byte offsets in ptrmask0. throw("rootBlockBytes must be a multiple of 8*ptrSize") } b := b0 + uintptr(shard)*rootBlockBytes if b >= b0+n0 { return } ptrmask := (*uint8)(add(unsafe.Pointer(ptrmask0), uintptr(shard)*(rootBlockBytes/(8*sys.PtrSize)))) n := uintptr(rootBlockBytes) if b+n > b0+n0 { n = b0 + n0 - b } // Scan this shard. scanblock(b, n, ptrmask, gcw) } // markrootFreeGStacks frees stacks of dead Gs. // // This does not free stacks of dead Gs cached on Ps, but having a few // cached stacks around isn't a problem. // //TODO go:nowritebarrier func markrootFreeGStacks() { // Take list of dead Gs with stacks. lock(&sched.gflock) list := sched.gfreeStack sched.gfreeStack = nil unlock(&sched.gflock) if list == nil { return } // Free stacks. tail := list for gp := list; gp != nil; gp = gp.schedlink.ptr() { shrinkstack(gp) tail = gp } // Put Gs back on the free list. lock(&sched.gflock) tail.schedlink.set(sched.gfreeNoStack) sched.gfreeNoStack = list unlock(&sched.gflock) } // markrootSpans marks roots for one shard of work.spans. // //go:nowritebarrier func markrootSpans(gcw *gcWork, shard int) { // Objects with finalizers have two GC-related invariants: // // 1) Everything reachable from the object must be marked. // This ensures that when we pass the object to its finalizer, // everything the finalizer can reach will be retained. // // 2) Finalizer specials (which are not in the garbage // collected heap) are roots. In practice, this means the fn // field must be scanned. // // TODO(austin): There are several ideas for making this more // efficient in issue #11485. if work.markrootDone { throw("markrootSpans during second markroot") } sg := mheap_.sweepgen spans := mheap_.sweepSpans[mheap_.sweepgen/2%2].block(shard) // Note that work.spans may not include spans that were // allocated between entering the scan phase and now. This is // okay because any objects with finalizers in those spans // must have been allocated and given finalizers after we // entered the scan phase, so addfinalizer will have ensured // the above invariants for them. for _, s := range spans { if s.state != mSpanInUse { continue } if !useCheckmark && s.sweepgen != sg { // sweepgen was updated (+2) during non-checkmark GC pass print("sweep ", s.sweepgen, " ", sg, "\n") throw("gc: unswept span") } // Speculatively check if there are any specials // without acquiring the span lock. This may race with // adding the first special to a span, but in that // case addfinalizer will observe that the GC is // active (which is globally synchronized) and ensure // the above invariants. We may also ensure the // invariants, but it's okay to scan an object twice. if s.specials == nil { continue } // Lock the specials to prevent a special from being // removed from the list while we're traversing it. lock(&s.speciallock) for sp := s.specials; sp != nil; sp = sp.next { if sp.kind != _KindSpecialFinalizer { continue } // don't mark finalized object, but scan it so we // retain everything it points to. spf := (*specialfinalizer)(unsafe.Pointer(sp)) // A finalizer can be set for an inner byte of an object, find object beginning. p := s.base() + uintptr(spf.special.offset)/s.elemsize*s.elemsize // Mark everything that can be reached from // the object (but *not* the object itself or // we'll never collect it). scanobject(p, gcw) // The special itself is a root. scanblock(uintptr(unsafe.Pointer(&spf.fn)), sys.PtrSize, &oneptrmask[0], gcw) } unlock(&s.speciallock) } } // gcAssistAlloc performs GC work to make gp's assist debt positive. // gp must be the calling user gorountine. // // This must be called with preemption enabled. func gcAssistAlloc(gp *g) { // Don't assist in non-preemptible contexts. These are // generally fragile and won't allow the assist to block. if getg() == gp.m.g0 { return } if mp := getg().m; mp.locks > 0 || mp.preemptoff != "" { return } traced := false retry: // Compute the amount of scan work we need to do to make the // balance positive. When the required amount of work is low, // we over-assist to build up credit for future allocations // and amortize the cost of assisting. debtBytes := -gp.gcAssistBytes scanWork := int64(gcController.assistWorkPerByte * float64(debtBytes)) if scanWork < gcOverAssistWork { scanWork = gcOverAssistWork debtBytes = int64(gcController.assistBytesPerWork * float64(scanWork)) } // Steal as much credit as we can from the background GC's // scan credit. This is racy and may drop the background // credit below 0 if two mutators steal at the same time. This // will just cause steals to fail until credit is accumulated // again, so in the long run it doesn't really matter, but we // do have to handle the negative credit case. bgScanCredit := atomic.Loadint64(&gcController.bgScanCredit) stolen := int64(0) if bgScanCredit > 0 { if bgScanCredit < scanWork { stolen = bgScanCredit gp.gcAssistBytes += 1 + int64(gcController.assistBytesPerWork*float64(stolen)) } else { stolen = scanWork gp.gcAssistBytes += debtBytes } atomic.Xaddint64(&gcController.bgScanCredit, -stolen) scanWork -= stolen if scanWork == 0 { // We were able to steal all of the credit we // needed. if traced { traceGCMarkAssistDone() } return } } if trace.enabled && !traced { traced = true traceGCMarkAssistStart() } // Perform assist work systemstack(func() { gcAssistAlloc1(gp, scanWork) // The user stack may have moved, so this can't touch // anything on it until it returns from systemstack. }) completed := gp.param != nil gp.param = nil if completed { gcMarkDone() } if gp.gcAssistBytes < 0 { // We were unable steal enough credit or perform // enough work to pay off the assist debt. We need to // do one of these before letting the mutator allocate // more to prevent over-allocation. // // If this is because we were preempted, reschedule // and try some more. if gp.preempt { Gosched() goto retry } // Add this G to an assist queue and park. When the GC // has more background credit, it will satisfy queued // assists before flushing to the global credit pool. // // Note that this does *not* get woken up when more // work is added to the work list. The theory is that // there wasn't enough work to do anyway, so we might // as well let background marking take care of the // work that is available. if !gcParkAssist() { goto retry } // At this point either background GC has satisfied // this G's assist debt, or the GC cycle is over. } if traced { traceGCMarkAssistDone() } } // gcAssistAlloc1 is the part of gcAssistAlloc that runs on the system // stack. This is a separate function to make it easier to see that // we're not capturing anything from the user stack, since the user // stack may move while we're in this function. // // gcAssistAlloc1 indicates whether this assist completed the mark // phase by setting gp.param to non-nil. This can't be communicated on // the stack since it may move. // //go:systemstack func gcAssistAlloc1(gp *g, scanWork int64) { // Clear the flag indicating that this assist completed the // mark phase. gp.param = nil if atomic.Load(&gcBlackenEnabled) == 0 { // The gcBlackenEnabled check in malloc races with the // store that clears it but an atomic check in every malloc // would be a performance hit. // Instead we recheck it here on the non-preemptable system // stack to determine if we should perform an assist. // GC is done, so ignore any remaining debt. gp.gcAssistBytes = 0 return } // Track time spent in this assist. Since we're on the // system stack, this is non-preemptible, so we can // just measure start and end time. startTime := nanotime() decnwait := atomic.Xadd(&work.nwait, -1) if decnwait == work.nproc { println("runtime: work.nwait =", decnwait, "work.nproc=", work.nproc) throw("nwait > work.nprocs") } // gcDrainN requires the caller to be preemptible. casgstatus(gp, _Grunning, _Gwaiting) gp.waitreason = waitReasonGCAssistMarking // drain own cached work first in the hopes that it // will be more cache friendly. gcw := &getg().m.p.ptr().gcw workDone := gcDrainN(gcw, scanWork) // If we are near the end of the mark phase // dispose of the gcw. if gcBlackenPromptly { gcw.dispose() } casgstatus(gp, _Gwaiting, _Grunning) // Record that we did this much scan work. // // Back out the number of bytes of assist credit that // this scan work counts for. The "1+" is a poor man's // round-up, to ensure this adds credit even if // assistBytesPerWork is very low. gp.gcAssistBytes += 1 + int64(gcController.assistBytesPerWork*float64(workDone)) // If this is the last worker and we ran out of work, // signal a completion point. incnwait := atomic.Xadd(&work.nwait, +1) if incnwait > work.nproc { println("runtime: work.nwait=", incnwait, "work.nproc=", work.nproc, "gcBlackenPromptly=", gcBlackenPromptly) throw("work.nwait > work.nproc") } if incnwait == work.nproc && !gcMarkWorkAvailable(nil) { // This has reached a background completion point. Set // gp.param to a non-nil value to indicate this. It // doesn't matter what we set it to (it just has to be // a valid pointer). gp.param = unsafe.Pointer(gp) } duration := nanotime() - startTime _p_ := gp.m.p.ptr() _p_.gcAssistTime += duration if _p_.gcAssistTime > gcAssistTimeSlack { atomic.Xaddint64(&gcController.assistTime, _p_.gcAssistTime) _p_.gcAssistTime = 0 } } // gcWakeAllAssists wakes all currently blocked assists. This is used // at the end of a GC cycle. gcBlackenEnabled must be false to prevent // new assists from going to sleep after this point. func gcWakeAllAssists() { lock(&work.assistQueue.lock) injectglist(work.assistQueue.head.ptr()) work.assistQueue.head.set(nil) work.assistQueue.tail.set(nil) unlock(&work.assistQueue.lock) } // gcParkAssist puts the current goroutine on the assist queue and parks. // // gcParkAssist returns whether the assist is now satisfied. If it // returns false, the caller must retry the assist. // //go:nowritebarrier func gcParkAssist() bool { lock(&work.assistQueue.lock) // If the GC cycle finished while we were getting the lock, // exit the assist. The cycle can't finish while we hold the // lock. if atomic.Load(&gcBlackenEnabled) == 0 { unlock(&work.assistQueue.lock) return true } gp := getg() oldHead, oldTail := work.assistQueue.head, work.assistQueue.tail if oldHead == 0 { work.assistQueue.head.set(gp) } else { oldTail.ptr().schedlink.set(gp) } work.assistQueue.tail.set(gp) gp.schedlink.set(nil) // Recheck for background credit now that this G is in // the queue, but can still back out. This avoids a // race in case background marking has flushed more // credit since we checked above. if atomic.Loadint64(&gcController.bgScanCredit) > 0 { work.assistQueue.head = oldHead work.assistQueue.tail = oldTail if oldTail != 0 { oldTail.ptr().schedlink.set(nil) } unlock(&work.assistQueue.lock) return false } // Park. goparkunlock(&work.assistQueue.lock, waitReasonGCAssistWait, traceEvGoBlockGC, 2) return true } // gcFlushBgCredit flushes scanWork units of background scan work // credit. This first satisfies blocked assists on the // work.assistQueue and then flushes any remaining credit to // gcController.bgScanCredit. // // Write barriers are disallowed because this is used by gcDrain after // it has ensured that all work is drained and this must preserve that // condition. // //go:nowritebarrierrec func gcFlushBgCredit(scanWork int64) { if work.assistQueue.head == 0 { // Fast path; there are no blocked assists. There's a // small window here where an assist may add itself to // the blocked queue and park. If that happens, we'll // just get it on the next