Source file src/runtime/mgcmark.go

Documentation: runtime

     1  // Copyright 2009 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  // Garbage collector: marking and scanning
     6  
     7  package runtime
     8  
     9  import (
    10  	"internal/abi"
    11  	"internal/goarch"
    12  	"internal/goexperiment"
    13  	"runtime/internal/atomic"
    14  	"runtime/internal/sys"
    15  	"unsafe"
    16  )
    17  
    18  const (
    19  	fixedRootFinalizers = iota
    20  	fixedRootFreeGStacks
    21  	fixedRootCount
    22  
    23  	// rootBlockBytes is the number of bytes to scan per data or
    24  	// BSS root.
    25  	rootBlockBytes = 256 << 10
    26  
    27  	// maxObletBytes is the maximum bytes of an object to scan at
    28  	// once. Larger objects will be split up into "oblets" of at
    29  	// most this size. Since we can scan 1–2 MB/ms, 128 KB bounds
    30  	// scan preemption at ~100 µs.
    31  	//
    32  	// This must be > _MaxSmallSize so that the object base is the
    33  	// span base.
    34  	maxObletBytes = 128 << 10
    35  
    36  	// drainCheckThreshold specifies how many units of work to do
    37  	// between self-preemption checks in gcDrain. Assuming a scan
    38  	// rate of 1 MB/ms, this is ~100 µs. Lower values have higher
    39  	// overhead in the scan loop (the scheduler check may perform
    40  	// a syscall, so its overhead is nontrivial). Higher values
    41  	// make the system less responsive to incoming work.
    42  	drainCheckThreshold = 100000
    43  
    44  	// pagesPerSpanRoot indicates how many pages to scan from a span root
    45  	// at a time. Used by special root marking.
    46  	//
    47  	// Higher values improve throughput by increasing locality, but
    48  	// increase the minimum latency of a marking operation.
    49  	//
    50  	// Must be a multiple of the pageInUse bitmap element size and
    51  	// must also evenly divide pagesPerArena.
    52  	pagesPerSpanRoot = 512
    53  )
    54  
    55  // gcMarkRootPrepare queues root scanning jobs (stacks, globals, and
    56  // some miscellany) and initializes scanning-related state.
    57  //
    58  // The world must be stopped.
    59  func gcMarkRootPrepare() {
    60  	assertWorldStopped()
    61  
    62  	// Compute how many data and BSS root blocks there are.
    63  	nBlocks := func(bytes uintptr) int {
    64  		return int(divRoundUp(bytes, rootBlockBytes))
    65  	}
    66  
    67  	work.nDataRoots = 0
    68  	work.nBSSRoots = 0
    69  
    70  	// Scan globals.
    71  	for _, datap := range activeModules() {
    72  		nDataRoots := nBlocks(datap.edata - datap.data)
    73  		if nDataRoots > work.nDataRoots {
    74  			work.nDataRoots = nDataRoots
    75  		}
    76  	}
    77  
    78  	for _, datap := range activeModules() {
    79  		nBSSRoots := nBlocks(datap.ebss - datap.bss)
    80  		if nBSSRoots > work.nBSSRoots {
    81  			work.nBSSRoots = nBSSRoots
    82  		}
    83  	}
    84  
    85  	// Scan span roots for finalizer specials.
    86  	//
    87  	// We depend on addfinalizer to mark objects that get
    88  	// finalizers after root marking.
    89  	//
    90  	// We're going to scan the whole heap (that was available at the time the
    91  	// mark phase started, i.e. markArenas) for in-use spans which have specials.
    92  	//
    93  	// Break up the work into arenas, and further into chunks.
    94  	//
    95  	// Snapshot allArenas as markArenas. This snapshot is safe because allArenas
    96  	// is append-only.
    97  	mheap_.markArenas = mheap_.allArenas[:len(mheap_.allArenas):len(mheap_.allArenas)]
    98  	work.nSpanRoots = len(mheap_.markArenas) * (pagesPerArena / pagesPerSpanRoot)
    99  
   100  	// Scan stacks.
   101  	//
   102  	// Gs may be created after this point, but it's okay that we
   103  	// ignore them because they begin life without any roots, so
   104  	// there's nothing to scan, and any roots they create during
   105  	// the concurrent phase will be caught by the write barrier.
   106  	work.stackRoots = allGsSnapshot()
   107  	work.nStackRoots = len(work.stackRoots)
   108  
   109  	work.markrootNext = 0
   110  	work.markrootJobs = uint32(fixedRootCount + work.nDataRoots + work.nBSSRoots + work.nSpanRoots + work.nStackRoots)
   111  
   112  	// Calculate base indexes of each root type
   113  	work.baseData = uint32(fixedRootCount)
   114  	work.baseBSS = work.baseData + uint32(work.nDataRoots)
   115  	work.baseSpans = work.baseBSS + uint32(work.nBSSRoots)
   116  	work.baseStacks = work.baseSpans + uint32(work.nSpanRoots)
   117  	work.baseEnd = work.baseStacks + uint32(work.nStackRoots)
   118  }
   119  
   120  // gcMarkRootCheck checks that all roots have been scanned. It is
   121  // purely for debugging.
   122  func gcMarkRootCheck() {
   123  	if work.markrootNext < work.markrootJobs {
   124  		print(work.markrootNext, " of ", work.markrootJobs, " markroot jobs done\n")
   125  		throw("left over markroot jobs")
   126  	}
   127  
   128  	// Check that stacks have been scanned.
   129  	//
   130  	// We only check the first nStackRoots Gs that we should have scanned.
   131  	// Since we don't care about newer Gs (see comment in
   132  	// gcMarkRootPrepare), no locking is required.
   133  	i := 0
   134  	forEachGRace(func(gp *g) {
   135  		if i >= work.nStackRoots {
   136  			return
   137  		}
   138  
   139  		if !gp.gcscandone {
   140  			println("gp", gp, "goid", gp.goid,
   141  				"status", readgstatus(gp),
   142  				"gcscandone", gp.gcscandone)
   143  			throw("scan missed a g")
   144  		}
   145  
   146  		i++
   147  	})
   148  }
   149  
   150  // ptrmask for an allocation containing a single pointer.
   151  var oneptrmask = [...]uint8{1}
   152  
   153  // markroot scans the i'th root.
   154  //
   155  // Preemption must be disabled (because this uses a gcWork).
   156  //
   157  // Returns the amount of GC work credit produced by the operation.
   158  // If flushBgCredit is true, then that credit is also flushed
   159  // to the background credit pool.
   160  //
   161  // nowritebarrier is only advisory here.
   162  //
   163  //go:nowritebarrier
   164  func markroot(gcw *gcWork, i uint32, flushBgCredit bool) int64 {
   165  	// Note: if you add a case here, please also update heapdump.go:dumproots.
   166  	var workDone int64
   167  	var workCounter *atomic.Int64
   168  	switch {
   169  	case work.baseData <= i && i < work.baseBSS:
   170  		workCounter = &gcController.globalsScanWork
   171  		for _, datap := range activeModules() {
   172  			workDone += markrootBlock(datap.data, datap.edata-datap.data, datap.gcdatamask.bytedata, gcw, int(i-work.baseData))
   173  		}
   174  
   175  	case work.baseBSS <= i && i < work.baseSpans:
   176  		workCounter = &gcController.globalsScanWork
   177  		for _, datap := range activeModules() {
   178  			workDone += markrootBlock(datap.bss, datap.ebss-datap.bss, datap.gcbssmask.bytedata, gcw, int(i-work.baseBSS))
   179  		}
   180  
   181  	case i == fixedRootFinalizers:
   182  		for fb := allfin; fb != nil; fb = fb.alllink {
   183  			cnt := uintptr(atomic.Load(&fb.cnt))
   184  			scanblock(uintptr(unsafe.Pointer(&fb.fin[0])), cnt*unsafe.Sizeof(fb.fin[0]), &finptrmask[0], gcw, nil)
   185  		}
   186  
   187  	case i == fixedRootFreeGStacks:
   188  		// Switch to the system stack so we can call
   189  		// stackfree.
   190  		systemstack(markrootFreeGStacks)
   191  
   192  	case work.baseSpans <= i && i < work.baseStacks:
   193  		// mark mspan.specials
   194  		markrootSpans(gcw, int(i-work.baseSpans))
   195  
   196  	default:
   197  		// the rest is scanning goroutine stacks
   198  		workCounter = &gcController.stackScanWork
   199  		if i < work.baseStacks || work.baseEnd <= i {
   200  			printlock()
   201  			print("runtime: markroot index ", i, " not in stack roots range [", work.baseStacks, ", ", work.baseEnd, ")\n")
   202  			throw("markroot: bad index")
   203  		}
   204  		gp := work.stackRoots[i-work.baseStacks]
   205  
   206  		// remember when we've first observed the G blocked
   207  		// needed only to output in traceback
   208  		status := readgstatus(gp) // We are not in a scan state
   209  		if (status == _Gwaiting || status == _Gsyscall) && gp.waitsince == 0 {
   210  			gp.waitsince = work.tstart
   211  		}
   212  
   213  		// scanstack must be done on the system stack in case
   214  		// we're trying to scan our own stack.
