Source: trace.go in package runtime

Go execution tracer. The tracer captures a wide range of execution events like goroutine creation/blocking/unblocking, syscall enter/exit/block, GC-related events, changes of heap size, processor start/stop, etc and writes them to a buffer in a compact form. A precise nanosecond-precision timestamp and a stack trace is captured for most events. See https://golang.org/s/go15trace for more info.


package runtime

import (
	"runtime/internal/atomic"
	"runtime/internal/sys"
	"unsafe"
)

Event types in the trace, args are given in square brackets.

const (
	traceEvNone              = 0  // unused
	traceEvBatch             = 1  // start of per-P batch of events [pid, timestamp]
	traceEvFrequency         = 2  // contains tracer timer frequency [frequency (ticks per second)]
	traceEvStack             = 3  // stack [stack id, number of PCs, array of {PC, func string ID, file string ID, line}]
	traceEvGomaxprocs        = 4  // current value of GOMAXPROCS [timestamp, GOMAXPROCS, stack id]
	traceEvProcStart         = 5  // start of P [timestamp, thread id]
	traceEvProcStop          = 6  // stop of P [timestamp]
	traceEvGCStart           = 7  // GC start [timestamp, seq, stack id]
	traceEvGCDone            = 8  // GC done [timestamp]
	traceEvGCSTWStart        = 9  // GC STW start [timestamp, kind]
	traceEvGCSTWDone         = 10 // GC STW done [timestamp]
	traceEvGCSweepStart      = 11 // GC sweep start [timestamp, stack id]
	traceEvGCSweepDone       = 12 // GC sweep done [timestamp, swept, reclaimed]
	traceEvGoCreate          = 13 // goroutine creation [timestamp, new goroutine id, new stack id, stack id]
	traceEvGoStart           = 14 // goroutine starts running [timestamp, goroutine id, seq]
	traceEvGoEnd             = 15 // goroutine ends [timestamp]
	traceEvGoStop            = 16 // goroutine stops (like in select{}) [timestamp, stack]
	traceEvGoSched           = 17 // goroutine calls Gosched [timestamp, stack]
	traceEvGoPreempt         = 18 // goroutine is preempted [timestamp, stack]
	traceEvGoSleep           = 19 // goroutine calls Sleep [timestamp, stack]
	traceEvGoBlock           = 20 // goroutine blocks [timestamp, stack]
	traceEvGoUnblock         = 21 // goroutine is unblocked [timestamp, goroutine id, seq, stack]
	traceEvGoBlockSend       = 22 // goroutine blocks on chan send [timestamp, stack]
	traceEvGoBlockRecv       = 23 // goroutine blocks on chan recv [timestamp, stack]
	traceEvGoBlockSelect     = 24 // goroutine blocks on select [timestamp, stack]
	traceEvGoBlockSync       = 25 // goroutine blocks on Mutex/RWMutex [timestamp, stack]
	traceEvGoBlockCond       = 26 // goroutine blocks on Cond [timestamp, stack]
	traceEvGoBlockNet        = 27 // goroutine blocks on network [timestamp, stack]
	traceEvGoSysCall         = 28 // syscall enter [timestamp, stack]
	traceEvGoSysExit         = 29 // syscall exit [timestamp, goroutine id, seq, real timestamp]
	traceEvGoSysBlock        = 30 // syscall blocks [timestamp]
	traceEvGoWaiting         = 31 // denotes that goroutine is blocked when tracing starts [timestamp, goroutine id]
	traceEvGoInSyscall       = 32 // denotes that goroutine is in syscall when tracing starts [timestamp, goroutine id]
	traceEvHeapAlloc         = 33 // memstats.heap_live change [timestamp, heap_alloc]
	traceEvNextGC            = 34 // memstats.next_gc change [timestamp, next_gc]
	traceEvTimerGoroutine    = 35 // not currently used; previously denoted timer goroutine [timer goroutine id]
	traceEvFutileWakeup      = 36 // denotes that the previous wakeup of this goroutine was futile [timestamp]
	traceEvString            = 37 // string dictionary entry [ID, length, string]
	traceEvGoStartLocal      = 38 // goroutine starts running on the same P as the last event [timestamp, goroutine id]
	traceEvGoUnblockLocal    = 39 // goroutine is unblocked on the same P as the last event [timestamp, goroutine id, stack]
	traceEvGoSysExitLocal    = 40 // syscall exit on the same P as the last event [timestamp, goroutine id, real timestamp]
	traceEvGoStartLabel      = 41 // goroutine starts running with label [timestamp, goroutine id, seq, label string id]
	traceEvGoBlockGC         = 42 // goroutine blocks on GC assist [timestamp, stack]
	traceEvGCMarkAssistStart = 43 // GC mark assist start [timestamp, stack]
	traceEvGCMarkAssistDone  = 44 // GC mark assist done [timestamp]
	traceEvUserTaskCreate    = 45 // trace.NewContext [timestamp, internal task id, internal parent task id, stack, name string]
	traceEvUserTaskEnd       = 46 // end of a task [timestamp, internal task id, stack]
	traceEvUserRegion        = 47 // trace.WithRegion [timestamp, internal task id, mode(0:start, 1:end), stack, name string]
	traceEvUserLog           = 48 // trace.Log [timestamp, internal task id, key string id, stack, value string]

Byte is used but only 6 bits are available for event type. The remaining 2 bits are used to specify the number of arguments. That means, the max event type value is 63.

