runtime: enable allocheaders by default

[gostls13.git] / src / runtime / trace.go

// Copyright 2014 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// 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 (
        "internal/abi"
        "internal/goarch"
        "internal/goos"
        "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]
        traceEvSTWStart          = 9  // STW start [timestamp, kind]
        traceEvSTWDone           = 10 // 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 // gcController.heapLive change [timestamp, heap_alloc]
        traceEvHeapGoal          = 34 // gcController.heapGoal() (formerly next_gc) change [timestamp, heap goal in bytes]
        traceEvTimerGoroutine    = 35 // not currently used; previously denoted timer goroutine [timer goroutine id]
        traceEvFutileWakeup      = 36 // not currently used; 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.NewTask [timestamp, internal task id, internal parent task id, name string, stack]
        traceEvUserTaskEnd       = 46 // end of a task [timestamp, internal task id, stack]
        traceEvUserRegion        = 47 // trace.WithRegion [timestamp, internal task id, mode(0:start, 1:end), name string, stack]
        traceEvUserLog           = 48 // trace.Log [timestamp, internal task id, key string id, stack, value string]
        traceEvCPUSample         = 49 // CPU profiling sample [timestamp, real timestamp, real P id (-1 when absent), goroutine id, stack]
        traceEvCount             = 50
        // 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.
)

// traceBlockReason is an enumeration of reasons a goroutine might block.
// This is the interface the rest of the runtime uses to tell the
// tracer why a goroutine blocked. The tracer then propagates this information
// into the trace however it sees fit.
//
// Note that traceBlockReasons should not be compared, since reasons that are
// distinct by name may *not* be distinct by value.
type traceBlockReason uint8

// For maximal efficiency, just map the trace block reason directly to a trace
// event.
const (
        traceBlockGeneric         traceBlockReason = traceEvGoBlock
        traceBlockForever                          = traceEvGoStop
        traceBlockNet                              = traceEvGoBlockNet
        traceBlockSelect                           = traceEvGoBlockSelect
        traceBlockCondWait                         = traceEvGoBlockCond
        traceBlockSync                             = traceEvGoBlockSync
        traceBlockChanSend                         = traceEvGoBlockSend
        traceBlockChanRecv                         = traceEvGoBlockRecv
        traceBlockGCMarkAssist                     = traceEvGoBlockGC
        traceBlockGCSweep                          = traceEvGoBlock
        traceBlockSystemGoroutine                  = traceEvGoBlock
        traceBlockPreempted                        = traceEvGoBlock
        traceBlockDebugCall                        = traceEvGoBlock
        traceBlockUntilGCEnds                      = traceEvGoBlock
        traceBlockSleep                            = traceEvGoSleep
)

const (
        // 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.
        traceTimeDiv = 16 + 48*(goarch.Is386|goarch.IsAmd64)
        // 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.
        traceStackSize = 128
        // Identifier of a fake P that is used when we trace without a real P.
        traceGlobProc = -1
        // Maximum number of bytes to encode uint64 in base-128.
        traceBytesPerNumber = 10
        // Shift of the number of arguments in the first event byte.
        traceArgCountShift = 6
)

// trace is global tracing context.
var trace struct {
        // trace.lock must only be acquired on the system stack where
        // stack splits cannot happen while it is held.
        lock          mutex       // protects the following members
        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
        startTicks    int64       // cputicks when tracing was started
        endTicks      int64       // cputicks when tracing was stopped
        startNanotime int64       // nanotime when tracing was started
        endNanotime   int64       // nanotime when tracing was stopped
        startTime     traceTime   // traceClockNow when tracing started
        endTime       traceTime   // traceClockNow when tracing 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
        stackTab      traceStackTable // maps stack traces to unique ids
        // cpuLogRead accepts CPU profile samples from the signal handler where
        // they're generated. It uses a two-word header to hold the IDs of the P and
        // G (respectively) that were active at the time of the sample. Because
        // profBuf uses a record with all zeros in its header to indicate overflow,
        // we make sure to make the P field always non-zero: The ID of a real P will
        // start at bit 1, and bit 0 will be set. Samples that arrive while no P is
        // running (such as near syscalls) will set the first header field to 0b10.
        // This careful handling of the first header field allows us to store ID of
        // the active G directly in the second field, even though that will be 0
        // when sampling g0.
        cpuLogRead *profBuf
        // cpuLogBuf is a trace buffer to hold events corresponding to CPU profile
        // samples, which arrive out of band and not directly connected to a
        // specific P.
        cpuLogBuf traceBufPtr

        reader atomic.Pointer[g] // goroutine that called ReadTrace, or nil

        signalLock  atomic.Uint32 // protects use of the following member, only usable in signal handlers
        cpuLogWrite *profBuf      // copy of cpuLogRead for use in signal handlers, set without signalLock

        // 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
}

// gTraceState is per-G state for the tracer.
type gTraceState struct {
        sysExitTime        traceTime // timestamp when syscall has returned
        tracedSyscallEnter bool      // syscall or cgo was entered while trace was enabled or StartTrace has emitted EvGoInSyscall about this goroutine
        seq                uint64    // trace event sequencer
        lastP              puintptr  // last P emitted an event for this goroutine
}

// mTraceState is per-M state for the tracer.
type mTraceState struct {
        startingTrace  bool // this M is in TraceStart, potentially before traceEnabled is true
        tracedSTWStart bool // this M traced a STW start, so it should trace an end
}