flush. atomic.Xaddint64(&gcController.bgScanCredit, scanWork) return } scanBytes := int64(float64(scanWork) * gcController.assistBytesPerWork) lock(&work.assistQueue.lock) gp := work.assistQueue.head.ptr() for gp != nil && scanBytes > 0 { // Note that gp.gcAssistBytes is negative because gp // is in debt. Think carefully about the signs below. if scanBytes+gp.gcAssistBytes >= 0 { // Satisfy this entire assist debt. scanBytes += gp.gcAssistBytes gp.gcAssistBytes = 0 xgp := gp gp = gp.schedlink.ptr() // It's important that we *not* put xgp in // runnext. Otherwise, it's possible for user // code to exploit the GC worker's high // scheduler priority to get itself always run // before other goroutines and always in the // fresh quantum started by GC. ready(xgp, 0, false) } else { // Partially satisfy this assist. gp.gcAssistBytes += scanBytes scanBytes = 0 // As a heuristic, we move this assist to the // back of the queue so that large assists // can't clog up the assist queue and // substantially delay small assists. xgp := gp gp = gp.schedlink.ptr() if gp == nil { // gp is the only assist in the queue. gp = xgp } else { xgp.schedlink = 0 work.assistQueue.tail.ptr().schedlink.set(xgp) work.assistQueue.tail.set(xgp) } break } } work.assistQueue.head.set(gp) if gp == nil { work.assistQueue.tail.set(nil) } if scanBytes > 0 { // Convert from scan bytes back to work. scanWork = int64(float64(scanBytes) * gcController.assistWorkPerByte) atomic.Xaddint64(&gcController.bgScanCredit, scanWork) } unlock(&work.assistQueue.lock) } // scanstack scans gp's stack, greying all pointers found on the stack. // // scanstack is marked go:systemstack because it must not be preempted // while using a workbuf. // //go:nowritebarrier //go:systemstack func scanstack(gp *g, gcw *gcWork) { if gp.gcscanvalid { return } if readgstatus(gp)&_Gscan == 0 { print("runtime:scanstack: gp=", gp, ", goid=", gp.goid, ", gp->atomicstatus=", hex(readgstatus(gp)), "\n") throw("scanstack - bad status") } switch readgstatus(gp) &^ _Gscan { default: print("runtime: gp=", gp, ", goid=", gp.goid, ", gp->atomicstatus=", readgstatus(gp), "\n") throw("mark - bad status") case _Gdead: return case _Grunning: print("runtime: gp=", gp, ", goid=", gp.goid, ", gp->atomicstatus=", readgstatus(gp), "\n") throw("scanstack: goroutine not stopped") case _Grunnable, _Gsyscall, _Gwaiting: // ok } if gp == getg() { throw("can't scan our own stack") } mp := gp.m if mp != nil && mp.helpgc != 0 { throw("can't scan gchelper stack") } // Shrink the stack if not much of it is being used. During // concurrent GC, we can do this during concurrent mark. if !work.markrootDone { shrinkstack(gp) } // Scan the saved context register. This is effectively a live // register that gets moved back and forth between the // register and sched.ctxt without a write barrier. if gp.sched.ctxt != nil { scanblock(uintptr(unsafe.Pointer(&gp.sched.ctxt)), sys.PtrSize, &oneptrmask[0], gcw) } // Scan the stack. var cache pcvalueCache scanframe := func(frame *stkframe, unused unsafe.Pointer) bool { scanframeworker(frame, &cache, gcw) return true } gentraceback(^uintptr(0), ^uintptr(0), 0, gp, 0, nil, 0x7fffffff, scanframe, nil, 0) tracebackdefers(gp, scanframe, nil) gp.gcscanvalid = true } // Scan a stack frame: local variables and function arguments/results. //go:nowritebarrier func scanframeworker(frame *stkframe, cache *pcvalueCache, gcw *gcWork) { if _DebugGC > 1 && frame.continpc != 0 { print("scanframe ", funcname(frame.fn), "\n") } locals, args := getStackMap(frame, cache, false) // Scan local variables if stack frame has been allocated. if locals.n > 0 { size := uintptr(locals.n) * sys.PtrSize scanblock(frame.varp-size, size, locals.bytedata, gcw) } // Scan arguments. if args.n > 0 { scanblock(frame.argp, uintptr(args.n)*sys.PtrSize, args.bytedata, gcw) } } type