   215  		systemstack(func() {
   216  			// If this is a self-scan, put the user G in
   217  			// _Gwaiting to prevent self-deadlock. It may
   218  			// already be in _Gwaiting if this is a mark
   219  			// worker or we're in mark termination.
   220  			userG := getg().m.curg
   221  			selfScan := gp == userG && readgstatus(userG) == _Grunning
   222  			if selfScan {
   223  				casGToWaiting(userG, _Grunning, waitReasonGarbageCollectionScan)
   224  			}
   225  
   226  			// TODO: suspendG blocks (and spins) until gp
   227  			// stops, which may take a while for
   228  			// running goroutines. Consider doing this in
   229  			// two phases where the first is non-blocking:
   230  			// we scan the stacks we can and ask running
   231  			// goroutines to scan themselves; and the
   232  			// second blocks.
   233  			stopped := suspendG(gp)
   234  			if stopped.dead {
   235  				gp.gcscandone = true
   236  				return
   237  			}
   238  			if gp.gcscandone {
   239  				throw("g already scanned")
   240  			}
   241  			workDone += scanstack(gp, gcw)
   242  			gp.gcscandone = true
   243  			resumeG(stopped)
   244  
   245  			if selfScan {
   246  				casgstatus(userG, _Gwaiting, _Grunning)
   247  			}
   248  		})
   249  	}
   250  	if workCounter != nil && workDone != 0 {
   251  		workCounter.Add(workDone)
   252  		if flushBgCredit {
   253  			gcFlushBgCredit(workDone)
   254  		}
   255  	}
   256  	return workDone
   257  }
   258  
   259  // markrootBlock scans the shard'th shard of the block of memory [b0,
   260  // b0+n0), with the given pointer mask.
   261  //
   262  // Returns the amount of work done.
   263  //
   264  //go:nowritebarrier
   265  func markrootBlock(b0, n0 uintptr, ptrmask0 *uint8, gcw *gcWork, shard int) int64 {
   266  	if rootBlockBytes%(8*goarch.PtrSize) != 0 {
   267  		// This is necessary to pick byte offsets in ptrmask0.
   268  		throw("rootBlockBytes must be a multiple of 8*ptrSize")
   269  	}
   270  
   271  	// Note that if b0 is toward the end of the address space,
   272  	// then b0 + rootBlockBytes might wrap around.
   273  	// These tests are written to avoid any possible overflow.
   274  	off := uintptr(shard) * rootBlockBytes
   275  	if off >= n0 {
   276  		return 0
   277  	}
   278  	b := b0 + off
   279  	ptrmask := (*uint8)(add(unsafe.Pointer(ptrmask0), uintptr(shard)*(rootBlockBytes/(8*goarch.PtrSize))))
   280  	n := uintptr(rootBlockBytes)
   281  	if off+n > n0 {
   282  		n = n0 - off
   283  	}
   284  
   285  	// Scan this shard.
   286  	scanblock(b, n, ptrmask, gcw, nil)
   287  	return int64(n)
   288  }
   289  
   290  // markrootFreeGStacks frees stacks of dead Gs.
   291  //
   292  // This does not free stacks of dead Gs cached on Ps, but having a few
   293  // cached stacks around isn't a problem.
   294  func markrootFreeGStacks() {
   295  	// Take list of dead Gs with stacks.
   296  	lock(&sched.gFree.lock)
   297  	list := sched.gFree.stack
   298  	sched.gFree.stack = gList{}
   299  	unlock(&sched.gFree.lock)
   300  	if list.empty() {
   301  		return
   302  	}
   303  
   304  	// Free stacks.
   305  	q := gQueue{list.head, list.head}
   306  	for gp := list.head.ptr(); gp != nil; gp = gp.schedlink.ptr() {
   307  		stackfree(gp.stack)
   308  		gp.stack.lo = 0
   309  		gp.stack.hi = 0
   310  		// Manipulate the queue directly since the Gs are
   311  		// already all linked the right way.
   312  		q.tail.set(gp)
   313  	}
   314  
   315  	// Put Gs back on the free list.
   316  	lock(&sched.gFree.lock)
   317  	sched.gFree.noStack.pushAll(q)
   318  	unlock(&sched.gFree.lock)
   319  }
   320  
   321  // markrootSpans marks roots for one shard of markArenas.
   322  //
   323  //go:nowritebarrier
   324  func markrootSpans(gcw *gcWork, shard int) {
   325  	// Objects with finalizers have two GC-related invariants:
   326  	//
   327  	// 1) Everything reachable from the object must be marked.
   328  	// This ensures that when we pass the object to its finalizer,
   329  	// everything the finalizer can reach will be retained.
   330  	//
   331  	// 2) Finalizer specials (which are not in the garbage
   332  	// collected heap) are roots. In practice, this means the fn
   333  	// field must be scanned.
   334  	sg := mheap_.sweepgen
   335  
   336  	// Find the arena and page index into that arena for this shard.
   337  	ai := mheap_.markArenas[shard/(pagesPerArena/pagesPerSpanRoot)]
   338  	ha := mheap_.arenas[ai.l1()][ai.l2()]
   339  	arenaPage := uint(uintptr(shard) * pagesPerSpanRoot % pagesPerArena)
   340  
   341  	// Construct slice of bitmap which we'll iterate over.
   342  	specialsbits := ha.pageSpecials[arenaPage/8:]
   343  	specialsbits = specialsbits[:pagesPerSpanRoot/8]
   344  	for i := range specialsbits {
   345  		// Find set bits, which correspond to spans with specials.
   346  		specials := atomic.Load8(&specialsbits[i])
   347  		if specials == 0 {
   348  			continue
   349  		}
   350  		for j := uint(0); j < 8; j++ {
   351  			if specials&(1<<j) == 0 {
   352  				continue
   353  			}
   354  			// Find the span for this bit.
   355  			//
   356  			// This value is guaranteed to be non-nil because having
   357  			// specials implies that the span is in-use, and since we're
   358  			// currently marking we can be sure that we don't have to worry
   359  			// about the span being freed and re-used.
   360  			s := ha.spans[arenaPage+uint(i)*8+j]
   361  
   362  			// The state must be mSpanInUse if the specials bit is set, so
   363  			// sanity check that.
   364  			if state := s.state.get(); state != mSpanInUse {
   365  				print("s.state = ", state, "\n")
   366  				throw("non in-use span found with specials bit set")
   367  			}
   368  			// Check that this span was swept (it may be cached or uncached).
   369  			if !useCheckmark && !(s.sweepgen == sg || s.sweepgen == sg+3) {
   370  				// sweepgen was updated (+2) during non-checkmark GC pass
   371  				print("sweep ", s.sweepgen, " ", sg, "\n")
   372  				throw("gc: unswept span")
   373  			}
   374  
   375  			// Lock the specials to prevent a special from being
   376  			// removed from the list while we're traversing it.
   377  			lock(&s.speciallock)
   378  			for sp := s.specials; sp != nil; sp = sp.next {
   379  				if sp.kind != _KindSpecialFinalizer {
   380  					continue
   381  				}
   382  				// don't mark finalized object, but scan it so we
   383  				// retain everything it points to.
   384  				spf := (*specialfinalizer)(unsafe.Pointer(sp))
   385  				// A finalizer can be set for an inner byte of an object, find object beginning.
   386  				p := s.base() + uintptr(spf.special.offset)/s.elemsize*s.elemsize
   387  
   388  				// Mark everything that can be reached from
   389  				// the object (but *not* the object itself or
   390  				// we'll never collect it).
   391  				if !s.spanclass.noscan() {
   392  					scanobject(p, gcw)
   393  				}
   394  
   395  				// The special itself is a root.
   396  				scanblock(uintptr(unsafe.Pointer(&spf.fn)), goarch.PtrSize, &oneptrmask[0], gcw, nil)
   397  			}
   398  			unlock(&s.speciallock)
   399  		}
   400  	}
   401  }
   402  
   403  // gcAssistAlloc performs GC work to make gp's assist debt positive.
   404  // gp must be the calling user goroutine.
   405  //
   406  // This must be called with preemption enabled.
   407  func gcAssistAlloc(gp *g) {
   408  	// Don't assist in non-preemptible contexts. These are
   409  	// generally fragile and won't allow the assist to block.
   410  	if getg() == gp.m.g0 {
   411  		return
   412  	}
   413  	if mp := getg().m; mp.locks > 0 || mp.preemptoff != "" {
   414  		return
   415  	}
   416  
   417  	// This extremely verbose boolean indicates whether we've
   418  	// entered mark assist from the perspective of the tracer.
   419  	//
   420  	// In the old tracer, this is just before we call gcAssistAlloc1
   421  	// *and* tracing is enabled. Because the old tracer doesn't
   422  	// do any extra tracking, we need to be careful to not emit an
   423  	// "end" event if there was no corresponding "begin" for the
   424  	// mark assist.
   425  	//
   426  	// In the new tracer, this is just before we call gcAssistAlloc1
   427  	// *regardless* of whether tracing is enabled. This is because
   428  	// the new tracer allows for tracing to begin (and advance
   429  	// generations) in the middle of a GC mark phase, so we need to
   430  	// record some state so that the tracer can pick it up to ensure
   431  	// a consistent trace result.
   432  	//
   433  	// TODO(mknyszek): Hide the details of inMarkAssist in tracer
   434  	// functions and simplify all the state tracking. This is a lot.