// pTraceState is per-P state for the tracer.
type pTraceState struct {
        buf traceBufPtr

        // inSweep indicates the sweep events should be traced.
        // This is used to defer the sweep start event until a span
        // has actually been swept.
        inSweep bool

        // swept and reclaimed track the number of bytes swept and reclaimed
        // by sweeping in the current sweep loop (while inSweep was true).
        swept, reclaimed uintptr
}

// traceLockInit initializes global trace locks.
func traceLockInit() {
        lockInit(&trace.bufLock, lockRankTraceBuf)
        lockInit(&trace.stringsLock, lockRankTraceStrings)
        lockInit(&trace.lock, lockRankTrace)
        lockInit(&trace.stackTab.lock, lockRankTraceStackTab)
}

// traceBufHeader is per-P tracing buffer.
type traceBufHeader struct {
        link     traceBufPtr             // in trace.empty/full
        lastTime traceTime               // 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.
type traceBuf struct {
        _ sys.NotInHeap
        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 embedded runtime/internal/sys.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))
}

// traceEnabled returns true if the trace is currently enabled.
//
//go:nosplit
func traceEnabled() bool {
        return trace.enabled
}

// traceShuttingDown returns true if the trace is currently shutting down.
//
//go:nosplit
func traceShuttingDown() bool {
        return trace.shutdown
}

// 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.
func StartTrace() error {
        // 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(stwStartTrace)

        // 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 trace.startingTrace.
        // trace.enabled is set afterwards once we have emitted all preliminary events.
        mp := getg().m
        mp.trace.startingTrace = true

        // Obtain current stack ID to use in all traceEvGoCreate events below.
        stkBuf := make([]uintptr, traceStackSize)
        stackID := traceStackID(mp, stkBuf, 2)

        profBuf := newProfBuf(2, profBufWordCount, profBufTagCount) // after the timestamp, header is [pp.id, gp.goid]
        trace.cpuLogRead = profBuf

        // We must not acquire trace.signalLock outside of a signal handler: a
        // profiling signal may arrive at any time and try to acquire it, leading to
        // deadlock. Because we can't use that lock to protect updates to
        // trace.cpuLogWrite (only use of the structure it references), reads and
        // writes of the pointer must be atomic. (And although this field is never
        // the sole pointer to the profBuf value, it's best to allow a write barrier
        // here.)
        atomicstorep(unsafe.Pointer(&trace.cpuLogWrite), unsafe.Pointer(profBuf))

        // World is stopped, no need to lock.
        forEachGRace(func(gp *g) {
                status := readgstatus(gp)
                if status != _Gdead {
                        gp.trace.seq = 0
                        gp.trace.lastP = getg().m.p
                        // +PCQuantum because traceFrameForPC expects return PCs and subtracts PCQuantum.
                        id := trace.stackTab.put([]uintptr{logicalStackSentinel, startPCforTrace(gp.startpc) + sys.PCQuantum})
                        traceEvent(traceEvGoCreate, -1, gp.goid, uint64(id), stackID)
                }
                if status == _Gwaiting {
                        // traceEvGoWaiting is implied to have seq=1.
                        gp.trace.seq++
                        traceEvent(traceEvGoWaiting, -1, gp.goid)
                }
                if status == _Gsyscall {
                        gp.trace.seq++
                        gp.trace.tracedSyscallEnter = true
                        traceEvent(traceEvGoInSyscall, -1, gp.goid)
                } else if status == _Gdead && gp.m != nil && gp.m.isextra {
                        // Trigger two trace events for the dead g in the extra m,
                        // since the next event of the g will be traceEvGoSysExit in exitsyscall,
                        // while calling from C thread to Go.
                        gp.trace.seq = 0
                        gp.trace.lastP = getg().m.p
                        // +PCQuantum because traceFrameForPC expects return PCs and subtracts PCQuantum.
                        id := trace.stackTab.put([]uintptr{logicalStackSentinel, startPCforTrace(0) + sys.PCQuantum}) // no start pc
                        traceEvent(traceEvGoCreate, -1, gp.goid, uint64(id), stackID)
                        gp.trace.seq++
                        gp.trace.tracedSyscallEnter = true
                        traceEvent(traceEvGoInSyscall, -1, gp.goid)
                } else {
                        // We need to explicitly clear the flag. A previous trace might have ended with a goroutine
                        // not emitting a GoSysExit and clearing the flag, leaving it in a stale state. Clearing
                        // it here makes it unambiguous to any goroutine exiting a syscall racing with us that
                        // no EvGoInSyscall event was emitted for it. (It's not racy to set this flag here, because
                        // it'll only get checked when the goroutine runs again, which will be after the world starts
                        // again.)
                        gp.trace.tracedSyscallEnter = false
                }
        })
        traceProcStart()
        traceGoStart()
        // Note: startTicks needs to be set after we emit traceEvGoInSyscall events.
        // If we do it the other way around, it is possible that exitsyscall will
        // query sysExitTime after startTicks but before traceEvGoInSyscall timestamp.
        // It will lead to a false conclusion that cputicks is broken.
        trace.startTime = traceClockNow()
        trace.startTicks = cputicks()
        trace.startNanotime = 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
        mp.trace.startingTrace = false
        trace.enabled = true