Timestamps in trace are cputicks/traceTickDiv. This makes absolute values of timestamp diffs smaller, and so they are encoded in less number of bytes. 64 on x86 is somewhat arbitrary (one tick is ~20ns on a 3GHz machine). The suggested increment frequency for PowerPC's time base register is 512 MHz according to Power ISA v2.07 section 6.2, so we use 16 on ppc64 and ppc64le. Tracing won't work reliably for architectures where cputicks is emulated by nanotime, so the value doesn't matter for those architectures.

Maximum number of PCs in a single stack trace. Since events contain only stack id rather than whole stack trace, we can allow quite large values here.

Identifier of a fake P that is used when we trace without a real P.

Maximum number of bytes to encode uint64 in base-128.

Shift of the number of arguments in the first event byte.

Flag passed to traceGoPark to denote that the previous wakeup of this goroutine was futile. For example, a goroutine was unblocked on a mutex, but another goroutine got ahead and acquired the mutex before the first goroutine is scheduled, so the first goroutine has to block again. Such wakeups happen on buffered channels and sync.Mutex, but are generally not interesting for end user.

	traceFutileWakeup byte = 128
)

trace is global tracing context.

var trace struct {
	lock          mutex       // protects the following members
	lockOwner     *g          // to avoid deadlocks during recursive lock locks
	enabled       bool        // when set runtime traces events
	shutdown      bool        // set when we are waiting for trace reader to finish after setting enabled to false
	headerWritten bool        // whether ReadTrace has emitted trace header
	footerWritten bool        // whether ReadTrace has emitted trace footer
	shutdownSema  uint32      // used to wait for ReadTrace completion
	seqStart      uint64      // sequence number when tracing was started
	ticksStart    int64       // cputicks when tracing was started
	ticksEnd      int64       // cputicks when tracing was stopped
	timeStart     int64       // nanotime when tracing was started
	timeEnd       int64       // nanotime when tracing was stopped
	seqGC         uint64      // GC start/done sequencer
	reading       traceBufPtr // buffer currently handed off to user
	empty         traceBufPtr // stack of empty buffers
	fullHead      traceBufPtr // queue of full buffers
	fullTail      traceBufPtr
	reader        guintptr        // goroutine that called ReadTrace, or nil
	stackTab      traceStackTable // maps stack traces to unique ids

Dictionary for traceEvString. TODO: central lock to access the map is not ideal. option: pre-assign ids to all user annotation region names and tags option: per-P cache option: sync.Map like data structure

	stringsLock mutex
	strings     map[string]uint64
	stringSeq   uint64

markWorkerLabels maps gcMarkWorkerMode to string ID.

	markWorkerLabels [len(gcMarkWorkerModeStrings)]uint64

	bufLock mutex       // protects buf
	buf     traceBufPtr // global trace buffer, used when running without a p
}

traceBufHeader is per-P tracing buffer.

type traceBufHeader struct {
	link      traceBufPtr             // in trace.empty/full
	lastTicks uint64                  // when we wrote the last event
	pos       int                     // next write offset in arr
	stk       [traceStackSize]uintptr // scratch buffer for traceback
}

traceBuf is per-P tracing buffer.go:notinheap

type traceBuf struct {
	traceBufHeader
	arr [64<<10 - unsafe.Sizeof(traceBufHeader{})]byte // underlying buffer for traceBufHeader.buf
}

traceBufPtr is a *traceBuf that is not traced by the garbage collector and doesn't have write barriers. traceBufs are not allocated from the GC'd heap, so this is safe, and are often manipulated in contexts where write barriers are not allowed, so this is necessary. TODO: Since traceBuf is now go:notinheap, this isn't necessary.

type traceBufPtr uintptr

func (tp traceBufPtr) ptr() *traceBuf   { return (*traceBuf)(unsafe.Pointer(tp)) }
func (tp *traceBufPtr) set(b *traceBuf) { *tp = traceBufPtr(unsafe.Pointer(b)) }
func traceBufPtrOf(b *traceBuf) traceBufPtr {
	return traceBufPtr(unsafe.Pointer(b))
}

StartTrace enables tracing for the current process. While tracing, the data will be buffered and available via ReadTrace. StartTrace returns an error if tracing is already enabled. Most clients should use the runtime/trace package or the testing package's -test.trace flag instead of calling StartTrace directly.

Stop the world so that we can take a consistent snapshot of all goroutines at the beginning of the trace. Do not stop the world during GC so we ensure we always see a consistent view of GC-related events (e.g. a start is always paired with an end).

	stopTheWorldGC("start tracing")

Prevent sysmon from running any code that could generate events.

	lock(&sched.sysmonlock)

We are in stop-the-world, but syscalls can finish and write to trace concurrently. Exitsyscall could check trace.enabled long before and then suddenly wake up and decide to write to trace at a random point in time. However, such syscall will use the global trace.buf buffer, because we've acquired all p's by doing stop-the-world. So this protects us from such races.

	lock(&trace.bufLock)

	if trace.enabled || trace.shutdown {
		unlock(&trace.bufLock)
		unlock(&sched.sysmonlock)
		startTheWorldGC()
		return errorString("tracing is already enabled")
	}

Can't set trace.enabled yet. While the world is stopped, exitsyscall could already emit a delayed event (see exitTicks in exitsyscall) if we set trace.enabled here. That would lead to an inconsistent trace: - either GoSysExit appears before EvGoInSyscall, - or GoSysExit appears for a goroutine for which we don't emit EvGoInSyscall below. To instruct traceEvent that it must not ignore events below, we set startingtrace. trace.enabled is set afterwards once we have emitted all preliminary events.

