runtime: scan register backing store on ia64

[official-gcc.git] / libgo / go / runtime / mprof.go

// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Malloc profiling.
// Patterned after tcmalloc's algorithms; shorter code.

package runtime

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

// NOTE(rsc): Everything here could use cas if contention became an issue.
var proflock mutex

// All memory allocations are local and do not escape outside of the profiler.
// The profiler is forbidden from referring to garbage-collected memory.

const (
        // profile types
        memProfile bucketType = 1 + iota
        blockProfile
        mutexProfile

        // size of bucket hash table
        buckHashSize = 179999

        // max depth of stack to record in bucket
        maxStack = 32
)

type bucketType int

// A bucket holds per-call-stack profiling information.
// The representation is a bit sleazy, inherited from C.
// This struct defines the bucket header. It is followed in
// memory by the stack words and then the actual record
// data, either a memRecord or a blockRecord.
//
// Per-call-stack profiling information.
// Lookup by hashing call stack into a linked-list hash table.
//
// No heap pointers.
//
//go:notinheap
type bucket struct {
        next    *bucket
        allnext *bucket
        typ     bucketType // memBucket or blockBucket (includes mutexProfile)
        hash    uintptr
        size    uintptr
        nstk    uintptr
}

// A memRecord is the bucket data for a bucket of type memProfile,
// part of the memory profile.
type memRecord struct {
        // The following complex 3-stage scheme of stats accumulation
        // is required to obtain a consistent picture of mallocs and frees
        // for some point in time.
        // The problem is that mallocs come in real time, while frees
        // come only after a GC during concurrent sweeping. So if we would
        // naively count them, we would get a skew toward mallocs.
        //
        // Hence, we delay information to get consistent snapshots as
        // of mark termination. Allocations count toward the next mark
        // termination's snapshot, while sweep frees count toward the
        // previous mark termination's snapshot:
        //
        //              MT          MT          MT          MT
        //             .·|         .·|         .·|         .·|
        //          .·˙  |      .·˙  |      .·˙  |      .·˙  |
        //       .·˙     |   .·˙     |   .·˙     |   .·˙     |
        //    .·˙        |.·˙        |.·˙        |.·˙        |
        //
        //       alloc → ▲ ← free
        //               ┠┅┅┅┅┅┅┅┅┅┅┅P
        //       C+2     →    C+1    →  C
        //
        //                   alloc → ▲ ← free
        //                           ┠┅┅┅┅┅┅┅┅┅┅┅P
        //                   C+2     →    C+1    →  C
        //
        // Since we can't publish a consistent snapshot until all of
        // the sweep frees are accounted for, we wait until the next
        // mark termination ("MT" above) to publish the previous mark
        // termination's snapshot ("P" above). To do this, allocation
        // and free events are accounted to *future* heap profile
        // cycles ("C+n" above) and we only publish a cycle once all
        // of the events from that cycle must be done. Specifically:
        //
        // Mallocs are accounted to cycle C+2.
        // Explicit frees are accounted to cycle C+2.
        // GC frees (done during sweeping) are accounted to cycle C+1.
        //
        // After mark termination, we increment the global heap
        // profile cycle counter and accumulate the stats from cycle C
        // into the active profile.

        // active is the currently published profile. A profiling
        // cycle can be accumulated into active once its complete.
        active memRecordCycle

        // future records the profile events we're counting for cycles
        // that have not yet been published. This is ring buffer
        // indexed by the global heap profile cycle C and stores
        // cycles C, C+1, and C+2. Unlike active, these counts are
        // only for a single cycle; they are not cumulative across
        // cycles.
        //
        // We store cycle C here because there's a window between when
        // C becomes the active cycle and when we've flushed it to
        // active.
        future [3]memRecordCycle
}

// memRecordCycle
type memRecordCycle struct {
        allocs, frees           uintptr
        alloc_bytes, free_bytes uintptr
}

// add accumulates b into a. It does not zero b.
func (a *memRecordCycle) add(b *memRecordCycle) {
        a.allocs += b.allocs
        a.frees += b.frees
        a.alloc_bytes += b.alloc_bytes
        a.free_bytes += b.free_bytes
}

// A blockRecord is the bucket data for a bucket of type blockProfile,
// which is used in blocking and mutex profiles.
type blockRecord struct {
        count  int64
        cycles int64
}

var (
        mbuckets  *bucket // memory profile buckets
        bbuckets  *bucket // blocking profile buckets
        xbuckets  *bucket // mutex profile buckets
        buckhash  *[179999]*bucket
        bucketmem uintptr

        mProf struct {
                // All fields in mProf are protected by proflock.

                // cycle is the global heap profile cycle. This wraps
                // at mProfCycleWrap.
                cycle uint32
                // flushed indicates that future[cycle] in all buckets
                // has been flushed to the active profile.
                flushed bool
        }
)

const mProfCycleWrap = uint32(len(memRecord{}.future)) * (2 << 24)

// newBucket allocates a bucket with the given type and number of stack entries.
func newBucket(typ bucketType, nstk int) *bucket {
        size := unsafe.Sizeof(bucket{}) + uintptr(nstk)*unsafe.Sizeof(location{})
        switch typ {
        default:
                throw("invalid profile bucket type")
        case memProfile:
                size += unsafe.Sizeof(memRecord{})
        case blockProfile, mutexProfile:
                size += unsafe.Sizeof(blockRecord{})
        }

        b := (*bucket)(persistentalloc(size, 0, &memstats.buckhash_sys))
        bucketmem += size
        b.typ = typ
        b.nstk = uintptr(nstk)
        return b
}