gcDrainFlags int const ( gcDrainUntilPreempt gcDrainFlags = 1 << iota gcDrainNoBlock gcDrainFlushBgCredit gcDrainIdle gcDrainFractional // gcDrainBlock means neither gcDrainUntilPreempt or // gcDrainNoBlock. It is the default, but callers should use // the constant for documentation purposes. gcDrainBlock gcDrainFlags = 0 ) // gcDrain scans roots and objects in work buffers, blackening grey // objects until all roots and work buffers have been drained. // // If flags&gcDrainUntilPreempt != 0, gcDrain returns when g.preempt // is set. This implies gcDrainNoBlock. // // If flags&gcDrainIdle != 0, gcDrain returns when there is other work // to do. This implies gcDrainNoBlock. // // If flags&gcDrainFractional != 0, gcDrain self-preempts when // pollFractionalWorkerExit() returns true. This implies // gcDrainNoBlock. // // If flags&gcDrainNoBlock != 0, gcDrain returns as soon as it is // unable to get more work. Otherwise, it will block until all // blocking calls are blocked in gcDrain. // // If flags&gcDrainFlushBgCredit != 0, gcDrain flushes scan work // credit to gcController.bgScanCredit every gcCreditSlack units of // scan work. // //go:nowritebarrier func gcDrain(gcw *gcWork, flags gcDrainFlags) { if !writeBarrier.needed { throw("gcDrain phase incorrect") } gp := getg().m.curg preemptible := flags&gcDrainUntilPreempt != 0 blocking := flags&(gcDrainUntilPreempt|gcDrainIdle|gcDrainFractional|gcDrainNoBlock) == 0 flushBgCredit := flags&gcDrainFlushBgCredit != 0 idle := flags&gcDrainIdle != 0 initScanWork := gcw.scanWork // checkWork is the scan work before performing the next // self-preempt check. checkWork := int64(1<<63 - 1) var check func() bool if flags&(gcDrainIdle|gcDrainFractional) != 0 { checkWork = initScanWork + drainCheckThreshold if idle { check = pollWork } else if flags&gcDrainFractional != 0 { check = pollFractionalWorkerExit } } // Drain root marking jobs. if work.markrootNext < work.markrootJobs { for !(preemptible && gp.preempt) { job := atomic.Xadd(&work.markrootNext, +1) - 1 if job >= work.markrootJobs { break } markroot(gcw, job) if check != nil && check() { goto done } } } // Drain heap marking jobs. for !(preemptible && gp.preempt) { // Try to keep work available on the global queue. We used to // check if there were waiting workers, but it's better to // just keep work available than to make workers wait. In the // worst case, we'll do O(log(_WorkbufSize)) unnecessary // balances. if work.full == 0 { gcw.balance() } var b uintptr if blocking { b = gcw.get() } else { b = gcw.tryGetFast() if b == 0 { b = gcw.tryGet() } } if b == 0 { // work barrier reached or tryGet failed. break } scanobject(b, gcw) // Flush background scan work credit to the global // account if we've accumulated enough locally so // mutator assists can draw on it. if gcw.scanWork >= gcCreditSlack { atomic.Xaddint64(&gcController.scanWork, gcw.scanWork) if flushBgCredit { gcFlushBgCredit(gcw.scanWork - initScanWork) initScanWork = 0 } checkWork -= gcw.scanWork gcw.scanWork = 0 if checkWork <= 0 { checkWork += drainCheckThreshold if check != nil && check() { break } } } } // In blocking mode, write barriers are not allowed after this // point because we must preserve the condition that the work // buffers are empty. done: // Flush remaining scan work credit. if gcw.scanWork > 0 { atomic.Xaddint64(&gcController.scanWork, gcw.scanWork) if flushBgCredit { gcFlushBgCredit(gcw.scanWork - initScanWork) } gcw.scanWork = 0 } } // gcDrainN blackens grey objects until it has performed roughly // scanWork units of scan work or the G is preempted. This is // best-effort, so it may perform less work if it fails to get a work // buffer. Otherwise, it will perform at least n units of work, but // may perform more because scanning is always done in whole object // increments. It returns the amount of scan