	_g_ := getg()
	_g_.m.startingtrace = true

Obtain current stack ID to use in all traceEvGoCreate events below.

	mp := acquirem()
	stkBuf := make([]uintptr, traceStackSize)
	stackID := traceStackID(mp, stkBuf, 2)
	releasem(mp)

	for _, gp := range allgs {
		status := readgstatus(gp)
		if status != _Gdead {
			gp.traceseq = 0

+PCQuantum because traceFrameForPC expects return PCs and subtracts PCQuantum.

			id := trace.stackTab.put([]uintptr{gp.startpc + sys.PCQuantum})
			traceEvent(traceEvGoCreate, -1, uint64(gp.goid), uint64(id), stackID)
		}

traceEvGoWaiting is implied to have seq=1.

			gp.traceseq++
			traceEvent(traceEvGoWaiting, -1, uint64(gp.goid))
		}
		if status == _Gsyscall {
			gp.traceseq++
			traceEvent(traceEvGoInSyscall, -1, uint64(gp.goid))
		} else {
			gp.sysblocktraced = false
		}
	}
	traceProcStart()

Note: ticksStart needs to be set after we emit traceEvGoInSyscall events. If we do it the other way around, it is possible that exitsyscall will query sysexitticks after ticksStart but before traceEvGoInSyscall timestamp. It will lead to a false conclusion that cputicks is broken.

	trace.ticksStart = cputicks()
	trace.timeStart = nanotime()
	trace.headerWritten = false
	trace.footerWritten = false

string to id mapping 0 : reserved for an empty string remaining: other strings registered by traceString

	trace.stringSeq = 0
	trace.strings = make(map[string]uint64)

	trace.seqGC = 0
	_g_.m.startingtrace = false
	trace.enabled = true

	_, pid, bufp := traceAcquireBuffer()
	for i, label := range gcMarkWorkerModeStrings[:] {
		trace.markWorkerLabels[i], bufp = traceString(bufp, pid, label)
	}
	traceReleaseBuffer(pid)

	unlock(&trace.bufLock)

	unlock(&sched.sysmonlock)

	startTheWorldGC()
	return nil
}

StopTrace stops tracing, if it was previously enabled. StopTrace only returns after all the reads for the trace have completed.

Stop the world so that we can collect the trace buffers from all p's below, and also to avoid races with traceEvent.

	stopTheWorldGC("stop tracing")

See the comment in StartTrace.

	lock(&sched.sysmonlock)

See the comment in StartTrace.

	lock(&trace.bufLock)

	if !trace.enabled {
		unlock(&trace.bufLock)
		unlock(&sched.sysmonlock)
		startTheWorldGC()
		return
	}

	traceGoSched()

Loop over all allocated Ps because dead Ps may still have trace buffers.

	for _, p := range allp[:cap(allp)] {
		buf := p.tracebuf
		if buf != 0 {
			traceFullQueue(buf)
			p.tracebuf = 0
		}
	}
	if trace.buf != 0 {
		buf := trace.buf
		trace.buf = 0
		if buf.ptr().pos != 0 {
			traceFullQueue(buf)
		}
	}

	for {
		trace.ticksEnd = cputicks()

Windows time can tick only every 15ms, wait for at least one tick.

		if trace.timeEnd != trace.timeStart {
			break
		}
		osyield()
	}

	trace.enabled = false
	trace.shutdown = true
	unlock(&trace.bufLock)

	unlock(&sched.sysmonlock)

	startTheWorldGC()

The world is started but we've set trace.shutdown, so new tracing can't start. Wait for the trace reader to flush pending buffers and stop.

	semacquire(&trace.shutdownSema)
	if raceenabled {
		raceacquire(unsafe.Pointer(&trace.shutdownSema))
	}

The lock protects us from races with StartTrace/StopTrace because they do stop-the-world.

	lock(&trace.lock)
	for _, p := range allp[:cap(allp)] {
		if p.tracebuf != 0 {
			throw("trace: non-empty trace buffer in proc")
		}
	}
	if trace.buf != 0 {
		throw("trace: non-empty global trace buffer")
	}
	if trace.fullHead != 0 || trace.fullTail != 0 {
		throw("trace: non-empty full trace buffer")
	}
	if trace.reading != 0 || trace.reader != 0 {
		throw("trace: reading after shutdown")
	}
	for trace.empty != 0 {
		buf := trace.empty
		trace.empty = buf.ptr().link
		sysFree(unsafe.Pointer(buf), unsafe.Sizeof(*buf.ptr()), &memstats.other_sys)
	}
	trace.strings = nil
	trace.shutdown = false
	unlock(&trace.lock)
}

ReadTrace returns the next chunk of binary tracing data, blocking until data is available. If tracing is turned off and all the data accumulated while it was on has been returned, ReadTrace returns nil. The caller must copy the returned data before calling ReadTrace again. ReadTrace must be called from one goroutine at a time.