   435  	enteredMarkAssistForTracing := false
   436  retry:
   437  	if gcCPULimiter.limiting() {
   438  		// If the CPU limiter is enabled, intentionally don't
   439  		// assist to reduce the amount of CPU time spent in the GC.
   440  		if enteredMarkAssistForTracing {
   441  			trace := traceAcquire()
   442  			if trace.ok() {
   443  				trace.GCMarkAssistDone()
   444  				// Set this *after* we trace the end to make sure
   445  				// that we emit an in-progress event if this is
   446  				// the first event for the goroutine in the trace
   447  				// or trace generation. Also, do this between
   448  				// acquire/release because this is part of the
   449  				// goroutine's trace state, and it must be atomic
   450  				// with respect to the tracer.
   451  				gp.inMarkAssist = false
   452  				traceRelease(trace)
   453  			} else {
   454  				// This state is tracked even if tracing isn't enabled.
   455  				// It's only used by the new tracer.
   456  				// See the comment on enteredMarkAssistForTracing.
   457  				gp.inMarkAssist = false
   458  			}
   459  		}
   460  		return
   461  	}
   462  	// Compute the amount of scan work we need to do to make the
   463  	// balance positive. When the required amount of work is low,
   464  	// we over-assist to build up credit for future allocations
   465  	// and amortize the cost of assisting.
   466  	assistWorkPerByte := gcController.assistWorkPerByte.Load()
   467  	assistBytesPerWork := gcController.assistBytesPerWork.Load()
   468  	debtBytes := -gp.gcAssistBytes
   469  	scanWork := int64(assistWorkPerByte * float64(debtBytes))
   470  	if scanWork < gcOverAssistWork {
   471  		scanWork = gcOverAssistWork
   472  		debtBytes = int64(assistBytesPerWork * float64(scanWork))
   473  	}
   474  
   475  	// Steal as much credit as we can from the background GC's
   476  	// scan credit. This is racy and may drop the background
   477  	// credit below 0 if two mutators steal at the same time. This
   478  	// will just cause steals to fail until credit is accumulated
   479  	// again, so in the long run it doesn't really matter, but we
   480  	// do have to handle the negative credit case.
   481  	bgScanCredit := gcController.bgScanCredit.Load()
   482  	stolen := int64(0)
   483  	if bgScanCredit > 0 {
   484  		if bgScanCredit < scanWork {
   485  			stolen = bgScanCredit
   486  			gp.gcAssistBytes += 1 + int64(assistBytesPerWork*float64(stolen))
   487  		} else {
   488  			stolen = scanWork
   489  			gp.gcAssistBytes += debtBytes
   490  		}
   491  		gcController.bgScanCredit.Add(-stolen)
   492  
   493  		scanWork -= stolen
   494  
   495  		if scanWork == 0 {
   496  			// We were able to steal all of the credit we
   497  			// needed.
   498  			if enteredMarkAssistForTracing {
   499  				trace := traceAcquire()
   500  				if trace.ok() {
   501  					trace.GCMarkAssistDone()
   502  					// Set this *after* we trace the end to make sure
   503  					// that we emit an in-progress event if this is
   504  					// the first event for the goroutine in the trace
   505  					// or trace generation. Also, do this between
   506  					// acquire/release because this is part of the
   507  					// goroutine's trace state, and it must be atomic
   508  					// with respect to the tracer.
   509  					gp.inMarkAssist = false
   510  					traceRelease(trace)
   511  				} else {
   512  					// This state is tracked even if tracing isn't enabled.
   513  					// It's only used by the new tracer.
   514  					// See the comment on enteredMarkAssistForTracing.
   515  					gp.inMarkAssist = false
   516  				}
   517  			}
   518  			return
   519  		}
   520  	}
   521  	if !enteredMarkAssistForTracing {
   522  		trace := traceAcquire()
   523  		if trace.ok() {
   524  			if !goexperiment.ExecTracer2 {
   525  				// In the old tracer, enter mark assist tracing only
   526  				// if we actually traced an event. Otherwise a goroutine
   527  				// waking up from mark assist post-GC might end up
   528  				// writing a stray "end" event.
   529  				//
   530  				// This means inMarkAssist will not be meaningful
   531  				// in the old tracer; that's OK, it's unused.
   532  				//
   533  				// See the comment on enteredMarkAssistForTracing.
   534  				enteredMarkAssistForTracing = true
   535  			}
   536  			trace.GCMarkAssistStart()
   537  			// Set this *after* we trace the start, otherwise we may
   538  			// emit an in-progress event for an assist we're about to start.
   539  			gp.inMarkAssist = true
   540  			traceRelease(trace)
   541  		} else {
   542  			gp.inMarkAssist = true
   543  		}
   544  		if goexperiment.ExecTracer2 {
   545  			// In the new tracer, set enter mark assist tracing if we
   546  			// ever pass this point, because we must manage inMarkAssist
   547  			// correctly.
   548  			//
   549  			// See the comment on enteredMarkAssistForTracing.
   550  			enteredMarkAssistForTracing = true
   551  		}
   552  	}
   553  
   554  	// Perform assist work
   555  	systemstack(func() {
   556  		gcAssistAlloc1(gp, scanWork)
   557  		// The user stack may have moved, so this can't touch
   558  		// anything on it until it returns from systemstack.
   559  	})
   560  
   561  	completed := gp.param != nil
   562  	gp.param = nil
   563  	if completed {
   564  		gcMarkDone()
   565  	}
   566  
   567  	if gp.gcAssistBytes < 0 {
   568  		// We were unable steal enough credit or perform
   569  		// enough work to pay off the assist debt. We need to
   570  		// do one of these before letting the mutator allocate
   571  		// more to prevent over-allocation.
   572  		//
   573  		// If this is because we were preempted, reschedule
   574  		// and try some more.
   575  		if gp.preempt {
   576  			Gosched()
   577  			goto retry
   578  		}
   579  
   580  		// Add this G to an assist queue and park. When the GC
   581  		// has more background credit, it will satisfy queued
   582  		// assists before flushing to the global credit pool.
   583  		//
   584  		// Note that this does *not* get woken up when more
   585  		// work is added to the work list. The theory is that
   586  		// there wasn't enough work to do anyway, so we might
   587  		// as well let background marking take care of the
   588  		// work that is available.
   589  		if !gcParkAssist() {
   590  			goto retry
   591  		}
   592  
   593  		// At this point either background GC has satisfied
   594  		// this G's assist debt, or the GC cycle is over.
   595  	}
   596  	if enteredMarkAssistForTracing {
   597  		trace := traceAcquire()
   598  		if trace.ok() {
   599  			trace.GCMarkAssistDone()
   600  			// Set this *after* we trace the end to make sure
   601  			// that we emit an in-progress event if this is
   602  			// the first event for the goroutine in the trace
   603  			// or trace generation. Also, do this between
   604  			// acquire/release because this is part of the
   605  			// goroutine's trace state, and it must be atomic
   606  			// with respect to the tracer.
   607  			gp.inMarkAssist = false
   608  			traceRelease(trace)
   609  		} else {
   610  			// This state is tracked even if tracing isn't enabled.
   611  			// It's only used by the new tracer.
   612  			// See the comment on enteredMarkAssistForTracing.
   613  			gp.inMarkAssist = false
   614  		}
   615  	}
   616  }
   617  
   618  // gcAssistAlloc1 is the part of gcAssistAlloc that runs on the system
   619  // stack. This is a separate function to make it easier to see that
   620  // we're not capturing anything from the user stack, since the user
   621  // stack may move while we're in this function.
   622  //
   623  // gcAssistAlloc1 indicates whether this assist completed the mark
   624  // phase by setting gp.param to non-nil. This can't be communicated on
   625  // the stack since it may move.
   626  //
   627  //go:systemstack
   628  func gcAssistAlloc1(gp *g, scanWork int64) {
   629  	// Clear the flag indicating that this assist completed the
   630  	// mark phase.
   631  	gp.param = nil
   632  
   633  	if atomic.Load(&gcBlackenEnabled) == 0 {
   634  		// The gcBlackenEnabled check in malloc races with the
   635  		// store that clears it but an atomic check in every malloc
   636  		// would be a performance hit.
   637  		// Instead we recheck it here on the non-preemptible system
   638  		// stack to determine if we should perform an assist.
   639  
   640  		// GC is done, so ignore any remaining debt.
   641  		gp.gcAssistBytes = 0
   642  		return
   643  	}
   644  	// Track time spent in this assist. Since we're on the
   645  	// system stack, this is non-preemptible, so we can
   646  	// just measure start and end time.
   647  	//
   648  	// Limiter event tracking might be disabled if we end up here
   649  	// while on a mark worker.
   650  	startTime := nanotime()
   651  	trackLimiterEvent := gp.m.p.ptr().limiterEvent.start(limiterEventMarkAssist, startTime)
   652  
   653  	decnwait := atomic.Xadd(&work.nwait, -1)
   654  	if decnwait == work.nproc {
   655  		println("runtime: work.nwait =", decnwait, "work.nproc=", work.nproc)
   656  		throw("nwait > work.nprocs")
   657  	}
   658  
   659  	// gcDrainN requires the caller to be preemptible.
   660  	casGToWaiting(gp, _Grunning, waitReasonGCAssistMarking)
   661  
   662  	// drain own cached work first in the hopes that it
   663  	// will be more cache friendly.