work performed. // // The caller goroutine must be in a preemptible state (e.g., // _Gwaiting) to prevent deadlocks during stack scanning. As a // consequence, this must be called on the system stack. // //go:nowritebarrier //go:systemstack func gcDrainN(gcw *gcWork, scanWork int64) int64 { if !writeBarrier.needed { throw("gcDrainN phase incorrect") } // There may already be scan work on the gcw, which we don't // want to claim was done by this call. workFlushed := -gcw.scanWork gp := getg().m.curg for !gp.preempt && workFlushed+gcw.scanWork < scanWork { // See gcDrain comment. if work.full == 0 { gcw.balance() } // This might be a good place to add prefetch code... // if(wbuf.nobj > 4) { // PREFETCH(wbuf->obj[wbuf.nobj - 3]; // } // b := gcw.tryGetFast() if b == 0 { b = gcw.tryGet() } if b == 0 { // Try to do a root job. // // TODO: Assists should get credit for this // work. if work.markrootNext < work.markrootJobs { job := atomic.Xadd(&work.markrootNext, +1) - 1 if job < work.markrootJobs { markroot(gcw, job) continue } } // No heap or root jobs. break } scanobject(b, gcw) // Flush background scan work credit. if gcw.scanWork >= gcCreditSlack { atomic.Xaddint64(&gcController.scanWork, gcw.scanWork) workFlushed += gcw.scanWork gcw.scanWork = 0 } } // Unlike gcDrain, there's no need to flush remaining work // here because this never flushes to bgScanCredit and // gcw.dispose will flush any remaining work to scanWork. return workFlushed + gcw.scanWork } // scanblock scans b as scanobject would, but using an explicit // pointer bitmap instead of the heap bitmap. // // This is used to scan non-heap roots, so it does not update // gcw.bytesMarked or gcw.scanWork. // //go:nowritebarrier func scanblock(b0, n0 uintptr, ptrmask *uint8, gcw *gcWork) { // Use local copies of original parameters, so that a stack trace // due to one of the throws below shows the original block // base and extent. b := b0 n := n0 for i := uintptr(0); i < n; { // Find bits for the next word. bits := uint32(*addb(ptrmask, i/(sys.PtrSize*8))) if bits == 0 { i += sys.PtrSize * 8 continue } for j := 0; j < 8 && i < n; j++ { if bits&1 != 0 { // Same work as in scanobject; see comments there. obj := *(*uintptr)(unsafe.Pointer(b + i)) if obj != 0 { if obj, span, objIndex := findObject(obj, b, i); obj != 0 { greyobject(obj, b, i, span, gcw, objIndex) } } } bits >>= 1 i += sys.PtrSize } } } // scanobject scans the object starting at b, adding pointers to gcw. // b must point to the beginning of a heap object or an oblet. // scanobject consults the GC bitmap for the pointer mask and the // spans for the size of the object. // //go:nowritebarrier func scanobject(b uintptr, gcw *gcWork) { // Find the bits for b and the size of the object at b. // // b is either the beginning of an object, in which case this // is the size of the object to scan, or it points to an // oblet, in which case we compute the size to scan below. hbits := heapBitsForAddr(b) s := spanOfUnchecked(b) n := s.elemsize if n == 0 { throw("scanobject n == 0") } if n > maxObletBytes { // Large object. Break into oblets for better // parallelism and lower latency. if b == s.base() { // It's possible this is a noscan object (not // from greyobject, but from other code // paths), in which case we must *not* enqueue // oblets since their bitmaps will be // uninitialized. if s.spanclass.noscan() { // Bypass the whole scan. gcw.bytesMarked += uint64(n) return } // Enqueue the other oblets to scan later. // Some oblets may be in b's scalar tail, but // these will be marked as "no more pointers", // so we'll drop out immediately when we go to // scan those. for oblet := b + maxObletBytes; oblet < s.base()+s.elemsize; oblet += maxObletBytes { if !gcw.putFast(oblet) { gcw.put(oblet) } } } // Compute the size of the oblet. Since this object // must be a large