This function may need to lock trace.lock recursively (goparkunlock -> traceGoPark -> traceEvent -> traceFlush). To allow this we use trace.lockOwner. Also this function must not allocate while holding trace.lock: allocation can call heap allocate, which will try to emit a trace event while holding heap lock.

	lock(&trace.lock)
	trace.lockOwner = getg()

More than one goroutine reads trace. This is bad. But we rather do not crash the program because of tracing, because tracing can be enabled at runtime on prod servers.

		trace.lockOwner = nil
		unlock(&trace.lock)
		println("runtime: ReadTrace called from multiple goroutines simultaneously")
		return nil

Recycle the old buffer.

	if buf := trace.reading; buf != 0 {
		buf.ptr().link = trace.empty
		trace.empty = buf
		trace.reading = 0

Write trace header.

	if !trace.headerWritten {
		trace.headerWritten = true
		trace.lockOwner = nil
		unlock(&trace.lock)
		return []byte("go 1.11 trace\x00\x00\x00")

Wait for new data.

	if trace.fullHead == 0 && !trace.shutdown {
		trace.reader.set(getg())
		goparkunlock(&trace.lock, waitReasonTraceReaderBlocked, traceEvGoBlock, 2)
		lock(&trace.lock)

Write a buffer.

	if trace.fullHead != 0 {
		buf := traceFullDequeue()
		trace.reading = buf
		trace.lockOwner = nil
		unlock(&trace.lock)
		return buf.ptr().arr[:buf.ptr().pos]

Write footer with timer frequency.

	if !trace.footerWritten {

Use float64 because (trace.ticksEnd - trace.ticksStart) * 1e9 can overflow int64.

		freq := float64(trace.ticksEnd-trace.ticksStart) * 1e9 / float64(trace.timeEnd-trace.timeStart) / traceTickDiv
		trace.lockOwner = nil
		unlock(&trace.lock)
		var data []byte
		data = append(data, traceEvFrequency|0<<traceArgCountShift)

This will emit a bunch of full buffers, we will pick them up on the next iteration.

		trace.stackTab.dump()
		return data

Done.

	if trace.shutdown {
		trace.lockOwner = nil
		unlock(&trace.lock)

Model synchronization on trace.shutdownSema, which race detector does not see. This is required to avoid false race reports on writer passed to trace.Start.

			racerelease(unsafe.Pointer(&trace.shutdownSema))

trace.enabled is already reset, so can call traceable functions.

		semrelease(&trace.shutdownSema)
		return nil

Also bad, but see the comment above.

	trace.lockOwner = nil
	unlock(&trace.lock)
	println("runtime: spurious wakeup of trace reader")
	return nil
}

traceReader returns the trace reader that should be woken up, if any.

func traceReader() *g {
	if trace.reader == 0 || (trace.fullHead == 0 && !trace.shutdown) {
		return nil
	}
	lock(&trace.lock)
	if trace.reader == 0 || (trace.fullHead == 0 && !trace.shutdown) {
		unlock(&trace.lock)
		return nil
	}
	gp := trace.reader.ptr()
	trace.reader.set(nil)
	unlock(&trace.lock)
	return gp
}

traceProcFree frees trace buffer associated with pp.

func traceProcFree(pp *p) {
	buf := pp.tracebuf
	pp.tracebuf = 0
	if buf == 0 {
		return
	}
	lock(&trace.lock)
	traceFullQueue(buf)
	unlock(&trace.lock)
}

traceFullQueue queues buf into queue of full buffers.

func traceFullQueue(buf traceBufPtr) {
	buf.ptr().link = 0
	if trace.fullHead == 0 {
		trace.fullHead = buf
	} else {
		trace.fullTail.ptr().link = buf
	}
	trace.fullTail = buf
}

traceFullDequeue dequeues from queue of full buffers.

func traceFullDequeue() traceBufPtr {
	buf := trace.fullHead
	if buf == 0 {
		return 0
	}
	trace.fullHead = buf.ptr().link
	if trace.fullHead == 0 {
		trace.fullTail = 0
	}
	buf.ptr().link = 0
	return buf
}

traceEvent writes a single event to trace buffer, flushing the buffer if necessary. ev is event type. If skip > 0, write current stack id as the last argument (skipping skip top frames). If skip = 0, this event type should contain a stack, but we don't want to collect and remember it for this particular call.

func traceEvent(ev byte, skip int, args ...uint64) {

Double-check trace.enabled now that we've done m.locks++ and acquired bufLock. This protects from races between traceEvent and StartTrace/StopTrace.

The caller checked that trace.enabled == true, but trace.enabled might have been turned off between the check and now. Check again. traceLockBuffer did mp.locks++, StopTrace does stopTheWorld, and stopTheWorld waits for mp.locks to go back to zero, so if we see trace.enabled == true now, we know it's true for the rest of the function. Exitsyscall can run even during stopTheWorld. The race with StartTrace/StopTrace during tracing in exitsyscall is resolved by locking trace.bufLock in traceLockBuffer. Note trace_userTaskCreate runs the same check.

	if !trace.enabled && !mp.startingtrace {
		traceReleaseBuffer(pid)
		return
	}

	if skip > 0 {
		if getg() == mp.curg {
			skip++ // +1 because stack is captured in traceEventLocked.
		}
	}
	traceEventLocked(0, mp, pid, bufp, ev, skip, args...)
	traceReleaseBuffer(pid)
}

func traceEventLocked(extraBytes int, mp *m, pid int32, bufp *traceBufPtr, ev byte, skip int, args ...uint64) {

TODO: test on non-zero extraBytes param.

	maxSize := 2 + 5*traceBytesPerNumber + extraBytes // event type, length, sequence, timestamp, stack id and two add params
	if buf == nil || len(buf.arr)-buf.pos < maxSize {
		buf = traceFlush(traceBufPtrOf(buf), pid).ptr()
		bufp.set(buf)
	}

	ticks := uint64(cputicks()) / traceTickDiv
	tickDiff := ticks - buf.lastTicks
	buf.lastTicks = ticks
	narg := byte(len(args))
	if skip >= 0 {
		narg++