        // Register runtime goroutine labels.
        _, pid, bufp := traceAcquireBuffer()
        for i, label := range gcMarkWorkerModeStrings[:] {
                trace.markWorkerLabels[i], bufp = traceString(bufp, pid, label)
        }
        traceReleaseBuffer(mp, pid)

        unlock(&trace.bufLock)

        unlock(&sched.sysmonlock)

        // Record the current state of HeapGoal to avoid information loss in trace.
        traceHeapGoal()

        startTheWorldGC()
        return nil
}

// StopTrace stops tracing, if it was previously enabled.
// StopTrace only returns after all the reads for the trace have completed.
func StopTrace() {
        // Stop the world so that we can collect the trace buffers from all p's below,
        // and also to avoid races with traceEvent.
        stopTheWorldGC(stwStopTrace)

        // 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()

        atomicstorep(unsafe.Pointer(&trace.cpuLogWrite), nil)
        trace.cpuLogRead.close()
        traceReadCPU()

        // Loop over all allocated Ps because dead Ps may still have
        // trace buffers.
        for _, p := range allp[:cap(allp)] {
                buf := p.trace.buf
                if buf != 0 {
                        traceFullQueue(buf)
                        p.trace.buf = 0
                }
        }
        if trace.buf != 0 {
                buf := trace.buf
                trace.buf = 0
                if buf.ptr().pos != 0 {
                        traceFullQueue(buf)
                }
        }
        if trace.cpuLogBuf != 0 {
                buf := trace.cpuLogBuf
                trace.cpuLogBuf = 0
                if buf.ptr().pos != 0 {
                        traceFullQueue(buf)
                }
        }

        // Wait for startNanotime != endNanotime. On Windows the default interval between
        // system clock ticks is typically between 1 and 15 milliseconds, which may not
        // have passed since the trace started. Without nanotime moving forward, trace
        // tooling has no way of identifying how much real time each cputicks time deltas
        // represent.
        for {
                trace.endTime = traceClockNow()
                trace.endTicks = cputicks()
                trace.endNanotime = nanotime()

                if trace.endNanotime != trace.startNanotime || faketime != 0 {
                        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))
        }

        systemstack(func() {
                // 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.trace.buf != 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.Load() != nil {
                        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
                trace.cpuLogRead = nil
                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.
func ReadTrace() []byte {
top:
        var buf []byte
        var park bool
        systemstack(func() {
                buf, park = readTrace0()
        })
        if park {
                gopark(func(gp *g, _ unsafe.Pointer) bool {
                        if !trace.reader.CompareAndSwapNoWB(nil, gp) {
                                // We're racing with another reader.
                                // Wake up and handle this case.
                                return false
                        }

                        if g2 := traceReader(); gp == g2 {
                                // New data arrived between unlocking
                                // and the CAS and we won the wake-up
                                // race, so wake up directly.
                                return false
                        } else if g2 != nil {
                                printlock()
                                println("runtime: got trace reader", g2, g2.goid)
                                throw("unexpected trace reader")
                        }

                        return true
                }, nil, waitReasonTraceReaderBlocked, traceBlockSystemGoroutine, 2)
                goto top
        }

        return buf
}

// readTrace0 is ReadTrace's continuation on g0. This must run on the
// system stack because it acquires trace.lock.
//
//go:systemstack
func readTrace0() (buf []byte, park bool) {
        if raceenabled {
                // g0 doesn't have a race context. Borrow the user G's.
                if getg().racectx != 0 {
                        throw("expected racectx == 0")
                }
                getg().racectx = getg().m.curg.racectx
                // (This defer should get open-coded, which is safe on
                // the system stack.)
                defer func() { getg().racectx = 0 }()
        }

        // Optimistically look for CPU profile samples. This may write new stack
        // records, and may write new tracing buffers. This must be done with the
        // trace lock not held. footerWritten and shutdown are safe to access
        // here. They are only mutated by this goroutine or during a STW.
        if !trace.footerWritten && !trace.shutdown {
                traceReadCPU()
        }

        // 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)

        if trace.reader.Load() != nil {
                // 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.
                unlock(&trace.lock)
                println("runtime: ReadTrace called from multiple goroutines simultaneously")
                return nil, false
        }
        // 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
                unlock(&trace.lock)
                return []byte("go 1.21 trace\x00\x00\x00"), false
        }
        // Wait for new data.
        if trace.fullHead == 0 && !trace.shutdown {
                // We don't simply use a note because the scheduler
                // executes this goroutine directly when it wakes up
                // (also a note would consume an M).
                unlock(&trace.lock)
                return nil, true
        }
newFull:
        assertLockHeld(&trace.lock)
        // Write a buffer.
        if trace.fullHead != 0 {
                buf := traceFullDequeue()
                trace.reading = buf
                unlock(&trace.lock)
                return buf.ptr().arr[:buf.ptr().pos], false
        }

        // Write footer with timer frequency.
        if !trace.footerWritten {
                trace.footerWritten = true
                freq := (float64(trace.endTicks-trace.startTicks) / traceTimeDiv) / (float64(trace.endNanotime-trace.startNanotime) / 1e9)
                if freq <= 0 {
                        throw("trace: ReadTrace got invalid frequency")
                }
                unlock(&trace.lock)

                // Write frequency event.
                bufp := traceFlush(0, 0)
                buf := bufp.ptr()
                buf.byte(traceEvFrequency | 0<<traceArgCountShift)
                buf.varint(uint64(freq))

