// 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 ( "internal/abi" "internal/goarch" "internal/goexperiment" "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 // 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 // pagesPerSpanRoot indicates how many pages to scan from a span root // at a time. Used by special root marking. // // Higher values improve throughput by increasing locality, but // increase the minimum latency of a marking operation. // // Must be a multiple of the pageInUse bitmap element size and // must also evenly divide pagesPerArena. pagesPerSpanRoot = 512 ) // gcMarkRootPrepare queues root scanning jobs (stacks, globals, and // some miscellany) and initializes scanning-related state. // // The world must be stopped. func gcMarkRootPrepare() { assertWorldStopped() // Compute how many data and BSS root blocks there are. nBlocks := func(bytes uintptr) int { return int(divRoundUp(bytes, rootBlockBytes)) } work.nDataRoots = 0 work.nBSSRoots = 0 // Scan globals. 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 } } // Scan span roots for finalizer specials. // // We depend on addfinalizer to mark objects that get // finalizers after root marking. // // We're going to scan the whole heap (that was available at the time the // mark phase started, i.e. markArenas) for in-use spans which have specials. // // Break up the work into arenas, and further into chunks. // // Snapshot allArenas as markArenas. This snapshot is safe because allArenas // is append-only. mheap_.markArenas = mheap_.allArenas[:len(mheap_.allArenas):len(mheap_.allArenas)] work.nSpanRoots = len(mheap_.markArenas) * (pagesPerArena / pagesPerSpanRoot) // Scan stacks. // // 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 caught by the write barrier. work.stackRoots = allGsSnapshot() work.nStackRoots = len(work.stackRoots) work.markrootNext = 0 work.markrootJobs = uint32(fixedRootCount + work.nDataRoots + work.nBSSRoots + work.nSpanRoots + work.nStackRoots) // Calculate base indexes of each root type work.baseData = uint32(fixedRootCount) work.baseBSS = work.baseData + uint32(work.nDataRoots) work.baseSpans = work.baseBSS + uint32(work.nBSSRoots) work.baseStacks = work.baseSpans + uint32(work.nSpanRoots) work.baseEnd = work.baseStacks + uint32(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") } // Check that stacks have been scanned. // // We only check the first nStackRoots Gs that we should have scanned. // Since we don't care about newer Gs (see comment in // gcMarkRootPrepare), no locking is required. i := 0 forEachGRace(func(gp *g) { if i >= work.nStackRoots { return } if !gp.gcscandone { println("gp", gp, "goid", gp.goid, "status", readgstatus(gp), "gcscandone", gp.gcscandone) throw("scan missed a g") } i++ }) } // 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). // // Returns the amount of GC work credit produced by the operation. // If flushBgCredit is true, then that credit is also flushed // to the background credit pool. // // nowritebarrier is only advisory here. // //go:nowritebarrier func markroot(gcw *gcWork, i uint32, flushBgCredit bool) int64 { // Note: if you add a case here, please also update heapdump.go:dumproots. var workDone int64 var workCounter *atomic.Int64 switch { case work.baseData <= i && i < work.baseBSS: workCounter = &gcController.globalsScanWork for _, datap := range activeModules() { workDone += markrootBlock(datap.data, datap.edata-datap.data, datap.gcdatamask.bytedata, gcw, int(i-work.baseData)) } case work.baseBSS <= i && i < work.baseSpans: workCounter = &gcController.globalsScanWork for _, datap := range activeModules() { workDone += markrootBlock(datap.bss, datap.ebss-datap.bss, datap.gcbssmask.bytedata, gcw, int(i-work.baseBSS)) } case i == fixedRootFinalizers: 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, nil) } case i == fixedRootFreeGStacks: // Switch to the system stack so we can call // stackfree. systemstack(markrootFreeGStacks) case work.baseSpans <= i && i < work.baseStacks: // mark mspan.specials markrootSpans(gcw, int(i-work.baseSpans)) default: // the rest is scanning goroutine stacks workCounter = &gcController.stackScanWork if i < work.baseStacks || work.baseEnd <= i { printlock() print("runtime: markroot index ", i, " not in stack roots range [", work.baseStacks, ", ", work.baseEnd, ")\n") throw("markroot: bad index") } gp := work.stackRoots[i-work.baseStacks] // 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 } // scanstack 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 { casGToWaiting(userG, _Grunning, waitReasonGarbageCollectionScan) } // TODO: suspendG blocks (and spins) until gp // stops, 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. stopped := suspendG(gp) if stopped.dead { gp.gcscandone = true return } if gp.gcscandone { throw("g already scanned") } workDone += scanstack(gp, gcw) gp.gcscandone = true resumeG(stopped) if selfScan { casgstatus(userG, _Gwaiting, _Grunning) } }) } if workCounter != nil && workDone != 0 { workCounter.Add(workDone) if flushBgCredit { gcFlushBgCredit(workDone) } } return workDone } // markrootBlock scans the shard'th shard of the block of memory [b0, // b0+n0), with the given pointer mask. // // Returns the amount of work done. // //go:nowritebarrier func markrootBlock(b0, n0 uintptr, ptrmask0 *uint8, gcw *gcWork, shard int) int64 { if rootBlockBytes%(8*goarch.PtrSize) != 0 { // This is necessary to pick byte offsets in ptrmask0. throw("rootBlockBytes must be a multiple of 8*ptrSize") } // Note that if b0 is toward the end of the address space, // then b0 + rootBlockBytes might wrap around. // These tests are written to avoid any possible overflow. off := uintptr(shard) * rootBlockBytes if off >= n0 { return 0 } b := b0 + off ptrmask := (*uint8)(add(unsafe.Pointer(ptrmask0), uintptr(shard)*(rootBlockBytes/(8*goarch.PtrSize)))) n := uintptr(rootBlockBytes) if off+n > n0 { n = n0 - off } // Scan this shard. scanblock(b, n, ptrmask, gcw, nil) return int64(n) } // 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. func markrootFreeGStacks() { // Take list of dead Gs with stacks. lock(&sched.gFree.lock) list := sched.gFree.stack sched.gFree.stack = gList{} unlock(&sched.gFree.lock) if list.empty() { return } // Free stacks. q := gQueue{list.head, list.head} for gp := list.head.ptr(); gp != nil; gp = gp.schedlink.ptr() { stackfree(gp.stack) gp.stack.lo = 0 gp.stack.hi = 0 // Manipulate the queue directly since the Gs are // already all linked the right way. q.tail.set(gp) } // Put Gs back on the free list. lock(&sched.gFree.lock) sched.gFree.noStack.pushAll(q) unlock(&sched.gFree.lock) } // markrootSpans marks roots for one shard of markArenas. // //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. sg := mheap_.sweepgen // Find the arena and page index into that arena for this shard. ai := mheap_.markArenas[shard/(pagesPerArena/pagesPerSpanRoot)] ha := mheap_.arenas[ai.l1()][ai.l2()] arenaPage := uint(uintptr(shard) * pagesPerSpanRoot % pagesPerArena) // Construct slice of bitmap which we'll iterate over. specialsbits := ha.pageSpecials[arenaPage/8:] specialsbits = specialsbits[:pagesPerSpanRoot/8] for i := range specialsbits { // Find set bits, which correspond to spans with specials. specials := atomic.Load8(&specialsbits[i]) if specials == 0 { continue } for j := uint(0); j < 8; j++ { if specials&(1< 0 || mp.preemptoff != "" { return } // This extremely verbose boolean indicates whether we've // entered mark assist from the perspective of the tracer. // // In the old tracer, this is just before we call gcAssistAlloc1 // *and* tracing is enabled. Because the old tracer doesn't // do any extra tracking, we need to be careful to not emit an // "end" event if there was no corresponding "begin" for the // mark assist. // // In the new tracer, this is just before we call gcAssistAlloc1 // *regardless* of whether tracing is enabled. This is because // the new tracer allows for tracing to begin (and advance // generations) in the middle of a GC mark phase, so we need to // record some state so that the tracer can pick it up to ensure // a consistent trace result. // // TODO(mknyszek): Hide the details of inMarkAssist in tracer // functions and simplify all the state tracking. This is a lot. enteredMarkAssistForTracing := false retry: if gcCPULimiter.limiting() { // If the CPU limiter is enabled, intentionally don't // assist to reduce the amount of CPU time spent in the GC. if enteredMarkAssistForTracing { trace := traceAcquire() if trace.ok() { trace.GCMarkAssistDone() // Set this *after* we trace the end to make sure // that we emit an in-progress event if this is // the first event for the goroutine in the trace // or trace