object, s.base() is the beginning // of the object. n = s.base() + s.elemsize - b if n > maxObletBytes { n = maxObletBytes } } var i uintptr for i = 0; i < n; i += sys.PtrSize { // Find bits for this word. if i != 0 { // Avoid needless hbits.next() on last iteration. hbits = hbits.next() } // Load bits once. See CL 22712 and issue 16973 for discussion. bits := hbits.bits() // During checkmarking, 1-word objects store the checkmark // in the type bit for the one word. The only one-word objects // are pointers, or else they'd be merged with other non-pointer // data into larger allocations. if i != 1*sys.PtrSize && bits&bitScan == 0 { break // no more pointers in this object } if bits&bitPointer == 0 { continue // not a pointer } // Work here is duplicated in scanblock and above. // If you make changes here, make changes there too. obj := *(*uintptr)(unsafe.Pointer(b + i)) // At this point we have extracted the next potential pointer. // Quickly filter out nil and pointers back to the current object. if obj != 0 && obj-b >= n { // Test if obj points into the Go heap and, if so, // mark the object. // // Note that it's possible for findObject to // fail if obj points to a just-allocated heap // object because of a race with growing the // heap. In this case, we know the object was // just allocated and hence will be marked by // allocation itself. if obj, span, objIndex := findObject(obj, b, i); obj != 0 { greyobject(obj, b, i, span, gcw, objIndex) } } } gcw.bytesMarked += uint64(n) gcw.scanWork += int64(i) } // Shade the object if it isn't already. // The object is not nil and known to be in the heap. // Preemption must be disabled. //go:nowritebarrier func shade(b uintptr) { if obj, span, objIndex := findObject(b, 0, 0); obj != 0 { gcw := &getg().m.p.ptr().gcw greyobject(obj, 0, 0, span, gcw, objIndex) if gcphase == _GCmarktermination || gcBlackenPromptly { // Ps aren't allowed to cache work during mark // termination. gcw.dispose() } } } // obj is the start of an object with mark mbits. // If it isn't already marked, mark it and enqueue into gcw. // base and off are for debugging only and could be removed. // // See also wbBufFlush1, which partially duplicates this logic. // //go:nowritebarrierrec func greyobject(obj, base, off uintptr, span *mspan, gcw *gcWork, objIndex uintptr) { // obj should be start of allocation, and so must be at least pointer-aligned. if obj&(sys.PtrSize-1) != 0 { throw("greyobject: obj not pointer-aligned") } mbits := span.markBitsForIndex(objIndex) if useCheckmark { if !mbits.isMarked() { printlock() print("runtime:greyobject: checkmarks finds unexpected unmarked object obj=", hex(obj), "\n") print("runtime: found obj at *(", hex(base), "+", hex(off), ")\n") // Dump the source (base) object gcDumpObject("base", base, off) // Dump the object gcDumpObject("obj", obj, ^uintptr(0)) getg().m.traceback = 2 throw("checkmark found unmarked object") } hbits := heapBitsForAddr(obj) if hbits.isCheckmarked(span.elemsize) { return } hbits.setCheckmarked(span.elemsize) if !hbits.isCheckmarked(span.elemsize) { throw("setCheckmarked and isCheckmarked disagree") } } else { if debug.gccheckmark > 0 && span.isFree(objIndex) { print("runtime: marking free object ", hex(obj), " found at *(", hex(base), "+", hex(off), ")\n") gcDumpObject("base", base, off) gcDumpObject("obj", obj, ^uintptr(0)) getg().m.traceback = 2 throw("marking free object") } // If marked we have nothing to do. if mbits.isMarked() { return } // mbits.setMarked() // Avoid extra call overhead with manual inlining. atomic.Or8(mbits.bytep, mbits.mask) // If this is a noscan object, fast-track it to black // instead of greying it. if span.spanclass.noscan() { gcw.bytesMarked += uint64(span.elemsize) return } } // Queue the obj for scanning. The PREFETCH(obj) logic has been removed but // seems like a nice