                // Dump stack table.
                // This will emit a bunch of full buffers, we will pick them up
                // on the next iteration.
                bufp = trace.stackTab.dump(bufp)

                // Flush final buffer.
                lock(&trace.lock)
                traceFullQueue(bufp)
                goto newFull // trace.lock should be held at newFull
        }
        // Done.
        if trace.shutdown {
                unlock(&trace.lock)
                if raceenabled {
                        // 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, false
        }
        // Also bad, but see the comment above.
        unlock(&trace.lock)
        println("runtime: spurious wakeup of trace reader")
        return nil, false
}

// traceReader returns the trace reader that should be woken up, if any.
// Callers should first check that trace.enabled or trace.shutdown is set.
//
// This must run on the system stack because it acquires trace.lock.
//
//go:systemstack
func traceReader() *g {
        // Optimistic check first
        if traceReaderAvailable() == nil {
                return nil
        }
        lock(&trace.lock)
        gp := traceReaderAvailable()
        if gp == nil || !trace.reader.CompareAndSwapNoWB(gp, nil) {
                unlock(&trace.lock)
                return nil
        }
        unlock(&trace.lock)
        return gp
}

// traceReaderAvailable returns the trace reader if it is not currently
// scheduled and should be. Callers should first check that trace.enabled
// or trace.shutdown is set.
func traceReaderAvailable() *g {
        if trace.fullHead != 0 || trace.shutdown {
                return trace.reader.Load()
        }
        return nil
}

// traceProcFree frees trace buffer associated with pp.
//
// This must run on the system stack because it acquires trace.lock.
//
//go:systemstack
func traceProcFree(pp *p) {
        buf := pp.trace.buf
        pp.trace.buf = 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) {
        mp, pid, bufp := traceAcquireBuffer()
        // 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.trace.startingTrace {
                traceReleaseBuffer(mp, pid)
                return
        }

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

// traceEventLocked writes a single event of type ev to the trace buffer bufp,
// flushing the buffer if necessary. pid is the id of the current P, or
// traceGlobProc if we're tracing without a real P.
//
// Preemption is disabled, and if running without a real P the global tracing
// buffer is locked.
//
// Events types that do not include a stack set skip to -1. Event types that
// include a stack may explicitly reference a stackID from the trace.stackTab
// (obtained by an earlier call to traceStackID). Without an explicit stackID,
// this function will automatically capture the stack of the goroutine currently
// running on mp, skipping skip top frames or, if skip is 0, writing out an
// empty stack record.
//
// It records the event's args to the traceBuf, and also makes an effort to
// reserve extraBytes bytes of additional space immediately following the event,
// in the same traceBuf.
func traceEventLocked(extraBytes int, mp *m, pid int32, bufp *traceBufPtr, ev byte, stackID uint32, skip int, args ...uint64) {
        buf := bufp.ptr()
        // 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 {
                systemstack(func() {
                        buf = traceFlush(traceBufPtrOf(buf), pid).ptr()
                })
                bufp.set(buf)
        }

        ts := traceClockNow()
        if ts <= buf.lastTime {
                ts = buf.lastTime + 1
        }
        tsDiff := uint64(ts - buf.lastTime)
        buf.lastTime = ts
        narg := byte(len(args))
        if stackID != 0 || 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
        if narg == 3 {
                // Reserve the byte for length assuming that length < 128.
                buf.varint(0)
                lenp = &buf.arr[buf.pos-1]
        }
        buf.varint(tsDiff)
        for _, a := range args {
                buf.varint(a)
        }
        if stackID != 0 {
                buf.varint(uint64(stackID))
        } else 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")
        }
        if lenp != nil {
                // Fill in actual length.
                *lenp = byte(evSize - 2)
        }
}

// traceCPUSample writes a CPU profile sample stack to the execution tracer's
// profiling buffer. It is called from a signal handler, so is limited in what
// it can do.
func traceCPUSample(gp *g, pp *p, stk []uintptr) {
        if !trace.enabled {
                // Tracing is usually turned off; don't spend time acquiring the signal
                // lock unless it's active.
                return
        }

        // Match the clock used in traceEventLocked
        now := traceClockNow()
        // The "header" here is the ID of the P that was running the profiled code,
        // followed by the ID of the goroutine. (For normal CPU profiling, it's
        // usually the number of samples with the given stack.) Near syscalls, pp
        // may be nil. Reporting goid of 0 is fine for either g0 or a nil gp.
        var hdr [2]uint64
        if pp != nil {
                // Overflow records in profBuf have all header values set to zero. Make
                // sure that real headers have at least one bit set.
                hdr[0] = uint64(pp.id)<<1 | 0b1
        } else {
                hdr[0] = 0b10
        }
        if gp != nil {
                hdr[1] = gp.goid
        }