generation. Also, do this between // acquire/release because this is part of the // goroutine's trace state, and it must be atomic // with respect to the tracer. gp.inMarkAssist = false traceRelease(trace) } else { // This state is tracked even if tracing isn't enabled. // It's only used by the new tracer. // See the comment on enteredMarkAssistForTracing. gp.inMarkAssist = false } } return } // 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. assistWorkPerByte := gcController.assistWorkPerByte.Load() assistBytesPerWork := gcController.assistBytesPerWork.Load() debtBytes := -gp.gcAssistBytes scanWork := int64(assistWorkPerByte * float64(debtBytes)) if scanWork < gcOverAssistWork { scanWork = gcOverAssistWork debtBytes = int64(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 := gcController.bgScanCredit.Load() stolen := int64(0) if bgScanCredit > 0 { if bgScanCredit < scanWork { stolen = bgScanCredit gp.gcAssistBytes += 1 + int64(assistBytesPerWork*float64(stolen)) } else { stolen = scanWork gp.gcAssistBytes += debtBytes } gcController.bgScanCredit.Add(-stolen) scanWork -= stolen if scanWork == 0 { // We were able to steal all of the credit we // needed. if enteredMarkAssistForTracing { trace := traceAcquire() if trace.ok() { trace.GCMarkAssistDone() // Set this *after* we trace the end to make sure // that we emit an in-progress event if this is // the first event for the goroutine in the trace // or trace generation. Also, do this between // acquire/release because this is part of the // goroutine's trace state, and it must be atomic // with respect to the tracer. gp.inMarkAssist = false traceRelease(trace) } else { // This state is tracked even if tracing isn't enabled. // It's only used by the new tracer. // See the comment on enteredMarkAssistForTracing. gp.inMarkAssist = false } } return } } if !enteredMarkAssistForTracing { trace := traceAcquire() if trace.ok() { if !goexperiment.ExecTracer2 { // In the old tracer, enter mark assist tracing only // if we actually traced an event. Otherwise a goroutine // waking up from mark assist post-GC might end up // writing a stray "end" event. // // This means inMarkAssist will not be meaningful // in the old tracer; that's OK, it's unused. // // See the comment on enteredMarkAssistForTracing. enteredMarkAssistForTracing = true } trace.GCMarkAssistStart() // Set this *after* we trace the start, otherwise we may // emit an in-progress event for an assist we're about to start. gp.inMarkAssist = true traceRelease(trace) } else { gp.inMarkAssist = true } if goexperiment.ExecTracer2 { // In the new tracer, set enter mark assist tracing if we // ever pass this point, because we must manage inMarkAssist // correctly. // // See the comment on enteredMarkAssistForTracing. enteredMarkAssistForTracing = true } } // 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 enteredMarkAssistForTracing { trace := traceAcquire() if trace.ok() { trace.GCMarkAssistDone() // Set this *after* we trace the end to make sure // that we emit an in-progress event if this is // the first event for the goroutine in the trace // or trace generation. Also, do this between // acquire/release because this is part of the // goroutine's trace state, and it must be atomic // with respect to the tracer. gp.inMarkAssist = false traceRelease(trace) } else { // This state is tracked even if tracing isn't enabled. // It's only used by the new tracer. // See the comment on enteredMarkAssistForTracing. gp.inMarkAssist = false } } } // 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-preemptible 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. // // Limiter event tracking might be disabled if we end up here // while on a mark worker. startTime := nanotime() trackLimiterEvent := gp.m.p.ptr().limiterEvent.start(limiterEventMarkAssist, startTime) 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. casGToWaiting(gp, _Grunning, 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) 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. assistBytesPerWork := gcController.assistBytesPerWork.Load() gp.gcAssistBytes += 1 + int64(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) 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) } now := nanotime() duration := now - startTime pp := gp.m.p.ptr() pp.gcAssistTime += duration if trackLimiterEvent { pp.limiterEvent.stop(limiterEventMarkAssist, now) } if pp.gcAssistTime > gcAssistTimeSlack { gcController.assistTime.Add(pp.gcAssistTime) gcCPULimiter.update(now) pp.