We have only 2 bits for number of arguments. If number is >= 3, then the event type is followed by event length in bytes.

	if narg > 3 {
		narg = 3
	}
	startPos := buf.pos
	buf.byte(ev | narg<<traceArgCountShift)
	var lenp *byte

Reserve the byte for length assuming that length < 128.

		buf.varint(0)
		lenp = &buf.arr[buf.pos-1]
	}
	buf.varint(tickDiff)
	for _, a := range args {
		buf.varint(a)
	}
	if skip == 0 {
		buf.varint(0)
	} else if skip > 0 {
		buf.varint(traceStackID(mp, buf.stk[:], skip))
	}
	evSize := buf.pos - startPos
	if evSize > maxSize {
		throw("invalid length of trace event")
	}

Fill in actual length.

		*lenp = byte(evSize - 2)
	}
}

func traceStackID(mp *m, buf []uintptr, skip int) uint64 {
	_g_ := getg()
	gp := mp.curg
	var nstk int
	if gp == _g_ {
		nstk = callers(skip+1, buf)
	} else if gp != nil {
		gp = mp.curg
		nstk = gcallers(gp, skip, buf)
	}
	if nstk > 0 {
		nstk-- // skip runtime.goexit
	}
	if nstk > 0 && gp.goid == 1 {
		nstk-- // skip runtime.main
	}
	id := trace.stackTab.put(buf[:nstk])
	return uint64(id)
}

traceAcquireBuffer returns trace buffer to use and, if necessary, locks it.

func traceAcquireBuffer() (mp *m, pid int32, bufp *traceBufPtr) {
	mp = acquirem()
	if p := mp.p.ptr(); p != nil {
		return mp, p.id, &p.tracebuf
	}
	lock(&trace.bufLock)
	return mp, traceGlobProc, &trace.buf
}

traceReleaseBuffer releases a buffer previously acquired with traceAcquireBuffer.

func traceReleaseBuffer(pid int32) {
	if pid == traceGlobProc {
		unlock(&trace.bufLock)
	}
	releasem(getg().m)
}

traceFlush puts buf onto stack of full buffers and returns an empty buffer.

func traceFlush(buf traceBufPtr, pid int32) traceBufPtr {
	owner := trace.lockOwner
	dolock := owner == nil || owner != getg().m.curg
	if dolock {
		lock(&trace.lock)
	}
	if buf != 0 {
		traceFullQueue(buf)
	}
	if trace.empty != 0 {
		buf = trace.empty
		trace.empty = buf.ptr().link
	} else {
		buf = traceBufPtr(sysAlloc(unsafe.Sizeof(traceBuf{}), &memstats.other_sys))
		if buf == 0 {
			throw("trace: out of memory")
		}
	}
	bufp := buf.ptr()
	bufp.link.set(nil)
	bufp.pos = 0

initialize the buffer for a new batch

	ticks := uint64(cputicks()) / traceTickDiv
	bufp.lastTicks = ticks
	bufp.byte(traceEvBatch | 1<<traceArgCountShift)
	bufp.varint(uint64(pid))
	bufp.varint(ticks)

	if dolock {
		unlock(&trace.lock)
	}
	return buf
}

traceString adds a string to the trace.strings and returns the id.

func traceString(bufp *traceBufPtr, pid int32, s string) (uint64, *traceBufPtr) {
	if s == "" {
		return 0, bufp
	}

	lock(&trace.stringsLock)

raceacquire is necessary because the map access below is race annotated.

		raceacquire(unsafe.Pointer(&trace.stringsLock))
	}

	if id, ok := trace.strings[s]; ok {
		if raceenabled {
			racerelease(unsafe.Pointer(&trace.stringsLock))
		}
		unlock(&trace.stringsLock)

		return id, bufp
	}

	trace.stringSeq++
	id := trace.stringSeq
	trace.strings[s] = id

	if raceenabled {
		racerelease(unsafe.Pointer(&trace.stringsLock))
	}
	unlock(&trace.stringsLock)

memory allocation in above may trigger tracing and cause *bufp changes. Following code now works with *bufp, so there must be no memory allocation or any activities that causes tracing after this point.


	buf := bufp.ptr()
	size := 1 + 2*traceBytesPerNumber + len(s)
	if buf == nil || len(buf.arr)-buf.pos < size {
		buf = traceFlush(traceBufPtrOf(buf), pid).ptr()
		bufp.set(buf)
	}
	buf.byte(traceEvString)
	buf.varint(id)

double-check the string and the length can fit. Otherwise, truncate the string.

	slen := len(s)
	if room := len(buf.arr) - buf.pos; room < slen+traceBytesPerNumber {
		slen = room
	}

	buf.varint(uint64(slen))
	buf.pos += copy(buf.arr[buf.pos:], s[:slen])

	bufp.set(buf)
	return id, bufp
}

traceAppend appends v to buf in little-endian-base-128 encoding.

func traceAppend(buf []byte, v uint64) []byte {
	for ; v >= 0x80; v >>= 7 {
		buf = append(buf, 0x80|byte(v))
	}
	buf = append(buf, byte(v))
	return buf
}

varint appends v to buf in little-endian-base-128 encoding.