   664  	gcw := &getg().m.p.ptr().gcw
   665  	workDone := gcDrainN(gcw, scanWork)
   666  
   667  	casgstatus(gp, _Gwaiting, _Grunning)
   668  
   669  	// Record that we did this much scan work.
   670  	//
   671  	// Back out the number of bytes of assist credit that
   672  	// this scan work counts for. The "1+" is a poor man's
   673  	// round-up, to ensure this adds credit even if
   674  	// assistBytesPerWork is very low.
   675  	assistBytesPerWork := gcController.assistBytesPerWork.Load()
   676  	gp.gcAssistBytes += 1 + int64(assistBytesPerWork*float64(workDone))
   677  
   678  	// If this is the last worker and we ran out of work,
   679  	// signal a completion point.
   680  	incnwait := atomic.Xadd(&work.nwait, +1)
   681  	if incnwait > work.nproc {
   682  		println("runtime: work.nwait=", incnwait,
   683  			"work.nproc=", work.nproc)
   684  		throw("work.nwait > work.nproc")
   685  	}
   686  
   687  	if incnwait == work.nproc && !gcMarkWorkAvailable(nil) {
   688  		// This has reached a background completion point. Set
   689  		// gp.param to a non-nil value to indicate this. It
   690  		// doesn't matter what we set it to (it just has to be
   691  		// a valid pointer).
   692  		gp.param = unsafe.Pointer(gp)
   693  	}
   694  	now := nanotime()
   695  	duration := now - startTime
   696  	pp := gp.m.p.ptr()
   697  	pp.gcAssistTime += duration
   698  	if trackLimiterEvent {
   699  		pp.limiterEvent.stop(limiterEventMarkAssist, now)
   700  	}
   701  	if pp.gcAssistTime > gcAssistTimeSlack {
   702  		gcController.assistTime.Add(pp.gcAssistTime)
   703  		gcCPULimiter.update(now)
   704  		pp.gcAssistTime = 0
   705  	}
   706  }
   707  
   708  // gcWakeAllAssists wakes all currently blocked assists. This is used
   709  // at the end of a GC cycle. gcBlackenEnabled must be false to prevent
   710  // new assists from going to sleep after this point.
   711  func gcWakeAllAssists() {
   712  	lock(&work.assistQueue.lock)
   713  	list := work.assistQueue.q.popList()
   714  	injectglist(&list)
   715  	unlock(&work.assistQueue.lock)
   716  }
   717  
   718  // gcParkAssist puts the current goroutine on the assist queue and parks.
   719  //
   720  // gcParkAssist reports whether the assist is now satisfied. If it
   721  // returns false, the caller must retry the assist.
   722  func gcParkAssist() bool {
   723  	lock(&work.assistQueue.lock)
   724  	// If the GC cycle finished while we were getting the lock,
   725  	// exit the assist. The cycle can't finish while we hold the
   726  	// lock.
   727  	if atomic.Load(&gcBlackenEnabled) == 0 {
   728  		unlock(&work.assistQueue.lock)
   729  		return true
   730  	}
   731  
   732  	gp := getg()
   733  	oldList := work.assistQueue.q
   734  	work.assistQueue.q.pushBack(gp)
   735  
   736  	// Recheck for background credit now that this G is in
   737  	// the queue, but can still back out. This avoids a
   738  	// race in case background marking has flushed more
   739  	// credit since we checked above.
   740  	if gcController.bgScanCredit.Load() > 0 {
   741  		work.assistQueue.q = oldList
   742  		if oldList.tail != 0 {
   743  			oldList.tail.ptr().schedlink.set(nil)
   744  		}
   745  		unlock(&work.assistQueue.lock)
   746  		return false
   747  	}
   748  	// Park.
   749  	goparkunlock(&work.assistQueue.lock, waitReasonGCAssistWait, traceBlockGCMarkAssist, 2)
   750  	return true
   751  }
   752  
   753  // gcFlushBgCredit flushes scanWork units of background scan work
   754  // credit. This first satisfies blocked assists on the
   755  // work.assistQueue and then flushes any remaining credit to
   756  // gcController.bgScanCredit.
   757  //
   758  // Write barriers are disallowed because this is used by gcDrain after
   759  // it has ensured that all work is drained and this must preserve that
   760  // condition.
   761  //
   762  //go:nowritebarrierrec
   763  func gcFlushBgCredit(scanWork int64) {
   764  	if work.assistQueue.q.empty() {
   765  		// Fast path; there are no blocked assists. There's a
   766  		// small window here where an assist may add itself to
   767  		// the blocked queue and park. If that happens, we'll
   768  		// just get it on the next flush.
   769  		gcController.bgScanCredit.Add(scanWork)
   770  		return
   771  	}
   772  
   773  	assistBytesPerWork := gcController.assistBytesPerWork.Load()
   774  	scanBytes := int64(float64(scanWork) * assistBytesPerWork)
   775  
   776  	lock(&work.assistQueue.lock)
   777  	for !work.assistQueue.q.empty() && scanBytes > 0 {
   778  		gp := work.assistQueue.q.pop()
   779  		// Note that gp.gcAssistBytes is negative because gp
   780  		// is in debt. Think carefully about the signs below.
   781  		if scanBytes+gp.gcAssistBytes >= 0 {
   782  			// Satisfy this entire assist debt.
   783  			scanBytes += gp.gcAssistBytes
   784  			gp.gcAssistBytes = 0
   785  			// It's important that we *not* put gp in
   786  			// runnext. Otherwise, it's possible for user
   787  			// code to exploit the GC worker's high
   788  			// scheduler priority to get itself always run
   789  			// before other goroutines and always in the
   790  			// fresh quantum started by GC.
   791  			ready(gp, 0, false)
   792  		} else {
   793  			// Partially satisfy this assist.
   794  			gp.gcAssistBytes += scanBytes
   795  			scanBytes = 0
   796  			// As a heuristic, we move this assist to the
   797  			// back of the queue so that large assists
   798  			// can't clog up the assist queue and
   799  			// substantially delay small assists.
   800  			work.assistQueue.q.pushBack(gp)
   801  			break
   802  		}
   803  	}
   804  
   805  	if scanBytes > 0 {
   806  		// Convert from scan bytes back to work.
   807  		assistWorkPerByte := gcController.assistWorkPerByte.Load()
   808  		scanWork = int64(float64(scanBytes) * assistWorkPerByte)
   809  		gcController.bgScanCredit.Add(scanWork)
   810  	}
   811  	unlock(&work.assistQueue.lock)
   812  }
   813  
   814  // scanstack scans gp's stack, greying all pointers found on the stack.
   815  //
   816  // Returns the amount of scan work performed, but doesn't update
   817  // gcController.stackScanWork or flush any credit. Any background credit produced
   818  // by this function should be flushed by its caller. scanstack itself can't
   819  // safely flush because it may result in trying to wake up a goroutine that
   820  // was just scanned, resulting in a self-deadlock.
   821  //
   822  // scanstack will also shrink the stack if it is safe to do so. If it
   823  // is not, it schedules a stack shrink for the next synchronous safe
   824  // point.
   825  //
   826  // scanstack is marked go:systemstack because it must not be preempted
   827  // while using a workbuf.
   828  //
   829  //go:nowritebarrier
   830  //go:systemstack
   831  func scanstack(gp *g, gcw *gcWork) int64 {
   832  	if readgstatus(gp)&_Gscan == 0 {
   833  		print("runtime:scanstack: gp=", gp, ", goid=", gp.goid, ", gp->atomicstatus=", hex(readgstatus(gp)), "\n")
   834  		throw("scanstack - bad status")
   835  	}
   836  
   837  	switch readgstatus(gp) &^ _Gscan {
   838  	default:
   839  		print("runtime: gp=", gp, ", goid=", gp.goid, ", gp->atomicstatus=", readgstatus(gp), "\n")
   840  		throw("mark - bad status")
   841  	case _Gdead:
   842  		return 0
   843  	case _Grunning:
   844  		print("runtime: gp=", gp, ", goid=", gp.goid, ", gp->atomicstatus=", readgstatus(gp), "\n")
   845  		throw("scanstack: goroutine not stopped")
   846  	case _Grunnable, _Gsyscall, _Gwaiting:
   847  		// ok
   848  	}
   849  
   850  	if gp == getg() {
   851  		throw("can't scan our own stack")
   852  	}
   853  
   854  	// scannedSize is the amount of work we'll be reporting.
   855  	//
   856  	// It is less than the allocated size (which is hi-lo).
   857  	var sp uintptr
   858  	if gp.syscallsp != 0 {
   859  		sp = gp.syscallsp // If in a system call this is the stack pointer (gp.sched.sp can be 0 in this case on Windows).
   860  	} else {
   861  		sp = gp.sched.sp
   862  	}
   863  	scannedSize := gp.stack.hi - sp
   864  
   865  	// Keep statistics for initial stack size calculation.
   866  	// Note that this accumulates the scanned size, not the allocated size.
   867  	p := getg().m.p.ptr()
   868  	p.scannedStackSize += uint64(scannedSize)
   869  	p.scannedStacks++
   870  
   871  	if isShrinkStackSafe(gp) {
   872  		// Shrink the stack if not much of it is being used.