        // Allow only one writer at a time
        for !trace.signalLock.CompareAndSwap(0, 1) {
                // TODO: Is it safe to osyield here? https://go.dev/issue/52672
                osyield()
        }

        if log := (*profBuf)(atomic.Loadp(unsafe.Pointer(&trace.cpuLogWrite))); log != nil {
                // Note: we don't pass a tag pointer here (how should profiling tags
                // interact with the execution tracer?), but if we did we'd need to be
                // careful about write barriers. See the long comment in profBuf.write.
                log.write(nil, int64(now), hdr[:], stk)
        }

        trace.signalLock.Store(0)
}

func traceReadCPU() {
        bufp := &trace.cpuLogBuf

        for {
                data, tags, _ := trace.cpuLogRead.read(profBufNonBlocking)
                if len(data) == 0 {
                        break
                }
                for len(data) > 0 {
                        if len(data) < 4 || data[0] > uint64(len(data)) {
                                break // truncated profile
                        }
                        if data[0] < 4 || tags != nil && len(tags) < 1 {
                                break // malformed profile
                        }
                        if len(tags) < 1 {
                                break // mismatched profile records and tags
                        }
                        timestamp := data[1]
                        ppid := data[2] >> 1
                        if hasP := (data[2] & 0b1) != 0; !hasP {
                                ppid = ^uint64(0)
                        }
                        goid := data[3]
                        stk := data[4:data[0]]
                        empty := len(stk) == 1 && data[2] == 0 && data[3] == 0
                        data = data[data[0]:]
                        // No support here for reporting goroutine tags at the moment; if
                        // that information is to be part of the execution trace, we'd
                        // probably want to see when the tags are applied and when they
                        // change, instead of only seeing them when we get a CPU sample.
                        tags = tags[1:]

                        if empty {
                                // Looks like an overflow record from the profBuf. Not much to
                                // do here, we only want to report full records.
                                //
                                // TODO: should we start a goroutine to drain the profBuf,
                                // rather than relying on a high-enough volume of tracing events
                                // to keep ReadTrace busy? https://go.dev/issue/52674
                                continue
                        }

                        buf := bufp.ptr()
                        if buf == nil {
                                systemstack(func() {
                                        *bufp = traceFlush(*bufp, 0)
                                })
                                buf = bufp.ptr()
                        }
                        nstk := 1
                        buf.stk[0] = logicalStackSentinel
                        for ; nstk < len(buf.stk) && nstk-1 < len(stk); nstk++ {
                                buf.stk[nstk] = uintptr(stk[nstk-1])
                        }
                        stackID := trace.stackTab.put(buf.stk[:nstk])

                        traceEventLocked(0, nil, 0, bufp, traceEvCPUSample, stackID, 1, timestamp, ppid, goid)
                }
        }
}

// logicalStackSentinel is a sentinel value at pcBuf[0] signifying that
// pcBuf[1:] holds a logical stack requiring no further processing. Any other
// value at pcBuf[0] represents a skip value to apply to the physical stack in
// pcBuf[1:] after inline expansion.
const logicalStackSentinel = ^uintptr(0)

// traceStackID captures a stack trace into pcBuf, registers it in the trace
// stack table, and returns its unique ID. pcBuf should have a length equal to
// traceStackSize. skip controls the number of leaf frames to omit in order to
// hide tracer internals from stack traces, see CL 5523.
func traceStackID(mp *m, pcBuf []uintptr, skip int) uint64 {
        gp := getg()
        curgp := mp.curg
        nstk := 1
        if tracefpunwindoff() || mp.hasCgoOnStack() {
                // Slow path: Unwind using default unwinder. Used when frame pointer
                // unwinding is unavailable or disabled (tracefpunwindoff), or might
                // produce incomplete results or crashes (hasCgoOnStack). Note that no
                // cgo callback related crashes have been observed yet. The main
                // motivation is to take advantage of a potentially registered cgo
                // symbolizer.
                pcBuf[0] = logicalStackSentinel
                if curgp == gp {
                        nstk += callers(skip+1, pcBuf[1:])
                } else if curgp != nil {
                        nstk += gcallers(curgp, skip, pcBuf[1:])
                }
        } else {
                // Fast path: Unwind using frame pointers.
                pcBuf[0] = uintptr(skip)
                if curgp == gp {
                        nstk += fpTracebackPCs(unsafe.Pointer(getfp()), pcBuf[1:])
                } else if curgp != nil {
                        // We're called on the g0 stack through mcall(fn) or systemstack(fn). To
                        // behave like gcallers above, we start unwinding from sched.bp, which
                        // points to the caller frame of the leaf frame on g's stack. The return
                        // address of the leaf frame is stored in sched.pc, which we manually
                        // capture here.
                        pcBuf[1] = curgp.sched.pc
                        nstk += 1 + fpTracebackPCs(unsafe.Pointer(curgp.sched.bp), pcBuf[2:])
                }
        }
        if nstk > 0 {
                nstk-- // skip runtime.goexit
        }
        if nstk > 0 && curgp.goid == 1 {
                nstk-- // skip runtime.main
        }
        id := trace.stackTab.put(pcBuf[:nstk])
        return uint64(id)
}

// tracefpunwindoff returns true if frame pointer unwinding for the tracer is
// disabled via GODEBUG or not supported by the architecture.
// TODO(#60254): support frame pointer unwinding on plan9/amd64.
func tracefpunwindoff() bool {
        return debug.tracefpunwindoff != 0 || (goarch.ArchFamily != goarch.AMD64 && goarch.ArchFamily != goarch.ARM64) || goos.IsPlan9 == 1
}