func (buf *traceBuf) varint(v uint64) {
	pos := buf.pos
	for ; v >= 0x80; v >>= 7 {
		buf.arr[pos] = 0x80 | byte(v)
		pos++
	}
	buf.arr[pos] = byte(v)
	pos++
	buf.pos = pos
}

byte appends v to buf.

func (buf *traceBuf) byte(v byte) {
	buf.arr[buf.pos] = v
	buf.pos++
}

traceStackTable maps stack traces (arrays of PC's) to unique uint32 ids. It is lock-free for reading.

type traceStackTable struct {
	lock mutex
	seq  uint32
	mem  traceAlloc
	tab  [1 << 13]traceStackPtr
}

traceStack is a single stack in traceStackTable.

type traceStack struct {
	link traceStackPtr
	hash uintptr
	id   uint32
	n    int
	stk  [0]uintptr // real type [n]uintptr
}

type traceStackPtr uintptr

func (tp traceStackPtr) ptr() *traceStack { return (*traceStack)(unsafe.Pointer(tp)) }

stack returns slice of PCs.

func (ts *traceStack) stack() []uintptr {
	return (*[traceStackSize]uintptr)(unsafe.Pointer(&ts.stk))[:ts.n]
}

put returns a unique id for the stack trace pcs and caches it in the table, if it sees the trace for the first time.

func (tab *traceStackTable) put(pcs []uintptr) uint32 {
	if len(pcs) == 0 {
		return 0
	}

First, search the hashtable w/o the mutex.

	if id := tab.find(pcs, hash); id != 0 {
		return id

Now, double check under the mutex.

	lock(&tab.lock)
	if id := tab.find(pcs, hash); id != 0 {
		unlock(&tab.lock)
		return id

Create new record.

	tab.seq++
	stk := tab.newStack(len(pcs))
	stk.hash = hash
	stk.id = tab.seq
	stk.n = len(pcs)
	stkpc := stk.stack()
	for i, pc := range pcs {
		stkpc[i] = pc
	}
	part := int(hash % uintptr(len(tab.tab)))
	stk.link = tab.tab[part]
	atomicstorep(unsafe.Pointer(&tab.tab[part]), unsafe.Pointer(stk))
	unlock(&tab.lock)
	return stk.id
}

find checks if the stack trace pcs is already present in the table.

func (tab *traceStackTable) find(pcs []uintptr, hash uintptr) uint32 {
	part := int(hash % uintptr(len(tab.tab)))
Search:
	for stk := tab.tab[part].ptr(); stk != nil; stk = stk.link.ptr() {
		if stk.hash == hash && stk.n == len(pcs) {
			for i, stkpc := range stk.stack() {
				if stkpc != pcs[i] {
					continue Search
				}
			}
			return stk.id
		}
	}
	return 0
}

newStack allocates a new stack of size n.

func (tab *traceStackTable) newStack(n int) *traceStack {
	return (*traceStack)(tab.mem.alloc(unsafe.Sizeof(traceStack{}) + uintptr(n)*sys.PtrSize))
}

allFrames returns all of the Frames corresponding to pcs.

func allFrames(pcs []uintptr) []Frame {
	frames := make([]Frame, 0, len(pcs))
	ci := CallersFrames(pcs)
	for {
		f, more := ci.Next()
		frames = append(frames, f)
		if !more {
			return frames
		}
	}
}

dump writes all previously cached stacks to trace buffers, releases all memory and resets state.

func (tab *traceStackTable) dump() {
	var tmp [(2 + 4*traceStackSize) * traceBytesPerNumber]byte
	bufp := traceFlush(0, 0)
	for _, stk := range tab.tab {
		stk := stk.ptr()
		for ; stk != nil; stk = stk.link.ptr() {
			tmpbuf := tmp[:0]
			tmpbuf = traceAppend(tmpbuf, uint64(stk.id))
			frames := allFrames(stk.stack())
			tmpbuf = traceAppend(tmpbuf, uint64(len(frames)))
			for _, f := range frames {
				var frame traceFrame
				frame, bufp = traceFrameForPC(bufp, 0, f)
				tmpbuf = traceAppend(tmpbuf, uint64(f.PC))
				tmpbuf = traceAppend(tmpbuf, uint64(frame.funcID))
				tmpbuf = traceAppend(tmpbuf, uint64(frame.fileID))
				tmpbuf = traceAppend(tmpbuf, uint64(frame.line))

Now copy to the buffer.

			size := 1 + traceBytesPerNumber + len(tmpbuf)
			if buf := bufp.ptr(); len(buf.arr)-buf.pos < size {
				bufp = traceFlush(bufp, 0)
			}
			buf := bufp.ptr()
			buf.byte(traceEvStack | 3<<traceArgCountShift)
			buf.varint(uint64(len(tmpbuf)))
			buf.pos += copy(buf.arr[buf.pos:], tmpbuf)
		}
	}

	lock(&trace.lock)
	traceFullQueue(bufp)
	unlock(&trace.lock)

	tab.mem.drop()
	*tab = traceStackTable{}
	lockInit(&((*tab).lock), lockRankTraceStackTab)
}

type traceFrame struct {
	funcID uint64
	fileID uint64
	line   uint64
}

traceFrameForPC records the frame information. It may allocate memory.

