| // 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/runtime/atomic" |
| "internal/runtime/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 |
| } |
| |
| 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 { |
| casGToWaitingForGC(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. |
| // |
| // Objects with weak handles have only one invariant related |
| // to this function: weak handle specials (which are not in the |
| // garbage collected heap) are roots. In practice, this means |
| // the handle field must be scanned. Note that the value the |
| // handle pointer referenced does *not* need to be scanned. See |
| // the definition of specialWeakHandle for details. |
| 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<<j) == 0 { |
| continue |
| } |
| // Find the span for this bit. |
| // |
| // This value is guaranteed to be non-nil because having |
| // specials implies that the span is in-use, and since we're |
| // currently marking we can be sure that we don't have to worry |
| // about the span being freed and re-used. |
| s := ha.spans[arenaPage+uint(i)*8+j] |
| |
| // The state must be mSpanInUse if the specials bit is set, so |
| // sanity check that. |
| if state := s.state.get(); state != mSpanInUse { |
| print("s.state = ", state, "\n") |
| throw("non in-use span found with specials bit set") |
| } |
| // Check that this span was swept (it may be cached or uncached). |
| if !useCheckmark && !(s.sweepgen == sg || s.sweepgen == sg+3) { |
| // sweepgen was updated (+2) during non-checkmark GC pass |
| print("sweep ", s.sweepgen, " ", sg, "\n") |
| throw("gc: unswept span") |
| } |
| |
| // 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 { |
| switch sp.kind { |
| case _KindSpecialFinalizer: |
| // 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). |
| if !s.spanclass.noscan() { |
| scanobject(p, gcw) |
| } |
| |
| // The special itself is a root. |
| scanblock(uintptr(unsafe.Pointer(&spf.fn)), goarch.PtrSize, &oneptrmask[0], gcw, nil) |
| case _KindSpecialWeakHandle: |
| // The special itself is a root. |
| spw := (*specialWeakHandle)(unsafe.Pointer(sp)) |
| scanblock(uintptr(unsafe.Pointer(&spw.handle)), goarch.PtrSize, &oneptrmask[0], gcw, nil) |
| } |
| } |
| unlock(&s.speciallock) |
| } |
| } |
| } |
| |
| // gcAssistAlloc performs GC work to make gp's assist debt positive. |
| // gp must be the calling user goroutine. |
| // |
| // 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 |
| } |
| |
| // This extremely verbose boolean indicates whether we've |
| // entered mark assist from the perspective of the tracer. |
| // |
| // In the tracer, this is just before we call gcAssistAlloc1 |
| // *regardless* of whether tracing is enabled. This is because |
| // the 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() { |
| 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 |
| } |
| // 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. |
| casGToWaitingForGC(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) |
| tp = s.typePointersOfUnchecked(s.base()) |
| tp = tp.fastForward(b-tp.addr, b+n) |
| } else { |
| tp = s.typePointersOfUnchecked(b) |
| } |
| |
| var scanSize uintptr |
| for { |
| var addr uintptr |
| if tp, addr = tp.nextFast(); addr == 0 { |
| if tp, addr = tp.next(b + n); 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) |
| } |
| } |