// fpTracebackPCs populates pcBuf with the return addresses for each frame and
// returns the number of PCs written to pcBuf. The returned PCs correspond to
// "physical frames" rather than "logical frames"; that is if A is inlined into
// B, this will return a PC for only B.
func fpTracebackPCs(fp unsafe.Pointer, pcBuf []uintptr) (i int) {
        for i = 0; i < len(pcBuf) && fp != nil; i++ {
                // return addr sits one word above the frame pointer
                pcBuf[i] = *(*uintptr)(unsafe.Pointer(uintptr(fp) + goarch.PtrSize))
                // follow the frame pointer to the next one
                fp = unsafe.Pointer(*(*uintptr)(fp))
        }
        return i
}

// traceAcquireBuffer returns trace buffer to use and, if necessary, locks it.
func traceAcquireBuffer() (mp *m, pid int32, bufp *traceBufPtr) {
        // Any time we acquire a buffer, we may end up flushing it,
        // but flushes are rare. Record the lock edge even if it
        // doesn't happen this time.
        lockRankMayTraceFlush()

        mp = acquirem()
        if p := mp.p.ptr(); p != nil {
                return mp, p.id, &p.trace.buf
        }
        lock(&trace.bufLock)
        return mp, traceGlobProc, &trace.buf
}

// traceReleaseBuffer releases a buffer previously acquired with traceAcquireBuffer.
func traceReleaseBuffer(mp *m, pid int32) {
        if pid == traceGlobProc {
                unlock(&trace.bufLock)
        }
        releasem(mp)
}

// lockRankMayTraceFlush records the lock ranking effects of a
// potential call to traceFlush.
func lockRankMayTraceFlush() {
        lockWithRankMayAcquire(&trace.lock, getLockRank(&trace.lock))
}

// traceFlush puts buf onto stack of full buffers and returns an empty buffer.
//
// This must run on the system stack because it acquires trace.lock.
//
//go:systemstack
func traceFlush(buf traceBufPtr, pid int32) traceBufPtr {
        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
        ts := traceClockNow()
        if ts <= bufp.lastTime {
                ts = bufp.lastTime + 1
        }
        bufp.lastTime = ts
        bufp.byte(traceEvBatch | 1<<traceArgCountShift)
        bufp.varint(uint64(pid))
        bufp.varint(uint64(ts))

        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)
        if raceenabled {
                // 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 {
                systemstack(func() {
                        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
}

// 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
}

// varintAt writes varint v at byte position pos in buf. This always
// consumes traceBytesPerNumber bytes. This is intended for when the
// caller needs to reserve space for a varint but can't populate it
// until later.
func (buf *traceBuf) varintAt(pos int, v uint64) {
        for i := 0; i < traceBytesPerNumber; i++ {
                if i < traceBytesPerNumber-1 {
                        buf.arr[pos] = 0x80 | byte(v)
                } else {
                        buf.arr[pos] = byte(v)
                }
                v >>= 7
                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 // Must be acquired on the system stack
        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
        }
        hash := memhash(unsafe.Pointer(&pcs[0]), 0, uintptr(len(pcs))*unsafe.Sizeof(pcs[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.
        // Switch to the system stack so we can acquire tab.lock
        var id uint32
        systemstack(func() {
                lock(&tab.lock)
                if id = tab.find(pcs, hash); id != 0 {
                        unlock(&tab.lock)
                        return
                }
                // Create new record.
                tab.seq++
                stk := tab.newStack(len(pcs))
                stk.hash = hash
                stk.id = tab.seq
                id = stk.id
                stk.n = len(pcs)
                stkpc := stk.stack()
                copy(stkpc, pcs)
                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 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)*goarch.PtrSize))
}

// traceFrames returns the frames corresponding to pcs. It may
// allocate and may emit trace events.
func traceFrames(bufp traceBufPtr, pcs []uintptr) ([]traceFrame, traceBufPtr) {
        frames := make([]traceFrame, 0, len(pcs))
        ci := CallersFrames(pcs)
        for {
                var frame traceFrame
                f, more := ci.Next()
                frame, bufp = traceFrameForPC(bufp, 0, f)
                frames = append(frames, frame)
                if !more {
                        return frames, bufp
                }
        }
}

// dump writes all previously cached stacks to trace buffers,
// releases all memory and resets state.
//
// This must run on the system stack because it calls traceFlush.
//
//go:systemstack
func (tab *traceStackTable) dump(bufp traceBufPtr) traceBufPtr {
        for i := range tab.tab {
                stk := tab.tab[i].ptr()
                for ; stk != nil; stk = stk.link.ptr() {
                        var frames []traceFrame
                        frames, bufp = traceFrames(bufp, fpunwindExpand(stk.stack()))

                        // Estimate the size of this record. This
                        // bound is pretty loose, but avoids counting
                        // lots of varint sizes.
                        maxSize := 1 + traceBytesPerNumber + (2+4*len(frames))*traceBytesPerNumber
                        // Make sure we have enough buffer space.
                        if buf := bufp.ptr(); len(buf.arr)-buf.pos < maxSize {
                                bufp = traceFlush(bufp, 0)
                        }

                        // Emit header, with space reserved for length.
                        buf := bufp.ptr()
                        buf.byte(traceEvStack | 3<<traceArgCountShift)
                        lenPos := buf.pos
                        buf.pos += traceBytesPerNumber

                        // Emit body.
                        recPos := buf.pos
                        buf.varint(uint64(stk.id))
                        buf.varint(uint64(len(frames)))
                        for _, frame := range frames {
                                buf.varint(uint64(frame.PC))
                                buf.varint(frame.funcID)
                                buf.varint(frame.fileID)
                                buf.varint(frame.line)
                        }