   873  		shrinkstack(gp)
   874  	} else {
   875  		// Otherwise, shrink the stack at the next sync safe point.
   876  		gp.preemptShrink = true
   877  	}
   878  
   879  	var state stackScanState
   880  	state.stack = gp.stack
   881  
   882  	if stackTraceDebug {
   883  		println("stack trace goroutine", gp.goid)
   884  	}
   885  
   886  	if debugScanConservative && gp.asyncSafePoint {
   887  		print("scanning async preempted goroutine ", gp.goid, " stack [", hex(gp.stack.lo), ",", hex(gp.stack.hi), ")\n")
   888  	}
   889  
   890  	// Scan the saved context register. This is effectively a live
   891  	// register that gets moved back and forth between the
   892  	// register and sched.ctxt without a write barrier.
   893  	if gp.sched.ctxt != nil {
   894  		scanblock(uintptr(unsafe.Pointer(&gp.sched.ctxt)), goarch.PtrSize, &oneptrmask[0], gcw, &state)
   895  	}
   896  
   897  	// Scan the stack. Accumulate a list of stack objects.
   898  	var u unwinder
   899  	for u.init(gp, 0); u.valid(); u.next() {
   900  		scanframeworker(&u.frame, &state, gcw)
   901  	}
   902  
   903  	// Find additional pointers that point into the stack from the heap.
   904  	// Currently this includes defers and panics. See also function copystack.
   905  
   906  	// Find and trace other pointers in defer records.
   907  	for d := gp._defer; d != nil; d = d.link {
   908  		if d.fn != nil {
   909  			// Scan the func value, which could be a stack allocated closure.
   910  			// See issue 30453.
   911  			scanblock(uintptr(unsafe.Pointer(&d.fn)), goarch.PtrSize, &oneptrmask[0], gcw, &state)
   912  		}
   913  		if d.link != nil {
   914  			// The link field of a stack-allocated defer record might point
   915  			// to a heap-allocated defer record. Keep that heap record live.
   916  			scanblock(uintptr(unsafe.Pointer(&d.link)), goarch.PtrSize, &oneptrmask[0], gcw, &state)
   917  		}
   918  		// Retain defers records themselves.
   919  		// Defer records might not be reachable from the G through regular heap
   920  		// tracing because the defer linked list might weave between the stack and the heap.
   921  		if d.heap {
   922  			scanblock(uintptr(unsafe.Pointer(&d)), goarch.PtrSize, &oneptrmask[0], gcw, &state)
   923  		}
   924  	}
   925  	if gp._panic != nil {
   926  		// Panics are always stack allocated.
   927  		state.putPtr(uintptr(unsafe.Pointer(gp._panic)), false)
   928  	}
   929  
   930  	// Find and scan all reachable stack objects.
   931  	//
   932  	// The state's pointer queue prioritizes precise pointers over
   933  	// conservative pointers so that we'll prefer scanning stack
   934  	// objects precisely.
   935  	state.buildIndex()
   936  	for {
   937  		p, conservative := state.getPtr()
   938  		if p == 0 {
   939  			break
   940  		}
   941  		obj := state.findObject(p)
   942  		if obj == nil {
   943  			continue
   944  		}
   945  		r := obj.r
   946  		if r == nil {
   947  			// We've already scanned this object.
   948  			continue
   949  		}
   950  		obj.setRecord(nil) // Don't scan it again.
   951  		if stackTraceDebug {
   952  			printlock()
   953  			print("  live stkobj at", hex(state.stack.lo+uintptr(obj.off)), "of size", obj.size)
   954  			if conservative {
   955  				print(" (conservative)")
   956  			}
   957  			println()
   958  			printunlock()
   959  		}
   960  		gcdata := r.gcdata()
   961  		var s *mspan
   962  		if r.useGCProg() {
   963  			// This path is pretty unlikely, an object large enough
   964  			// to have a GC program allocated on the stack.
   965  			// We need some space to unpack the program into a straight
   966  			// bitmask, which we allocate/free here.
   967  			// TODO: it would be nice if there were a way to run a GC
   968  			// program without having to store all its bits. We'd have
   969  			// to change from a Lempel-Ziv style program to something else.
   970  			// Or we can forbid putting objects on stacks if they require
   971  			// a gc program (see issue 27447).
   972  			s = materializeGCProg(r.ptrdata(), gcdata)
   973  			gcdata = (*byte)(unsafe.Pointer(s.startAddr))
   974  		}
   975  
   976  		b := state.stack.lo + uintptr(obj.off)
   977  		if conservative {
   978  			scanConservative(b, r.ptrdata(), gcdata, gcw, &state)
   979  		} else {
   980  			scanblock(b, r.ptrdata(), gcdata, gcw, &state)
   981  		}
   982  
   983  		if s != nil {
   984  			dematerializeGCProg(s)
   985  		}
   986  	}
   987  
   988  	// Deallocate object buffers.
   989  	// (Pointer buffers were all deallocated in the loop above.)
   990  	for state.head != nil {
   991  		x := state.head
   992  		state.head = x.next
   993  		if stackTraceDebug {
   994  			for i := 0; i < x.nobj; i++ {
   995  				obj := &x.obj[i]
   996  				if obj.r == nil { // reachable
   997  					continue
   998  				}
   999  				println("  dead stkobj at", hex(gp.stack.lo+uintptr(obj.off)), "of size", obj.r.size)
  1000  				// Note: not necessarily really dead - only reachable-from-ptr dead.
  1001  			}
  1002  		}
  1003  		x.nobj = 0
  1004  		putempty((*workbuf)(unsafe.Pointer(x)))
  1005  	}
  1006  	if state.buf != nil || state.cbuf != nil || state.freeBuf != nil {
  1007  		throw("remaining pointer buffers")
  1008  	}
  1009  	return int64(scannedSize)
  1010  }
  1011  
  1012  // Scan a stack frame: local variables and function arguments/results.
  1013  //
  1014  //go:nowritebarrier
  1015  func scanframeworker(frame *stkframe, state *stackScanState, gcw *gcWork) {
  1016  	if _DebugGC > 1 && frame.continpc != 0 {
  1017  		print("scanframe ", funcname(frame.fn), "\n")
  1018  	}
  1019  
  1020  	isAsyncPreempt := frame.fn.valid() && frame.fn.funcID == abi.FuncID_asyncPreempt
  1021  	isDebugCall := frame.fn.valid() && frame.fn.funcID == abi.FuncID_debugCallV2
  1022  	if state.conservative || isAsyncPreempt || isDebugCall {
  1023  		if debugScanConservative {
  1024  			println("conservatively scanning function", funcname(frame.fn), "at PC", hex(frame.continpc))
  1025  		}
  1026  
  1027  		// Conservatively scan the frame. Unlike the precise
  1028  		// case, this includes the outgoing argument space
  1029  		// since we may have stopped while this function was
  1030  		// setting up a call.
  1031  		//
  1032  		// TODO: We could narrow this down if the compiler
  1033  		// produced a single map per function of stack slots
  1034  		// and registers that ever contain a pointer.
  1035  		if frame.varp != 0 {
  1036  			size := frame.varp - frame.sp
  1037  			if size > 0 {
  1038  				scanConservative(frame.sp, size, nil, gcw, state)
  1039  			}
  1040  		}
  1041  
  1042  		// Scan arguments to this frame.
  1043  		if n := frame.argBytes(); n != 0 {
  1044  			// TODO: We could pass the entry argument map
  1045  			// to narrow this down further.
  1046  			scanConservative(frame.argp, n, nil, gcw, state)
  1047  		}
  1048  
  1049  		if isAsyncPreempt || isDebugCall {
  1050  			// This function's frame contained the
  1051  			// registers for the asynchronously stopped
  1052  			// parent frame. Scan the parent
  1053  			// conservatively.
  1054  			state.conservative = true
  1055  		} else {
  1056  			// We only wanted to scan those two frames
  1057  			// conservatively. Clear the flag for future
  1058  			// frames.
  1059  			state.conservative = false
  1060  		}
  1061  		return
  1062  	}
  1063  
  1064  	locals, args, objs := frame.getStackMap(false)
  1065  
  1066  	// Scan local variables if stack frame has been allocated.
  1067  	if locals.n > 0 {
  1068  		size := uintptr(locals.n) * goarch.PtrSize
  1069  		scanblock(frame.varp-size, size, locals.bytedata, gcw, state)
  1070  	}
  1071  
  1072  	// Scan arguments.
  1073  	if args.n > 0 {
  1074  		scanblock(frame.argp, uintptr(args.n)*goarch.PtrSize, args.bytedata, gcw, state)
  1075  	}
  1076  
  1077  	// Add all stack objects to the stack object list.
  1078  	if frame.varp != 0 {
  1079  		// varp is 0 for defers, where there are no locals.
  1080  		// In that case, there can't be a pointer to its args, either.
  1081  		// (And all args would be scanned above anyway.)
  1082  		for i := range objs {
  1083  			obj := &objs[i]
  1084  			off := obj.off
  1085  			base := frame.varp // locals base pointer
  1086  			if off >= 0 {
  1087  				base = frame.argp // arguments and return values base pointer
  1088  			}
  1089  			ptr := base + uintptr(off)
  1090  			if ptr < frame.sp {
  1091  				// object hasn't been allocated in the frame yet.