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) list := work.assistQueue.q.popList() injectglist(&list) unlock(&work.assistQueue.lock) } // gcParkAssist puts the current goroutine on the assist queue and parks. // // gcParkAssist reports whether the assist is now satisfied. If it // returns false, the caller must retry the assist. 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() oldList := work.assistQueue.q work.assistQueue.q.pushBack(gp) // 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 gcController.bgScanCredit.Load() > 0 { work.assistQueue.q = oldList if oldList.tail != 0 { oldList.tail.ptr().schedlink.set(nil) } unlock(&work.assistQueue.lock) return false } // Park. goparkunlock(&work.assistQueue.lock, waitReasonGCAssistWait, traceBlockGCMarkAssist, 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.q.empty() { // 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. gcController.bgScanCredit.Add(scanWork) return } assistBytesPerWork := gcController.assistBytesPerWork.Load() scanBytes := int64(float64(scanWork) * assistBytesPerWork) lock(&work.assistQueue.lock) for !work.assistQueue.q.empty() && scanBytes > 0 { gp := work.assistQueue.q.pop() // 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 // It's important that we *not* put gp 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(gp, 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. work.assistQueue.q.pushBack(gp) break } } if scanBytes > 0 { // Convert from scan bytes back to work. assistWorkPerByte := gcController.assistWorkPerByte.Load() scanWork = int64(float64(scanBytes) * assistWorkPerByte) gcController.bgScanCredit.Add(scanWork) } unlock(&work.assistQueue.lock) } // scanstack scans gp's stack, greying all pointers found on the stack. // // Returns the amount of scan work performed, but doesn't update // gcController.stackScanWork or flush any credit. Any background credit produced // by this function should be flushed by its caller. scanstack itself can't // safely flush because it may result in trying to wake up a goroutine that // was just scanned, resulting in a self-deadlock. // // scanstack will also shrink the stack if it is safe to do so. If it // is not, it schedules a stack shrink for the next synchronous safe // point. // // 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) int64 { 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 0 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") } // scannedSize is the amount of work we'll be reporting. // // It is less than the allocated size (which is hi-lo). var sp uintptr if gp.syscallsp != 0 { sp = gp.syscallsp // If in a system call this is the stack pointer (gp.sched.sp can be 0 in this case on Windows). } else { sp = gp.sched.sp } scannedSize := gp.stack.hi - sp // Keep statistics for initial stack size calculation. // Note that this accumulates the scanned size, not the allocated size. p := getg().m.p.ptr() p.scannedStackSize += uint64(scannedSize) p.scannedStacks++ if isShrinkStackSafe(gp) { // Shrink the stack if not much of it is being used. shrinkstack(gp) } else { // Otherwise, shrink the stack at the next sync safe point. gp.preemptShrink = true } var state stackScanState state.stack = gp.stack if stackTraceDebug { println("stack trace goroutine", gp.goid) } if debugScanConservative && gp.asyncSafePoint { print("scanning async preempted goroutine ", gp.goid, " stack [", hex(gp.stack.lo), ",", hex(gp.stack.hi), ")\n") } // 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)), goarch.PtrSize, &oneptrmask[0], gcw, &state) } // Scan the stack. Accumulate a list of stack objects. var u unwinder for u.init(gp, 0); u.valid(); u.next() { scanframeworker(&u.frame, &state, gcw) } // Find additional pointers that point into the stack from the heap. // Currently this includes defers and panics. See also function copystack. // Find and trace other pointers in defer records. for d := gp._defer; d != nil; d = d.link { if d.fn != nil { // Scan the func value, which could be a stack allocated closure. // See issue 30453. scanblock(uintptr(unsafe.Pointer(&d.fn)), goarch.PtrSize, &oneptrmask[0], gcw, &state) } if d.link != nil { // The link field of a stack-allocated defer