func traceFrameForPC(buf traceBufPtr, pid int32, f Frame) (traceFrame, traceBufPtr) {
	bufp := &buf
	var frame traceFrame

	fn := f.Function
	const maxLen = 1 << 10
	if len(fn) > maxLen {
		fn = fn[len(fn)-maxLen:]
	}
	frame.funcID, bufp = traceString(bufp, pid, fn)
	frame.line = uint64(f.Line)
	file := f.File
	if len(file) > maxLen {
		file = file[len(file)-maxLen:]
	}
	frame.fileID, bufp = traceString(bufp, pid, file)
	return frame, (*bufp)
}

traceAlloc is a non-thread-safe region allocator. It holds a linked list of traceAllocBlock.

type traceAlloc struct {
	head traceAllocBlockPtr
	off  uintptr
}

traceAllocBlock is a block in traceAlloc. traceAllocBlock is allocated from non-GC'd memory, so it must not contain heap pointers. Writes to pointers to traceAllocBlocks do not need write barriers.go:notinheap

type traceAllocBlock struct {
	next traceAllocBlockPtr
	data [64<<10 - sys.PtrSize]byte
}

TODO: Since traceAllocBlock is now go:notinheap, this isn't necessary.

type traceAllocBlockPtr uintptr

func (p traceAllocBlockPtr) ptr() *traceAllocBlock   { return (*traceAllocBlock)(unsafe.Pointer(p)) }
func (p *traceAllocBlockPtr) set(x *traceAllocBlock) { *p = traceAllocBlockPtr(unsafe.Pointer(x)) }

alloc allocates n-byte block.

func (a *traceAlloc) alloc(n uintptr) unsafe.Pointer {
	n = alignUp(n, sys.PtrSize)
	if a.head == 0 || a.off+n > uintptr(len(a.head.ptr().data)) {
		if n > uintptr(len(a.head.ptr().data)) {
			throw("trace: alloc too large")
		}
		block := (*traceAllocBlock)(sysAlloc(unsafe.Sizeof(traceAllocBlock{}), &memstats.other_sys))
		if block == nil {
			throw("trace: out of memory")
		}
		block.next.set(a.head.ptr())
		a.head.set(block)
		a.off = 0
	}
	p := &a.head.ptr().data[a.off]
	a.off += n
	return unsafe.Pointer(p)
}

drop frees all previously allocated memory and resets the allocator.

func (a *traceAlloc) drop() {
	for a.head != 0 {
		block := a.head.ptr()
		a.head.set(block.next.ptr())
		sysFree(unsafe.Pointer(block), unsafe.Sizeof(traceAllocBlock{}), &memstats.other_sys)
	}
}

The following functions write specific events to trace.


func traceGomaxprocs(procs int32) {
	traceEvent(traceEvGomaxprocs, 1, uint64(procs))
}

func traceProcStart() {
	traceEvent(traceEvProcStart, -1, uint64(getg().m.id))
}

Sysmon and stopTheWorld can stop Ps blocked in syscalls, to handle this we temporary employ the P.

	mp := acquirem()
	oldp := mp.p
	mp.p.set(pp)
	traceEvent(traceEvProcStop, -1)
	mp.p = oldp
	releasem(mp)
}

func traceGCStart() {
	traceEvent(traceEvGCStart, 3, trace.seqGC)
	trace.seqGC++
}

func traceGCDone() {
	traceEvent(traceEvGCDone, -1)
}

func traceGCSTWStart(kind int) {
	traceEvent(traceEvGCSTWStart, -1, uint64(kind))
}

func traceGCSTWDone() {
	traceEvent(traceEvGCSTWDone, -1)
}

traceGCSweepStart prepares to trace a sweep loop. This does not emit any events until traceGCSweepSpan is called. traceGCSweepStart must be paired with traceGCSweepDone and there must be no preemption points between these two calls.

Delay the actual GCSweepStart event until the first span sweep. If we don't sweep anything, don't emit any events.

	_p_ := getg().m.p.ptr()
	if _p_.traceSweep {
		throw("double traceGCSweepStart")
	}
	_p_.traceSweep, _p_.traceSwept, _p_.traceReclaimed = true, 0, 0
}

traceGCSweepSpan traces the sweep of a single page. This may be called outside a traceGCSweepStart/traceGCSweepDone pair; however, it will not emit any trace events in this case.

func traceGCSweepSpan(bytesSwept uintptr) {
	_p_ := getg().m.p.ptr()
	if _p_.traceSweep {
		if _p_.traceSwept == 0 {
			traceEvent(traceEvGCSweepStart, 1)
		}
		_p_.traceSwept += bytesSwept
	}
}

func traceGCSweepDone() {
	_p_ := getg().m.p.ptr()
	if !_p_.traceSweep {
		throw("missing traceGCSweepStart")
	}
	if _p_.traceSwept != 0 {
		traceEvent(traceEvGCSweepDone, -1, uint64(_p_.traceSwept), uint64(_p_.traceReclaimed))
	}
	_p_.traceSweep = false
}

func traceGCMarkAssistStart() {
	traceEvent(traceEvGCMarkAssistStart, 1)
}

func traceGCMarkAssistDone() {
	traceEvent(traceEvGCMarkAssistDone, -1)
}

func traceGoCreate(newg *g, pc uintptr) {
	newg.traceseq = 0

+PCQuantum because traceFrameForPC expects return PCs and subtracts PCQuantum.