  1092  				continue
  1093  			}
  1094  			if stackTraceDebug {
  1095  				println("stkobj at", hex(ptr), "of size", obj.size)
  1096  			}
  1097  			state.addObject(ptr, obj)
  1098  		}
  1099  	}
  1100  }
  1101  
  1102  type gcDrainFlags int
  1103  
  1104  const (
  1105  	gcDrainUntilPreempt gcDrainFlags = 1 << iota
  1106  	gcDrainFlushBgCredit
  1107  	gcDrainIdle
  1108  	gcDrainFractional
  1109  )
  1110  
  1111  // gcDrainMarkWorkerIdle is a wrapper for gcDrain that exists to better account
  1112  // mark time in profiles.
  1113  func gcDrainMarkWorkerIdle(gcw *gcWork) {
  1114  	gcDrain(gcw, gcDrainIdle|gcDrainUntilPreempt|gcDrainFlushBgCredit)
  1115  }
  1116  
  1117  // gcDrainMarkWorkerDedicated is a wrapper for gcDrain that exists to better account
  1118  // mark time in profiles.
  1119  func gcDrainMarkWorkerDedicated(gcw *gcWork, untilPreempt bool) {
  1120  	flags := gcDrainFlushBgCredit
  1121  	if untilPreempt {
  1122  		flags |= gcDrainUntilPreempt
  1123  	}
  1124  	gcDrain(gcw, flags)
  1125  }
  1126  
  1127  // gcDrainMarkWorkerFractional is a wrapper for gcDrain that exists to better account
  1128  // mark time in profiles.
  1129  func gcDrainMarkWorkerFractional(gcw *gcWork) {
  1130  	gcDrain(gcw, gcDrainFractional|gcDrainUntilPreempt|gcDrainFlushBgCredit)
  1131  }
  1132  
  1133  // gcDrain scans roots and objects in work buffers, blackening grey
  1134  // objects until it is unable to get more work. It may return before
  1135  // GC is done; it's the caller's responsibility to balance work from
  1136  // other Ps.
  1137  //
  1138  // If flags&gcDrainUntilPreempt != 0, gcDrain returns when g.preempt
  1139  // is set.
  1140  //
  1141  // If flags&gcDrainIdle != 0, gcDrain returns when there is other work
  1142  // to do.
  1143  //
  1144  // If flags&gcDrainFractional != 0, gcDrain self-preempts when
  1145  // pollFractionalWorkerExit() returns true. This implies
  1146  // gcDrainNoBlock.
  1147  //
  1148  // If flags&gcDrainFlushBgCredit != 0, gcDrain flushes scan work
  1149  // credit to gcController.bgScanCredit every gcCreditSlack units of
  1150  // scan work.
  1151  //
  1152  // gcDrain will always return if there is a pending STW or forEachP.
  1153  //
  1154  // Disabling write barriers is necessary to ensure that after we've
  1155  // confirmed that we've drained gcw, that we don't accidentally end
  1156  // up flipping that condition by immediately adding work in the form
  1157  // of a write barrier buffer flush.
  1158  //
  1159  // Don't set nowritebarrierrec because it's safe for some callees to
  1160  // have write barriers enabled.
  1161  //
  1162  //go:nowritebarrier
  1163  func gcDrain(gcw *gcWork, flags gcDrainFlags) {
  1164  	if !writeBarrier.enabled {
  1165  		throw("gcDrain phase incorrect")
  1166  	}
  1167  
  1168  	// N.B. We must be running in a non-preemptible context, so it's
  1169  	// safe to hold a reference to our P here.
  1170  	gp := getg().m.curg
  1171  	pp := gp.m.p.ptr()
  1172  	preemptible := flags&gcDrainUntilPreempt != 0
  1173  	flushBgCredit := flags&gcDrainFlushBgCredit != 0
  1174  	idle := flags&gcDrainIdle != 0
  1175  
  1176  	initScanWork := gcw.heapScanWork
  1177  
  1178  	// checkWork is the scan work before performing the next
  1179  	// self-preempt check.
  1180  	checkWork := int64(1<<63 - 1)
  1181  	var check func() bool
  1182  	if flags&(gcDrainIdle|gcDrainFractional) != 0 {
  1183  		checkWork = initScanWork + drainCheckThreshold
  1184  		if idle {
  1185  			check = pollWork
  1186  		} else if flags&gcDrainFractional != 0 {
  1187  			check = pollFractionalWorkerExit
  1188  		}
  1189  	}
  1190  
  1191  	// Drain root marking jobs.
  1192  	if work.markrootNext < work.markrootJobs {
  1193  		// Stop if we're preemptible, if someone wants to STW, or if
  1194  		// someone is calling forEachP.
  1195  		for !(gp.preempt && (preemptible || sched.gcwaiting.Load() || pp.runSafePointFn != 0)) {
  1196  			job := atomic.Xadd(&work.markrootNext, +1) - 1
  1197  			if job >= work.markrootJobs {
  1198  				break
  1199  			}
  1200  			markroot(gcw, job, flushBgCredit)
  1201  			if check != nil && check() {
  1202  				goto done
  1203  			}
  1204  		}
  1205  	}
  1206  
  1207  	// Drain heap marking jobs.
  1208  	//
  1209  	// Stop if we're preemptible, if someone wants to STW, or if
  1210  	// someone is calling forEachP.
  1211  	//
  1212  	// TODO(mknyszek): Consider always checking gp.preempt instead
  1213  	// of having the preempt flag, and making an exception for certain
  1214  	// mark workers in retake. That might be simpler than trying to
  1215  	// enumerate all the reasons why we might want to preempt, even
  1216  	// if we're supposed to be mostly non-preemptible.
  1217  	for !(gp.preempt && (preemptible || sched.gcwaiting.Load() || pp.runSafePointFn != 0)) {
  1218  		// Try to keep work available on the global queue. We used to
  1219  		// check if there were waiting workers, but it's better to
  1220  		// just keep work available than to make workers wait. In the
  1221  		// worst case, we'll do O(log(_WorkbufSize)) unnecessary
  1222  		// balances.
  1223  		if work.full == 0 {
  1224  			gcw.balance()
  1225  		}
  1226  
  1227  		b := gcw.tryGetFast()
  1228  		if b == 0 {
  1229  			b = gcw.tryGet()
  1230  			if b == 0 {
  1231  				// Flush the write barrier
  1232  				// buffer; this may create
  1233  				// more work.
  1234  				wbBufFlush()
  1235  				b = gcw.tryGet()
  1236  			}
  1237  		}
  1238  		if b == 0 {
  1239  			// Unable to get work.
  1240  			break
  1241  		}
  1242  		scanobject(b, gcw)
  1243  
  1244  		// Flush background scan work credit to the global
  1245  		// account if we've accumulated enough locally so
  1246  		// mutator assists can draw on it.
  1247  		if gcw.heapScanWork >= gcCreditSlack {
  1248  			gcController.heapScanWork.Add(gcw.heapScanWork)
  1249  			if flushBgCredit {
  1250  				gcFlushBgCredit(gcw.heapScanWork - initScanWork)
  1251  				initScanWork = 0
  1252  			}
  1253  			checkWork -= gcw.heapScanWork
  1254  			gcw.heapScanWork = 0
  1255  
  1256  			if checkWork <= 0 {
  1257  				checkWork += drainCheckThreshold
  1258  				if check != nil && check() {
  1259  					break
  1260  				}
  1261  			}
  1262  		}
  1263  	}
  1264  
  1265  done:
  1266  	// Flush remaining scan work credit.
  1267  	if gcw.heapScanWork > 0 {
  1268  		gcController.heapScanWork.Add(gcw.heapScanWork)
  1269  		if flushBgCredit {
  1270  			gcFlushBgCredit(gcw.heapScanWork - initScanWork)
  1271  		}
  1272  		gcw.heapScanWork = 0
  1273  	}
  1274  }
  1275  
  1276  // gcDrainN blackens grey objects until it has performed roughly
  1277  // scanWork units of scan work or the G is preempted. This is
  1278  // best-effort, so it may perform less work if it fails to get a work
  1279  // buffer. Otherwise, it will perform at least n units of work, but
  1280  // may perform more because scanning is always done in whole object
  1281  // increments. It returns the amount of scan work performed.
  1282  //
  1283  // The caller goroutine must be in a preemptible state (e.g.,
  1284  // _Gwaiting) to prevent deadlocks during stack scanning. As a
  1285  // consequence, this must be called on the system stack.
  1286  //
  1287  //go:nowritebarrier
  1288  //go:systemstack
  1289  func gcDrainN(gcw *gcWork, scanWork int64) int64 {
  1290  	if !writeBarrier.enabled {
  1291  		throw("gcDrainN phase incorrect")
  1292  	}
  1293  
  1294  	// There may already be scan work on the gcw, which we don't
  1295  	// want to claim was done by this call.
  1296  	workFlushed := -gcw.heapScanWork
  1297  
  1298  	// In addition to backing out because of a preemption, back out
  1299  	// if the GC CPU limiter is enabled.
  1300  	gp := getg().m.curg
  1301  	for !gp.preempt && !gcCPULimiter.limiting() && workFlushed+gcw.heapScanWork < scanWork {
  1302  		// See gcDrain comment.