record might point // to a heap-allocated defer record. Keep that heap record live. scanblock(uintptr(unsafe.Pointer(&d.link)), goarch.PtrSize, &oneptrmask[0], gcw, &state) } // Retain defers records themselves. // Defer records might not be reachable from the G through regular heap // tracing because the defer linked list might weave between the stack and the heap. if d.heap { scanblock(uintptr(unsafe.Pointer(&d)), goarch.PtrSize, &oneptrmask[0], gcw, &state) } } if gp._panic != nil { // Panics are always stack allocated. state.putPtr(uintptr(unsafe.Pointer(gp._panic)), false) } // Find and scan all reachable stack objects. // // The state's pointer queue prioritizes precise pointers over // conservative pointers so that we'll prefer scanning stack // objects precisely. state.buildIndex() for { p, conservative := state.getPtr() if p == 0 { break } obj := state.findObject(p) if obj == nil { continue } r := obj.r if r == nil { // We've already scanned this object. continue } obj.setRecord(nil) // Don't scan it again. if stackTraceDebug { printlock() print(" live stkobj at", hex(state.stack.lo+uintptr(obj.off)), "of size", obj.size) if conservative { print(" (conservative)") } println() printunlock() } gcdata := r.gcdata() var s *mspan if r.useGCProg() { // This path is pretty unlikely, an object large enough // to have a GC program allocated on the stack. // We need some space to unpack the program into a straight // bitmask, which we allocate/free here. // TODO: it would be nice if there were a way to run a GC // program without having to store all its bits. We'd have // to change from a Lempel-Ziv style program to something else. // Or we can forbid putting objects on stacks if they require // a gc program (see issue 27447). s = materializeGCProg(r.ptrdata(), gcdata) gcdata = (*byte)(unsafe.Pointer(s.startAddr)) } b := state.stack.lo + uintptr(obj.off) if conservative { scanConservative(b, r.ptrdata(), gcdata, gcw, &state) } else { scanblock(b, r.ptrdata(), gcdata, gcw, &state) } if s != nil { dematerializeGCProg(s) } } // Deallocate object buffers. // (Pointer buffers were all deallocated in the loop above.) for state.head != nil { x := state.head state.head = x.next if stackTraceDebug { for i := 0; i < x.nobj; i++ { obj := &x.obj[i] if obj.r == nil { // reachable continue } println(" dead stkobj at", hex(gp.stack.lo+uintptr(obj.off)), "of size", obj.r.size) // Note: not necessarily really dead - only reachable-from-ptr dead. } } x.nobj = 0 putempty((*workbuf)(unsafe.Pointer(x))) } if state.buf != nil || state.cbuf != nil || state.freeBuf != nil { throw("remaining pointer buffers") } return int64(scannedSize) } // Scan a stack frame: local variables and function arguments/results. // //go:nowritebarrier func scanframeworker(frame *stkframe, state *stackScanState, gcw *gcWork) { if _DebugGC > 1 && frame.continpc != 0 { print("scanframe ", funcname(frame.fn), "\n") } isAsyncPreempt := frame.fn.valid() && frame.fn.funcID == abi.FuncID_asyncPreempt isDebugCall := frame.fn.valid() && frame.fn.funcID == abi.FuncID_debugCallV2 if state.conservative || isAsyncPreempt || isDebugCall { if debugScanConservative { println("conservatively scanning function", funcname(frame.fn), "at PC", hex(frame.continpc)) } // Conservatively scan the frame. Unlike the precise // case, this includes the outgoing argument space // since we may have stopped while this function was // setting up a call. // // TODO: We could narrow this down if the compiler // produced a single map per function of stack slots // and registers that ever contain a pointer. if frame.varp != 0 { size := frame.varp - frame.sp if size > 0 { scanConservative(frame.sp, size, nil, gcw, state) } } // Scan arguments to this frame. if n := frame.argBytes(); n != 0 { // TODO: We could pass the entry argument map // to narrow this down further. scanConservative(frame.argp, n, nil, gcw, state) } if isAsyncPreempt || isDebugCall { // This function's frame contained the // registers for the asynchronously stopped // parent frame. Scan the parent // conservatively. state.conservative = true } else { // We only wanted to scan those two frames // conservatively. Clear the flag for future // frames. state.conservative = false } return } locals, args, objs := frame.getStackMap(false) // Scan local variables if stack frame has been allocated. if locals.n > 0 { size := uintptr(locals.n) * goarch.PtrSize scanblock(frame.varp-size, size, locals.bytedata, gcw, state) } // Scan arguments. if args.n > 0 { scanblock(frame.argp, uintptr(args.n)*goarch.PtrSize, args.bytedata, gcw, state) } // Add all stack objects to the stack object list. if frame.varp != 0 { // varp is 0 for defers, where there are no locals. // In that case, there can't be a pointer to its args, either. // (And all args would be scanned above anyway.) for i := range objs { obj := &objs[i] off := obj.off base := frame.varp // locals base pointer if off >= 0 { base = frame.argp // arguments and return values base pointer } ptr := base + uintptr(off) if ptr < frame.sp { // object hasn't been allocated in the frame yet. continue } if stackTraceDebug { println("stkobj at", hex(ptr), "of size", obj.size) } state.addObject(ptr, obj) } } } type gcDrainFlags int const ( gcDrainUntilPreempt gcDrainFlags = 1 << iota gcDrainFlushBgCredit gcDrainIdle gcDrainFractional ) // gcDrainMarkWorkerIdle is a wrapper for gcDrain that exists to better account // mark time in profiles. func gcDrainMarkWorkerIdle(gcw *gcWork) { gcDrain(gcw, gcDrainIdle|gcDrainUntilPreempt|gcDrainFlushBgCredit) } // gcDrainMarkWorkerDedicated is a wrapper for gcDrain that exists to better account // mark time in profiles. func gcDrainMarkWorkerDedicated(gcw *gcWork, untilPreempt bool) { flags := gcDrainFlushBgCredit if untilPreempt { flags |= gcDrainUntilPreempt } gcDrain(gcw, flags) } // gcDrainMarkWorkerFractional is a wrapper for gcDrain that exists to better account // mark time in profiles. func gcDrainMarkWorkerFractional(gcw *gcWork) { gcDrain(gcw, gcDrainFractional|gcDrainUntilPreempt|gcDrainFlushBgCredit) } // gcDrain scans roots and objects in work buffers, blackening grey // objects until it is unable to get more work. It may return before // GC is done; it's the caller's responsibility to balance work from // other Ps. // // If flags&gcDrainUntilPreempt != 0, gcDrain returns when g.preempt // is set. // // If flags&gcDrainIdle != 0, gcDrain returns when there is other work // to do. // // If flags&gcDrainFractional != 0, gcDrain self-preempts when // pollFractionalWorkerExit() returns true. This implies // gcDrainNoBlock. // // If flags&gcDrainFlushBgCredit != 0, gcDrain flushes scan work // credit to gcController.bgScanCredit every gcCreditSlack units of // scan work. // // gcDrain will always return if there is a pending STW or forEachP. // // Disabling write barriers is necessary to ensure that after we've // confirmed that we've drained gcw, that we don't accidentally end // up flipping that condition by immediately adding work in the form // of a write barrier buffer flush. // // Don't set nowritebarrierrec because it's safe for some callees to // have write barriers enabled. // //go:nowritebarrier func gcDrain(gcw *gcWork, flags gcDrainFlags) { if !writeBarrier.enabled { throw("gcDrain phase incorrect") } // N.B. We must be running in a non-preemptible context, so it's // safe to hold a reference to our P here. gp := getg().m.curg pp := gp.m.p.ptr() preemptible := flags&gcDrainUntilPreempt != 0 flushBgCredit := flags&gcDrainFlushBgCredit != 0 idle := flags&gcDrainIdle != 0 initScanWork := gcw.heapScanWork // 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 { // Stop if we're preemptible, if someone wants to STW, or if // someone is calling forEachP. for !(gp.preempt && (preemptible || sched.gcwaiting.Load() || pp.runSafePointFn != 0)) { job := atomic.Xadd(&work.markrootNext, +1) - 1 if job >= work.markrootJobs { break } markroot(gcw, job, flushBgCredit) if check != nil && check() { goto done } } } // Drain heap marking jobs. // // Stop if we're preemptible, if someone wants to STW, or if // someone is calling forEachP. // // TODO(mknyszek): Consider always checking gp.preempt instead // of having the preempt flag, and making an exception for certain // mark workers in retake. That might be simpler than trying to // enumerate all the reasons why we might want to preempt, even // if we're supposed to be mostly non-preemptible. for !(gp.preempt && (preemptible || sched.gcwaiting.Load() || pp.runSafePointFn != 0)) { // 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() } b := gcw.tryGetFast() if b == 0 { b = gcw.tryGet() if b == 0 { // Flush the write barrier // buffer; this may create // more work. wbBufFlush() b = gcw.tryGet() } } if b == 0 { // Unable to get work. 