	id := trace.stackTab.put([]uintptr{pc + sys.PCQuantum})
	traceEvent(traceEvGoCreate, 2, uint64(newg.goid), uint64(id))
}

func traceGoStart() {
	_g_ := getg().m.curg
	_p_ := _g_.m.p
	_g_.traceseq++
	if _p_.ptr().gcMarkWorkerMode != gcMarkWorkerNotWorker {
		traceEvent(traceEvGoStartLabel, -1, uint64(_g_.goid), _g_.traceseq, trace.markWorkerLabels[_p_.ptr().gcMarkWorkerMode])
	} else if _g_.tracelastp == _p_ {
		traceEvent(traceEvGoStartLocal, -1, uint64(_g_.goid))
	} else {
		_g_.tracelastp = _p_
		traceEvent(traceEvGoStart, -1, uint64(_g_.goid), _g_.traceseq)
	}
}

func traceGoEnd() {
	traceEvent(traceEvGoEnd, -1)
}

func traceGoSched() {
	_g_ := getg()
	_g_.tracelastp = _g_.m.p
	traceEvent(traceEvGoSched, 1)
}

func traceGoPreempt() {
	_g_ := getg()
	_g_.tracelastp = _g_.m.p
	traceEvent(traceEvGoPreempt, 1)
}

func traceGoPark(traceEv byte, skip int) {
	if traceEv&traceFutileWakeup != 0 {
		traceEvent(traceEvFutileWakeup, -1)
	}
	traceEvent(traceEv & ^traceFutileWakeup, skip)
}

func traceGoUnpark(gp *g, skip int) {
	_p_ := getg().m.p
	gp.traceseq++
	if gp.tracelastp == _p_ {
		traceEvent(traceEvGoUnblockLocal, skip, uint64(gp.goid))
	} else {
		gp.tracelastp = _p_
		traceEvent(traceEvGoUnblock, skip, uint64(gp.goid), gp.traceseq)
	}
}

func traceGoSysCall() {
	traceEvent(traceEvGoSysCall, 1)
}

func traceGoSysExit(ts int64) {

There is a race between the code that initializes sysexitticks (in exitsyscall, which runs without a P, and therefore is not stopped with the rest of the world) and the code that initializes a new trace. The recorded sysexitticks must therefore be treated as "best effort". If they are valid for this trace, then great, use them for greater accuracy. But if they're not valid for this trace, assume that the trace was started after the actual syscall exit (but before we actually managed to start the goroutine, aka right now), and assign a fresh time stamp to keep the log consistent.

		ts = 0
	}
	_g_ := getg().m.curg
	_g_.traceseq++
	_g_.tracelastp = _g_.m.p
	traceEvent(traceEvGoSysExit, -1, uint64(_g_.goid), _g_.traceseq, uint64(ts)/traceTickDiv)
}

Sysmon and stopTheWorld can declare syscalls running on remote Ps as blocked, to handle this we temporary employ the P.

	mp := acquirem()
	oldp := mp.p
	mp.p.set(pp)
	traceEvent(traceEvGoSysBlock, -1)
	mp.p = oldp
	releasem(mp)
}

func traceHeapAlloc() {
	traceEvent(traceEvHeapAlloc, -1, memstats.heap_live)
}

func traceNextGC() {

Heap-based triggering is disabled.

		traceEvent(traceEvNextGC, -1, 0)
	} else {
		traceEvent(traceEvNextGC, -1, nextGC)
	}
}

To access runtime functions from runtime/trace. See runtime/trace/annotation.go

go:linkname trace_userTaskCreate runtime/trace.userTaskCreate

func trace_userTaskCreate(id, parentID uint64, taskType string) {
	if !trace.enabled {
		return
	}

Same as in traceEvent.

	mp, pid, bufp := traceAcquireBuffer()
	if !trace.enabled && !mp.startingtrace {
		traceReleaseBuffer(pid)
		return
	}

	typeStringID, bufp := traceString(bufp, pid, taskType)
	traceEventLocked(0, mp, pid, bufp, traceEvUserTaskCreate, 3, id, parentID, typeStringID)
	traceReleaseBuffer(pid)
}

go:linkname trace_userTaskEnd runtime/trace.userTaskEnd

func trace_userTaskEnd(id uint64) {
	traceEvent(traceEvUserTaskEnd, 2, id)
}

go:linkname trace_userRegion runtime/trace.userRegion

func trace_userRegion(id, mode uint64, name string) {
	if !trace.enabled {
		return
	}

	mp, pid, bufp := traceAcquireBuffer()
	if !trace.enabled && !mp.startingtrace {
		traceReleaseBuffer(pid)
		return
	}

	nameStringID, bufp := traceString(bufp, pid, name)
	traceEventLocked(0, mp, pid, bufp, traceEvUserRegion, 3, id, mode, nameStringID)
	traceReleaseBuffer(pid)
}

go:linkname trace_userLog runtime/trace.userLog

func trace_userLog(id uint64, category, message string) {
	if !trace.enabled {
		return
	}

	mp, pid, bufp := traceAcquireBuffer()
	if !trace.enabled && !mp.startingtrace {
		traceReleaseBuffer(pid)
		return
	}

	categoryID, bufp := traceString(bufp, pid, category)

	extraSpace := traceBytesPerNumber + len(message) // extraSpace for the value string

traceEventLocked reserved extra space for val and len(val) in buf, so buf now has room for the following.

	buf := bufp.ptr()

double-check the message and its length can fit. Otherwise, truncate the message.

	slen := len(message)
	if room := len(buf.arr) - buf.pos; room < slen+traceBytesPerNumber {
		slen = room
	}
	buf.varint(uint64(slen))
	buf.pos += copy(buf.arr[buf.pos:], message[:slen])

	traceReleaseBuffer(pid)