  1303  		if work.full == 0 {
  1304  			gcw.balance()
  1305  		}
  1306  
  1307  		b := gcw.tryGetFast()
  1308  		if b == 0 {
  1309  			b = gcw.tryGet()
  1310  			if b == 0 {
  1311  				// Flush the write barrier buffer;
  1312  				// this may create more work.
  1313  				wbBufFlush()
  1314  				b = gcw.tryGet()
  1315  			}
  1316  		}
  1317  
  1318  		if b == 0 {
  1319  			// Try to do a root job.
  1320  			if work.markrootNext < work.markrootJobs {
  1321  				job := atomic.Xadd(&work.markrootNext, +1) - 1
  1322  				if job < work.markrootJobs {
  1323  					workFlushed += markroot(gcw, job, false)
  1324  					continue
  1325  				}
  1326  			}
  1327  			// No heap or root jobs.
  1328  			break
  1329  		}
  1330  
  1331  		scanobject(b, gcw)
  1332  
  1333  		// Flush background scan work credit.
  1334  		if gcw.heapScanWork >= gcCreditSlack {
  1335  			gcController.heapScanWork.Add(gcw.heapScanWork)
  1336  			workFlushed += gcw.heapScanWork
  1337  			gcw.heapScanWork = 0
  1338  		}
  1339  	}
  1340  
  1341  	// Unlike gcDrain, there's no need to flush remaining work
  1342  	// here because this never flushes to bgScanCredit and
  1343  	// gcw.dispose will flush any remaining work to scanWork.
  1344  
  1345  	return workFlushed + gcw.heapScanWork
  1346  }
  1347  
  1348  // scanblock scans b as scanobject would, but using an explicit
  1349  // pointer bitmap instead of the heap bitmap.
  1350  //
  1351  // This is used to scan non-heap roots, so it does not update
  1352  // gcw.bytesMarked or gcw.heapScanWork.
  1353  //
  1354  // If stk != nil, possible stack pointers are also reported to stk.putPtr.
  1355  //
  1356  //go:nowritebarrier
  1357  func scanblock(b0, n0 uintptr, ptrmask *uint8, gcw *gcWork, stk *stackScanState) {
  1358  	// Use local copies of original parameters, so that a stack trace
  1359  	// due to one of the throws below shows the original block
  1360  	// base and extent.
  1361  	b := b0
  1362  	n := n0
  1363  
  1364  	for i := uintptr(0); i < n; {
  1365  		// Find bits for the next word.
  1366  		bits := uint32(*addb(ptrmask, i/(goarch.PtrSize*8)))
  1367  		if bits == 0 {
  1368  			i += goarch.PtrSize * 8
  1369  			continue
  1370  		}
  1371  		for j := 0; j < 8 && i < n; j++ {
  1372  			if bits&1 != 0 {
  1373  				// Same work as in scanobject; see comments there.
  1374  				p := *(*uintptr)(unsafe.Pointer(b + i))
  1375  				if p != 0 {
  1376  					if obj, span, objIndex := findObject(p, b, i); obj != 0 {
  1377  						greyobject(obj, b, i, span, gcw, objIndex)
  1378  					} else if stk != nil && p >= stk.stack.lo && p < stk.stack.hi {
  1379  						stk.putPtr(p, false)
  1380  					}
  1381  				}
  1382  			}
  1383  			bits >>= 1
  1384  			i += goarch.PtrSize
  1385  		}
  1386  	}
  1387  }
  1388  
  1389  // scanobject scans the object starting at b, adding pointers to gcw.
  1390  // b must point to the beginning of a heap object or an oblet.
  1391  // scanobject consults the GC bitmap for the pointer mask and the
  1392  // spans for the size of the object.
  1393  //
  1394  //go:nowritebarrier
  1395  func scanobject(b uintptr, gcw *gcWork) {
  1396  	// Prefetch object before we scan it.
  1397  	//
  1398  	// This will overlap fetching the beginning of the object with initial
  1399  	// setup before we start scanning the object.
  1400  	sys.Prefetch(b)
  1401  
  1402  	// Find the bits for b and the size of the object at b.
  1403  	//
  1404  	// b is either the beginning of an object, in which case this
  1405  	// is the size of the object to scan, or it points to an
  1406  	// oblet, in which case we compute the size to scan below.
  1407  	s := spanOfUnchecked(b)
  1408  	n := s.elemsize
  1409  	if n == 0 {
  1410  		throw("scanobject n == 0")
  1411  	}
  1412  	if s.spanclass.noscan() {
  1413  		// Correctness-wise this is ok, but it's inefficient
  1414  		// if noscan objects reach here.
  1415  		throw("scanobject of a noscan object")
  1416  	}
  1417  
  1418  	var tp typePointers
  1419  	if n > maxObletBytes {
  1420  		// Large object. Break into oblets for better
  1421  		// parallelism and lower latency.
  1422  		if b == s.base() {
  1423  			// Enqueue the other oblets to scan later.
  1424  			// Some oblets may be in b's scalar tail, but
  1425  			// these will be marked as "no more pointers",
  1426  			// so we'll drop out immediately when we go to
  1427  			// scan those.
  1428  			for oblet := b + maxObletBytes; oblet < s.base()+s.elemsize; oblet += maxObletBytes {
  1429  				if !gcw.putFast(oblet) {
  1430  					gcw.put(oblet)
  1431  				}
  1432  			}
  1433  		}
  1434  
  1435  		// Compute the size of the oblet. Since this object
  1436  		// must be a large object, s.base() is the beginning
  1437  		// of the object.
  1438  		n = s.base() + s.elemsize - b
  1439  		n = min(n, maxObletBytes)
  1440  		if goexperiment.AllocHeaders {
  1441  			tp = s.typePointersOfUnchecked(s.base())
  1442  			tp = tp.fastForward(b-tp.addr, b+n)
  1443  		}
  1444  	} else {
  1445  		if goexperiment.AllocHeaders {
  1446  			tp = s.typePointersOfUnchecked(b)
  1447  		}
  1448  	}
  1449  
  1450  	var hbits heapBits
  1451  	if !goexperiment.AllocHeaders {
  1452  		hbits = heapBitsForAddr(b, n)
  1453  	}
  1454  	var scanSize uintptr
  1455  	for {
  1456  		var addr uintptr
  1457  		if goexperiment.AllocHeaders {
  1458  			if tp, addr = tp.nextFast(); addr == 0 {
  1459  				if tp, addr = tp.next(b + n); addr == 0 {
  1460  					break
  1461  				}
  1462  			}
  1463  		} else {
  1464  			if hbits, addr = hbits.nextFast(); addr == 0 {
  1465  				if hbits, addr = hbits.next(); addr == 0 {
  1466  					break
  1467  				}
  1468  			}
  1469  		}
  1470  
  1471  		// Keep track of farthest pointer we found, so we can
  1472  		// update heapScanWork. TODO: is there a better metric,
  1473  		// now that we can skip scalar portions pretty efficiently?
  1474  		scanSize = addr - b + goarch.PtrSize
  1475  
  1476  		// Work here is duplicated in scanblock and above.
  1477  		// If you make changes here, make changes there too.
  1478  		obj := *(*uintptr)(unsafe.Pointer(addr))
  1479  
  1480  		// At this point we have extracted the next potential pointer.
  1481  		// Quickly filter out nil and pointers back to the current object.
  1482  		if obj != 0 && obj-b >= n {
  1483  			// Test if obj points into the Go heap and, if so,
  1484  			// mark the object.
  1485  			//
  1486  			// Note that it's possible for findObject to
  1487  			// fail if obj points to a just-allocated heap
  1488  			// object because of a race with growing the
  1489  			// heap. In this case, we know the object was
  1490  			// just allocated and hence will be marked by
  1491  			// allocation itself.
  1492  			if obj, span, objIndex := findObject(obj, b, addr-b); obj != 0 {
  1493  				greyobject(obj, b, addr-b, span, gcw, objIndex)
  1494  			}
  1495  		}
  1496  	}
  1497  	gcw.bytesMarked += uint64(n)
  1498  	gcw.heapScanWork += int64(scanSize)
  1499  }
  1500  
  1501  // scanConservative scans block [b, b+n) conservatively, treating any
  1502  // pointer-like value in the block as a pointer.
  1503  //
  1504  // If ptrmask != nil, only words that are marked in ptrmask are
  1505  // considered as potential pointers.
  1506  //
  1507  // If state != nil, it's assumed that [b, b+n) is a block in the stack
  1508  // and may contain pointers to stack objects.
  1509  func scanConservative(b, n uintptr, ptrmask *uint8, gcw *gcWork, state *stackScanState) {
  1510  	if debugScanConservative {
  1511  		printlock()
  1512  		print("conservatively scanning [", hex(b), ",", hex(b+n), ")\n")
  1513  		hexdumpWords(b, b+n, func(p uintptr) byte {
  1514  			if ptrmask != nil {
  1515  				word := (p - b) / goarch.PtrSize
  1516  				bits := *addb(ptrmask, word/8)
  1517  				if (bits>>(word%8))&1 == 0 {
  1518  					return '$'
  1519  				}
  1520  			}
  1521  
  1522  			val := *(*uintptr)(unsafe.Pointer(p))
  1523  			if state != nil && state.stack.lo <= val && val < state.stack.hi {
  1524  				return '@'
  1525  			}
  1526  
  1527  			span := spanOfHeap(val)
  1528  			if span == nil {
  1529  				return ' '
  1530  			}
  1531  			idx := span.objIndex(val)
  1532  			if span.isFree(idx) {
  1533  				return ' '
  1534  			}
  1535  			return '*'
  1536  		})
  1537  		printunlock()
  1538  	}
  1539  
  1540  	for i := uintptr(0); i < n; i += goarch.PtrSize {
  1541  		if ptrmask != nil {
  1542  			word := i / goarch.PtrSize
  1543  			bits := *addb(ptrmask, word/8)
  1544  			if bits == 0 {
  1545  				// Skip 8 words (the loop increment will do the 8th)
  1546  				//
  1547  				// This must be the first time we've
  1548  				// seen this word of ptrmask, so i
  1549  				// must be 8-word-aligned, but check
  1550  				// our reasoning just in case.