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.heapScanWork >= gcCreditSlack { gcController.heapScanWork.Add(gcw.heapScanWork) if flushBgCredit { gcFlushBgCredit(gcw.heapScanWork - initScanWork) initScanWork = 0 } checkWork -= gcw.heapScanWork gcw.heapScanWork = 0 if checkWork <= 0 { checkWork += drainCheckThreshold if check != nil && check() { break } } } } done: // Flush remaining scan work credit. if gcw.heapScanWork > 0 { gcController.heapScanWork.Add(gcw.heapScanWork) if flushBgCredit { gcFlushBgCredit(gcw.heapScanWork - initScanWork) } gcw.heapScanWork = 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.enabled { 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.heapScanWork // In addition to backing out because of a preemption, back out // if the GC CPU limiter is enabled. gp := getg().m.curg for !gp.preempt && !gcCPULimiter.limiting() && workFlushed+gcw.heapScanWork < scanWork { // See gcDrain comment. if work.full == 0 { gcw.balance() } b := gcw.tryGetFast() if b == 0 { b = gcw.tryGet() if b == 0 { // Flush the write barrier buffer; // this may create more work. wbBufFlush() b = gcw.tryGet() } } if b == 0 { // Try to do a root job. if work.markrootNext < work.markrootJobs { job := atomic.Xadd(&work.markrootNext, +1) - 1 if job < work.markrootJobs { workFlushed += markroot(gcw, job, false) continue } } // No heap or root jobs. break } scanobject(b, gcw) // Flush background scan work credit. if gcw.heapScanWork >= gcCreditSlack { gcController.heapScanWork.Add(gcw.heapScanWork) workFlushed += gcw.heapScanWork gcw.heapScanWork = 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.heapScanWork } // 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.heapScanWork. // // If stk != nil, possible stack pointers are also reported to stk.putPtr. // //go:nowritebarrier func scanblock(b0, n0 uintptr, ptrmask *uint8, gcw *gcWork, stk *stackScanState) { // 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/(goarch.PtrSize*8))) if bits == 0 { i += goarch.PtrSize * 8 continue } for j := 0; j < 8 && i < n; j++ { if bits&1 != 0 { // Same work as in scanobject; see comments there. p := *(*uintptr)(unsafe.Pointer(b + i)) if p != 0 { if obj, span, objIndex := findObject(p, b, i); obj != 0 { greyobject(obj, b, i, span, gcw, objIndex) } else if stk != nil && p >= stk.stack.lo && p < stk.stack.hi { stk.putPtr(p, false) } } } bits >>= 1 i += goarch.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) { // Prefetch object before we scan it. // // This will overlap fetching the beginning of the object with initial // setup before we start scanning the object. sys.Prefetch(b) // 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. s := spanOfUnchecked(b) n := s.elemsize if n == 0 { throw("scanobject n == 0") } if s.spanclass.noscan() { // Correctness-wise this is ok, but it's inefficient // if noscan objects reach here. throw("scanobject of a noscan object") } var tp typePointers if n > maxObletBytes { // Large object. Break into oblets for better // parallelism and lower latency. if b == s.base() { // 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 n = min(n, maxObletBytes) if goexperiment.AllocHeaders { tp = s.typePointersOfUnchecked(s.base()) tp = tp.fastForward(b-tp.addr, b+n) } } else { if goexperiment.AllocHeaders { tp = s.typePointersOfUnchecked(b) } } var hbits heapBits if !goexperiment.AllocHeaders { hbits = heapBitsForAddr(b, n) } var scanSize uintptr for { var addr uintptr if goexperiment.AllocHeaders { if tp, addr = tp.nextFast(); addr == 0 { if tp, addr = tp.next(b + n); addr == 0 { break } } } else { if hbits, addr = hbits.nextFast(); addr == 0 { if hbits, addr = hbits.next(); addr == 0 { break } } } // Keep track of farthest pointer we found, so we can // update heapScanWork. TODO: is there a better metric, // now that we can skip scalar portions pretty efficiently? scanSize = addr - b + goarch.PtrSize // Work here is duplicated in scanblock and above. // If you make changes here, make changes there too. obj := *(*uintptr)(unsafe.Pointer(addr)) // 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, addr-b); obj != 0 { greyobject(obj, b, addr-b, span, gcw, objIndex) } } } gcw.bytesMarked += uint64(n) gcw.heapScanWork += int64(scanSize) } // scanConservative scans block [b, b+n) conservatively, treating any // pointer-like value in the block as a pointer. // // If ptrmask != nil, only words that are marked in ptrmask are // considered as potential pointers. // // If state != nil, it's assumed that [b, b+n) is a block in the stack // and may contain pointers to stack objects. func scanConservative(b, n uintptr, ptrmask *uint8, gcw *gcWork, state *stackScanState) { if debugScanConservative { printlock() print("conservatively scanning [", hex(b), ",", hex(b+n), ")\n") hexdumpWords(b, b+n, func(p uintptr) byte { if ptrmask != nil { word := (p - b) / goarch.PtrSize bits := *addb(ptrmask, word/8) if (bits>>(word%8))&1 == 0 { return '$' } } val := *(*uintptr)(unsafe.Pointer(p)) if state != nil && state.stack.lo <= val && val < state.stack.hi { return '@' } span := spanOfHeap(val) if span == nil { return ' ' } idx := span.objIndex(val) if span.isFree(idx) { return ' ' } return '*' }) printunlock() } for i := uintptr(0); i < n; i += goarch.PtrSize { if ptrmask != nil { word := i / goarch.PtrSize bits := *addb(ptrmask, word/8) if bits == 0 { // Skip 8 words (the loop increment will do the 8th) // // This must be the first time we've // seen this word of ptrmask, so i // must be 8-word-aligned, but check // our reasoning just in case. if i%(goarch.PtrSize*8) != 0 { throw("misaligned mask") } i += goarch.PtrSize*8 - goarch.PtrSize continue } if (bits>>(word%8))&1 == 0 { continue } } val := *(*uintptr)(unsafe.Pointer(b + i)) // Check if val points into the stack. if state != nil && state.stack.lo <= val && val < state.stack.hi { // val may point to a stack object. This // object may be dead from last cycle and // hence may contain pointers to unallocated // objects, but unlike heap objects we can't // tell if it's already dead. Hence, if all // pointers to this object are from // conservative scanning, we have to scan it // defensively, too. state.putPtr(val, true) continue } // Check if val points to a heap span. span := spanOfHeap(val) if span == nil { continue } // Check if val points to an allocated object. idx := span.objIndex(val) if span.isFree(idx) { continue } // val points to an allocated object. Mark it. obj := span.base() + idx*span.elemsize greyobject(obj, b, i, span, gcw, idx) } } // 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) } } // 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&(goarch.PtrSize-1) != 0 { throw("greyobject: obj not pointer-aligned") } mbits := span.markBitsForIndex(objIndex) if useCheckmark { if setCheckmark(obj, base, off, mbits) { // Already marked. return } } 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() // Mark span. arena, pageIdx, pageMask := pageIndexOf(span.base()) if arena.pageMarks[pageIdx]&pageMask == 0 { atomic.Or8(&arena.pageMarks[pageIdx], pageMask) } // 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 } } // We're adding obj to P's local workbuf, so it's likely // this object will be processed soon by the same P. // Even if the workbuf gets flushed, there will likely still be // some benefit on platforms with inclusive shared caches. sys.Prefetch(obj) // Queue the obj for scanning. 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 state := s.state.get(); 0 <= state && int(state) < len(mSpanStateNames) { print(mSpanStateNames[state], "\n") } else { print("unknown(", state, ")\n") } skipped := false size := s.elemsize if s.state.get() == 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 + goarch.PtrSize } for i := uintptr(0); i < size; i += goarch.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*goarch.PtrSize || off-16*goarch.PtrSize < i && i < off+16*goarch.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(span *mspan, obj uintptr) { if useCheckmark { // The world should be stopped so this should not happen. throw("gcmarknewobject called while doing checkmark") } // Mark object. objIndex := span.objIndex(obj) span.markBitsForIndex(objIndex).setMarked() // Mark span. arena, pageIdx, pageMask := pageIndexOf(span.base()) if arena.pageMarks[pageIdx]&pageMask == 0 { atomic.Or8(&arena.pageMarks[pageIdx], pageMask) } gcw := &getg().m.p.ptr().gcw gcw.bytesMarked += uint64(span.elemsize) } // gcMarkTinyAllocs greys all active tiny alloc blocks. // // The world must be stopped. func gcMarkTinyAllocs() { assertWorldStopped() 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) } }