                        // Fill in size header.
                        buf.varintAt(lenPos, uint64(buf.pos-recPos))
                }
        }

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

        return bufp
}

// fpunwindExpand checks if pcBuf contains logical frames (which include inlined
// frames) or physical frames (produced by frame pointer unwinding) using a
// sentinel value in pcBuf[0]. Logical frames are simply returned without the
// sentinel. Physical frames are turned into logical frames via inline unwinding
// and by applying the skip value that's stored in pcBuf[0].
func fpunwindExpand(pcBuf []uintptr) []uintptr {
        if len(pcBuf) > 0 && pcBuf[0] == logicalStackSentinel {
                // pcBuf contains logical rather than inlined frames, skip has already been
                // applied, just return it without the sentinel value in pcBuf[0].
                return pcBuf[1:]
        }

        var (
                lastFuncID = abi.FuncIDNormal
                newPCBuf   = make([]uintptr, 0, traceStackSize)
                skip       = pcBuf[0]
                // skipOrAdd skips or appends retPC to newPCBuf and returns true if more
                // pcs can be added.
                skipOrAdd = func(retPC uintptr) bool {
                        if skip > 0 {
                                skip--
                        } else {
                                newPCBuf = append(newPCBuf, retPC)
                        }
                        return len(newPCBuf) < cap(newPCBuf)
                }
        )

outer:
        for _, retPC := range pcBuf[1:] {
                callPC := retPC - 1
                fi := findfunc(callPC)
                if !fi.valid() {
                        // There is no funcInfo if callPC belongs to a C function. In this case
                        // we still keep the pc, but don't attempt to expand inlined frames.
                        if more := skipOrAdd(retPC); !more {
                                break outer
                        }
                        continue
                }

                u, uf := newInlineUnwinder(fi, callPC)
                for ; uf.valid(); uf = u.next(uf) {
                        sf := u.srcFunc(uf)
                        if sf.funcID == abi.FuncIDWrapper && elideWrapperCalling(lastFuncID) {
                                // ignore wrappers
                        } else if more := skipOrAdd(uf.pc + 1); !more {
                                break outer
                        }
                        lastFuncID = sf.funcID
                }
        }
        return newPCBuf
}

type traceFrame struct {
        PC     uintptr
        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
        frame.PC = f.PC

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

// TODO: Since traceAllocBlock is now embedded runtime/internal/sys.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, goarch.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))
}

func traceProcStop(pp *p) {
        // 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 traceSTWStart(reason stwReason) {
        // Don't trace if this STW is for trace start/stop, since traceEnabled
        // switches during a STW.
        if reason == stwStartTrace || reason == stwStopTrace {
                return
        }
        getg().m.trace.tracedSTWStart = true
        traceEvent(traceEvSTWStart, -1, uint64(reason))
}

func traceSTWDone() {
        mp := getg().m
        if !mp.trace.tracedSTWStart {
                return
        }
        mp.trace.tracedSTWStart = false
        traceEvent(traceEvSTWDone, -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.
func traceGCSweepStart() {
        // Delay the actual GCSweepStart event until the first span
        // sweep. If we don't sweep anything, don't emit any events.
        pp := getg().m.p.ptr()
        if pp.trace.inSweep {
                throw("double traceGCSweepStart")
        }
        pp.trace.inSweep, pp.trace.swept, pp.trace.reclaimed = 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) {
        pp := getg().m.p.ptr()
        if pp.trace.inSweep {
                if pp.trace.swept == 0 {
                        traceEvent(traceEvGCSweepStart, 1)
                }
                pp.trace.swept += bytesSwept
        }
}

func traceGCSweepDone() {
        pp := getg().m.p.ptr()
        if !pp.trace.inSweep {
                throw("missing traceGCSweepStart")
        }
        if pp.trace.swept != 0 {
                traceEvent(traceEvGCSweepDone, -1, uint64(pp.trace.swept), uint64(pp.trace.reclaimed))
        }
        pp.trace.inSweep = false
}

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

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

func traceGoCreate(newg *g, pc uintptr) {
        newg.trace.seq = 0
        newg.trace.lastP = getg().m.p
        // +PCQuantum because traceFrameForPC expects return PCs and subtracts PCQuantum.
        id := trace.stackTab.put([]uintptr{logicalStackSentinel, startPCforTrace(pc) + sys.PCQuantum})
        traceEvent(traceEvGoCreate, 2, newg.goid, uint64(id))
}

func traceGoStart() {
        gp := getg().m.curg
        pp := gp.m.p
        gp.trace.seq++
        if pp.ptr().gcMarkWorkerMode != gcMarkWorkerNotWorker {
                traceEvent(traceEvGoStartLabel, -1, gp.goid, gp.trace.seq, trace.markWorkerLabels[pp.ptr().gcMarkWorkerMode])
        } else if gp.trace.lastP == pp {
                traceEvent(traceEvGoStartLocal, -1, gp.goid)
        } else {
                gp.trace.lastP = pp
                traceEvent(traceEvGoStart, -1, gp.goid, gp.trace.seq)
        }
}