  1551  				if i%(goarch.PtrSize*8) != 0 {
  1552  					throw("misaligned mask")
  1553  				}
  1554  				i += goarch.PtrSize*8 - goarch.PtrSize
  1555  				continue
  1556  			}
  1557  			if (bits>>(word%8))&1 == 0 {
  1558  				continue
  1559  			}
  1560  		}
  1561  
  1562  		val := *(*uintptr)(unsafe.Pointer(b + i))
  1563  
  1564  		// Check if val points into the stack.
  1565  		if state != nil && state.stack.lo <= val && val < state.stack.hi {
  1566  			// val may point to a stack object. This
  1567  			// object may be dead from last cycle and
  1568  			// hence may contain pointers to unallocated
  1569  			// objects, but unlike heap objects we can't
  1570  			// tell if it's already dead. Hence, if all
  1571  			// pointers to this object are from
  1572  			// conservative scanning, we have to scan it
  1573  			// defensively, too.
  1574  			state.putPtr(val, true)
  1575  			continue
  1576  		}
  1577  
  1578  		// Check if val points to a heap span.
  1579  		span := spanOfHeap(val)
  1580  		if span == nil {
  1581  			continue
  1582  		}
  1583  
  1584  		// Check if val points to an allocated object.
  1585  		idx := span.objIndex(val)
  1586  		if span.isFree(idx) {
  1587  			continue
  1588  		}
  1589  
  1590  		// val points to an allocated object. Mark it.
  1591  		obj := span.base() + idx*span.elemsize
  1592  		greyobject(obj, b, i, span, gcw, idx)
  1593  	}
  1594  }
  1595  
  1596  // Shade the object if it isn't already.
  1597  // The object is not nil and known to be in the heap.
  1598  // Preemption must be disabled.
  1599  //
  1600  //go:nowritebarrier
  1601  func shade(b uintptr) {
  1602  	if obj, span, objIndex := findObject(b, 0, 0); obj != 0 {
  1603  		gcw := &getg().m.p.ptr().gcw
  1604  		greyobject(obj, 0, 0, span, gcw, objIndex)
  1605  	}
  1606  }
  1607  
  1608  // obj is the start of an object with mark mbits.
  1609  // If it isn't already marked, mark it and enqueue into gcw.
  1610  // base and off are for debugging only and could be removed.
  1611  //
  1612  // See also wbBufFlush1, which partially duplicates this logic.
  1613  //
  1614  //go:nowritebarrierrec
  1615  func greyobject(obj, base, off uintptr, span *mspan, gcw *gcWork, objIndex uintptr) {
  1616  	// obj should be start of allocation, and so must be at least pointer-aligned.
  1617  	if obj&(goarch.PtrSize-1) != 0 {
  1618  		throw("greyobject: obj not pointer-aligned")
  1619  	}
  1620  	mbits := span.markBitsForIndex(objIndex)
  1621  
  1622  	if useCheckmark {
  1623  		if setCheckmark(obj, base, off, mbits) {
  1624  			// Already marked.
  1625  			return
  1626  		}
  1627  	} else {
  1628  		if debug.gccheckmark > 0 && span.isFree(objIndex) {
  1629  			print("runtime: marking free object ", hex(obj), " found at *(", hex(base), "+", hex(off), ")\n")
  1630  			gcDumpObject("base", base, off)
  1631  			gcDumpObject("obj", obj, ^uintptr(0))
  1632  			getg().m.traceback = 2
  1633  			throw("marking free object")
  1634  		}
  1635  
  1636  		// If marked we have nothing to do.
  1637  		if mbits.isMarked() {
  1638  			return
  1639  		}
  1640  		mbits.setMarked()
  1641  
  1642  		// Mark span.
  1643  		arena, pageIdx, pageMask := pageIndexOf(span.base())
  1644  		if arena.pageMarks[pageIdx]&pageMask == 0 {
  1645  			atomic.Or8(&arena.pageMarks[pageIdx], pageMask)
  1646  		}
  1647  
  1648  		// If this is a noscan object, fast-track it to black
  1649  		// instead of greying it.
  1650  		if span.spanclass.noscan() {
  1651  			gcw.bytesMarked += uint64(span.elemsize)
  1652  			return
  1653  		}
  1654  	}
  1655  
  1656  	// We're adding obj to P's local workbuf, so it's likely
  1657  	// this object will be processed soon by the same P.
  1658  	// Even if the workbuf gets flushed, there will likely still be
  1659  	// some benefit on platforms with inclusive shared caches.
  1660  	sys.Prefetch(obj)
  1661  	// Queue the obj for scanning.
  1662  	if !gcw.putFast(obj) {
  1663  		gcw.put(obj)
  1664  	}
  1665  }
  1666  
  1667  // gcDumpObject dumps the contents of obj for debugging and marks the
  1668  // field at byte offset off in obj.
  1669  func gcDumpObject(label string, obj, off uintptr) {
  1670  	s := spanOf(obj)
  1671  	print(label, "=", hex(obj))
  1672  	if s == nil {
  1673  		print(" s=nil\n")
  1674  		return
  1675  	}
  1676  	print(" s.base()=", hex(s.base()), " s.limit=", hex(s.limit), " s.spanclass=", s.spanclass, " s.elemsize=", s.elemsize, " s.state=")
  1677  	if state := s.state.get(); 0 <= state && int(state) < len(mSpanStateNames) {
  1678  		print(mSpanStateNames[state], "\n")
  1679  	} else {
  1680  		print("unknown(", state, ")\n")
  1681  	}
  1682  
  1683  	skipped := false
  1684  	size := s.elemsize
  1685  	if s.state.get() == mSpanManual && size == 0 {
  1686  		// We're printing something from a stack frame. We
  1687  		// don't know how big it is, so just show up to an
  1688  		// including off.
  1689  		size = off + goarch.PtrSize
  1690  	}
  1691  	for i := uintptr(0); i < size; i += goarch.PtrSize {
  1692  		// For big objects, just print the beginning (because
  1693  		// that usually hints at the object's type) and the
  1694  		// fields around off.
  1695  		if !(i < 128*goarch.PtrSize || off-16*goarch.PtrSize < i && i < off+16*goarch.PtrSize) {
  1696  			skipped = true
  1697  			continue
  1698  		}
  1699  		if skipped {
  1700  			print(" ...\n")
  1701  			skipped = false
  1702  		}
  1703  		print(" *(", label, "+", i, ") = ", hex(*(*uintptr)(unsafe.Pointer(obj + i))))
  1704  		if i == off {
  1705  			print(" <==")
  1706  		}
  1707  		print("\n")
  1708  	}
  1709  	if skipped {
  1710  		print(" ...\n")
  1711  	}
  1712  }
  1713  
  1714  // gcmarknewobject marks a newly allocated object black. obj must
  1715  // not contain any non-nil pointers.
  1716  //
  1717  // This is nosplit so it can manipulate a gcWork without preemption.
  1718  //
  1719  //go:nowritebarrier
  1720  //go:nosplit
  1721  func gcmarknewobject(span *mspan, obj uintptr) {
  1722  	if useCheckmark { // The world should be stopped so this should not happen.
  1723  		throw("gcmarknewobject called while doing checkmark")
  1724  	}
  1725  
  1726  	// Mark object.
  1727  	objIndex := span.objIndex(obj)
  1728  	span.markBitsForIndex(objIndex).setMarked()
  1729  
  1730  	// Mark span.
  1731  	arena, pageIdx, pageMask := pageIndexOf(span.base())
  1732  	if arena.pageMarks[pageIdx]&pageMask == 0 {
  1733  		atomic.Or8(&arena.pageMarks[pageIdx], pageMask)
  1734  	}
  1735  
  1736  	gcw := &getg().m.p.ptr().gcw
  1737  	gcw.bytesMarked += uint64(span.elemsize)
  1738  }
  1739  
  1740  // gcMarkTinyAllocs greys all active tiny alloc blocks.
  1741  //
  1742  // The world must be stopped.
  1743  func gcMarkTinyAllocs() {
  1744  	assertWorldStopped()
  1745  
  1746  	for _, p := range allp {
  1747  		c := p.mcache
  1748  		if c == nil || c.tiny == 0 {
  1749  			continue
  1750  		}
  1751  		_, span, objIndex := findObject(c.tiny, 0, 0)
  1752  		gcw := &p.gcw
  1753  		greyobject(c.tiny, 0, 0, span, gcw, objIndex)
  1754  	}
  1755  }
  1756  

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