optimization that can be added back in. // There needs to be time between the PREFETCH and the use. // Previously we put the obj in an 8 element buffer that is drained at a rate // to give the PREFETCH time to do its work. // Use of PREFETCHNTA might be more appropriate than PREFETCH if !gcw.putFast(obj) { gcw.put(obj) } } // gcDumpObject dumps the contents of obj for debugging and marks the // field at byte offset off in obj. func gcDumpObject(label string, obj, off uintptr) { s := spanOf(obj) print(label, "=", hex(obj)) if s == nil { print(" s=nil\n") return } print(" s.base()=", hex(s.base()), " s.limit=", hex(s.limit), " s.spanclass=", s.spanclass, " s.elemsize=", s.elemsize, " s.state=") if 0 <= s.state && int(s.state) < len(mSpanStateNames) { print(mSpanStateNames[s.state], "\n") } else { print("unknown(", s.state, ")\n") } skipped := false size := s.elemsize if s.state == _MSpanManual && size == 0 { // We're printing something from a stack frame. We // don't know how big it is, so just show up to an // including off. size = off + sys.PtrSize } for i := uintptr(0); i < size; i += sys.PtrSize { // For big objects, just print the beginning (because // that usually hints at the object's type) and the // fields around off. if !(i < 128*sys.PtrSize || off-16*sys.PtrSize < i && i < off+16*sys.PtrSize) { skipped = true continue } if skipped { print(" ...\n") skipped = false } print(" *(", label, "+", i, ") = ", hex(*(*uintptr)(unsafe.Pointer(obj + i)))) if i == off { print(" <==") } print("\n") } if skipped { print(" ...\n") } } // gcmarknewobject marks a newly allocated object black. obj must // not contain any non-nil pointers. // // This is nosplit so it can manipulate a gcWork without preemption. // //go:nowritebarrier //go:nosplit func gcmarknewobject(obj, size, scanSize uintptr) { if useCheckmark && !gcBlackenPromptly { // The world should be stopped so this should not happen. throw("gcmarknewobject called while doing checkmark") } markBitsForAddr(obj).setMarked() gcw := &getg().m.p.ptr().gcw gcw.bytesMarked += uint64(size) gcw.scanWork += int64(scanSize) if gcBlackenPromptly { // There shouldn't be anything in the work queue, but // we still need to flush stats. gcw.dispose() } } // gcMarkTinyAllocs greys all active tiny alloc blocks. // // The world must be stopped. func gcMarkTinyAllocs() { for _, p := range allp { c := p.mcache if c == nil || c.tiny == 0 { continue } _, span, objIndex := findObject(c.tiny, 0, 0) gcw := &p.gcw greyobject(c.tiny, 0, 0, span, gcw, objIndex) if gcBlackenPromptly { gcw.dispose() } } } // Checkmarking // To help debug the concurrent GC we remark with the world // stopped ensuring that any object encountered has their normal // mark bit set. To do this we use an orthogonal bit // pattern to indicate the object is marked. The following pattern // uses the upper two bits in the object's boundary nibble. // 01: scalar not marked // 10: pointer not marked // 11: pointer marked // 00: scalar marked // Xoring with 01 will flip the pattern from marked to unmarked and vica versa. // The higher bit is 1 for pointers and 0 for scalars, whether the object // is marked or not. // The first nibble no longer holds the typeDead pattern indicating that the // there are no more pointers in the object. This information is held // in the second nibble. // If useCheckmark is true, marking of an object uses the // checkmark bits (encoding above) instead of the standard // mark bits. var useCheckmark = false //go:nowritebarrier func initCheckmarks() { useCheckmark = true for _, s := range mheap_.allspans { if s.state == _MSpanInUse { heapBitsForAddr(s.base()).initCheckmarkSpan(s.layout()) } } } func clearCheckmarks() { useCheckmark = false for _, s := range mheap_.allspans { if s.state == _MSpanInUse { heapBitsForAddr(s.base()).clearCheckmarkSpan(s.layout()) } } }