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

func traceGoSched() {
        gp := getg()
        gp.trace.lastP = gp.m.p
        traceEvent(traceEvGoSched, 1)
}

func traceGoPreempt() {
        gp := getg()
        gp.trace.lastP = gp.m.p
        traceEvent(traceEvGoPreempt, 1)
}

func traceGoPark(reason traceBlockReason, skip int) {
        // Convert the block reason directly to a trace event type.
        // See traceBlockReason for more information.
        traceEvent(byte(reason), skip)
}

func traceGoUnpark(gp *g, skip int) {
        pp := getg().m.p
        gp.trace.seq++
        if gp.trace.lastP == pp {
                traceEvent(traceEvGoUnblockLocal, skip, gp.goid)
        } else {
                gp.trace.lastP = pp
                traceEvent(traceEvGoUnblock, skip, gp.goid, gp.trace.seq)
        }
}

func traceGoSysCall() {
        var skip int
        switch {
        case tracefpunwindoff():
                // Unwind by skipping 1 frame relative to gp.syscallsp which is captured 3
                // frames above this frame. For frame pointer unwinding we produce the same
                // results by hard coding the number of frames in between our caller and the
                // actual syscall, see cases below.
                // TODO(felixge): Implement gp.syscallbp to avoid this workaround?
                skip = 1
        case GOOS == "solaris" || GOOS == "illumos":
                // These platforms don't use a libc_read_trampoline.
                skip = 3
        default:
                // Skip the extra trampoline frame used on most systems.
                skip = 4
        }
        getg().m.curg.trace.tracedSyscallEnter = true
        traceEvent(traceEvGoSysCall, skip)
}

func traceGoSysExit() {
        gp := getg().m.curg
        if !gp.trace.tracedSyscallEnter {
                // There was no syscall entry traced for us at all, so there's definitely
                // no EvGoSysBlock or EvGoInSyscall before us, which EvGoSysExit requires.
                return
        }
        gp.trace.tracedSyscallEnter = false
        ts := gp.trace.sysExitTime
        if ts != 0 && ts < trace.startTime {
                // There is a race between the code that initializes sysExitTimes
                // (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 sysExitTime 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
        }
        gp.trace.sysExitTime = 0
        gp.trace.seq++
        gp.trace.lastP = gp.m.p
        traceEvent(traceEvGoSysExit, -1, gp.goid, gp.trace.seq, uint64(ts))
}

func traceGoSysBlock(pp *p) {
        // 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(live uint64) {
        traceEvent(traceEvHeapAlloc, -1, live)
}

func traceHeapGoal() {
        heapGoal := gcController.heapGoal()
        if heapGoal == ^uint64(0) {
                // Heap-based triggering is disabled.
                traceEvent(traceEvHeapGoal, -1, 0)
        } else {
                traceEvent(traceEvHeapGoal, -1, heapGoal)
        }
}

// 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.trace.startingTrace {
                traceReleaseBuffer(mp, pid)
                return
        }

        typeStringID, bufp := traceString(bufp, pid, taskType)
        traceEventLocked(0, mp, pid, bufp, traceEvUserTaskCreate, 0, 3, id, parentID, typeStringID)
        traceReleaseBuffer(mp, 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.trace.startingTrace {
                traceReleaseBuffer(mp, pid)
                return
        }

        nameStringID, bufp := traceString(bufp, pid, name)
        traceEventLocked(0, mp, pid, bufp, traceEvUserRegion, 0, 3, id, mode, nameStringID)
        traceReleaseBuffer(mp, 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.trace.startingTrace {
                traceReleaseBuffer(mp, pid)
                return
        }

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

        // The log message is recorded after all of the normal trace event
        // arguments, including the task, category, and stack IDs. We must ask
        // traceEventLocked to reserve extra space for the length of the message
        // and the message itself.
        extraSpace := traceBytesPerNumber + len(message)
        traceEventLocked(extraSpace, mp, pid, bufp, traceEvUserLog, 0, 3, id, categoryID)
        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(mp, pid)
}

// the start PC of a goroutine for tracing purposes. If pc is a wrapper,
// it returns the PC of the wrapped function. Otherwise it returns pc.
func startPCforTrace(pc uintptr) uintptr {
        f := findfunc(pc)
        if !f.valid() {
                return pc // may happen for locked g in extra M since its pc is 0.
        }
        w := funcdata(f, abi.FUNCDATA_WrapInfo)
        if w == nil {
                return pc // not a wrapper
        }
        return f.datap.textAddr(*(*uint32)(w))
}

// traceOneNewExtraM registers the fact that a new extra M was created with
// the tracer. This matters if the M (which has an attached G) is used while
// the trace is still active because if it is, we need the fact that it exists
// to show up in the final trace.
func traceOneNewExtraM(gp *g) {
        // Trigger two trace events for the locked g in the extra m,
        // since the next event of the g will be traceEvGoSysExit in exitsyscall,
        // while calling from C thread to Go.
        traceGoCreate(gp, 0) // no start pc
        gp.trace.seq++
        traceEvent(traceEvGoInSyscall, -1, gp.goid)
}

// traceTime represents a timestamp for the trace.
type traceTime uint64

// traceClockNow returns a monotonic timestamp. The clock this function gets
// the timestamp from is specific to tracing, and shouldn't be mixed with other
// clock sources.
//
// nosplit because it's called from exitsyscall, which is nosplit.
//
//go:nosplit
func traceClockNow() traceTime {
        return traceTime(cputicks() / traceTimeDiv)
}