// stk returns the slice in b holding the stack.
func (b *bucket) stk() []location {
        stk := (*[maxStack]location)(add(unsafe.Pointer(b), unsafe.Sizeof(*b)))
        return stk[:b.nstk:b.nstk]
}

// mp returns the memRecord associated with the memProfile bucket b.
func (b *bucket) mp() *memRecord {
        if b.typ != memProfile {
                throw("bad use of bucket.mp")
        }
        data := add(unsafe.Pointer(b), unsafe.Sizeof(*b)+b.nstk*unsafe.Sizeof(location{}))
        return (*memRecord)(data)
}

// bp returns the blockRecord associated with the blockProfile bucket b.
func (b *bucket) bp() *blockRecord {
        if b.typ != blockProfile && b.typ != mutexProfile {
                throw("bad use of bucket.bp")
        }
        data := add(unsafe.Pointer(b), unsafe.Sizeof(*b)+b.nstk*unsafe.Sizeof(location{}))
        return (*blockRecord)(data)
}

// Return the bucket for stk[0:nstk], allocating new bucket if needed.
func stkbucket(typ bucketType, size uintptr, stk []location, alloc bool) *bucket {
        if buckhash == nil {
                buckhash = (*[buckHashSize]*bucket)(sysAlloc(unsafe.Sizeof(*buckhash), &memstats.buckhash_sys))
                if buckhash == nil {
                        throw("runtime: cannot allocate memory")
                }
        }

        // Hash stack.
        var h uintptr
        for _, loc := range stk {
                h += loc.pc
                h += h << 10
                h ^= h >> 6
        }
        // hash in size
        h += size
        h += h << 10
        h ^= h >> 6
        // finalize
        h += h << 3
        h ^= h >> 11

        i := int(h % buckHashSize)
        for b := buckhash[i]; b != nil; b = b.next {
                if b.typ == typ && b.hash == h && b.size == size && eqslice(b.stk(), stk) {
                        return b
                }
        }

        if !alloc {
                return nil
        }

        // Create new bucket.
        b := newBucket(typ, len(stk))
        copy(b.stk(), stk)
        b.hash = h
        b.size = size
        b.next = buckhash[i]
        buckhash[i] = b
        if typ == memProfile {
                b.allnext = mbuckets
                mbuckets = b
        } else if typ == mutexProfile {
                b.allnext = xbuckets
                xbuckets = b
        } else {
                b.allnext = bbuckets
                bbuckets = b
        }
        return b
}

func eqslice(x, y []location) bool {
        if len(x) != len(y) {
                return false
        }
        for i, xi := range x {
                if xi != y[i] {
                        return false
                }
        }
        return true
}

// mProf_NextCycle publishes the next heap profile cycle and creates a
// fresh heap profile cycle. This operation is fast and can be done
// during STW. The caller must call mProf_Flush before calling
// mProf_NextCycle again.
//
// This is called by mark termination during STW so allocations and
// frees after the world is started again count towards a new heap
// profiling cycle.
func mProf_NextCycle() {
        lock(&proflock)
        // We explicitly wrap mProf.cycle rather than depending on
        // uint wraparound because the memRecord.future ring does not
        // itself wrap at a power of two.
        mProf.cycle = (mProf.cycle + 1) % mProfCycleWrap
        mProf.flushed = false
        unlock(&proflock)
}

// mProf_Flush flushes the events from the current heap profiling
// cycle into the active profile. After this it is safe to start a new
// heap profiling cycle with mProf_NextCycle.
//
// This is called by GC after mark termination starts the world. In
// contrast with mProf_NextCycle, this is somewhat expensive, but safe
// to do concurrently.
func mProf_Flush() {
        lock(&proflock)
        if !mProf.flushed {
                mProf_FlushLocked()
                mProf.flushed = true
        }
        unlock(&proflock)
}

func mProf_FlushLocked() {
        c := mProf.cycle
        for b := mbuckets; b != nil; b = b.allnext {
                mp := b.mp()

                // Flush cycle C into the published profile and clear
                // it for reuse.
                mpc := &mp.future[c%uint32(len(mp.future))]
                mp.active.add(mpc)
                *mpc = memRecordCycle{}
        }
}

// mProf_PostSweep records that all sweep frees for this GC cycle have
// completed. This has the effect of publishing the heap profile
// snapshot as of the last mark termination without advancing the heap
// profile cycle.
func mProf_PostSweep() {
        lock(&proflock)
        // Flush cycle C+1 to the active profile so everything as of
        // the last mark termination becomes visible. *Don't* advance
        // the cycle, since we're still accumulating allocs in cycle
        // C+2, which have to become C+1 in the next mark termination
        // and so on.
        c := mProf.cycle
        for b := mbuckets; b != nil; b = b.allnext {
                mp := b.mp()
                mpc := &mp.future[(c+1)%uint32(len(mp.future))]
                mp.active.add(mpc)
                *mpc = memRecordCycle{}
        }
        unlock(&proflock)
}

// Called by malloc to record a profiled block.
func mProf_Malloc(p unsafe.Pointer, size uintptr) {
        var stk [maxStack]location
        nstk := callers(4, stk[:])
        lock(&proflock)
        b := stkbucket(memProfile, size, stk[:nstk], true)
        c := mProf.cycle
        mp := b.mp()
        mpc := &mp.future[(c+2)%uint32(len(mp.future))]
        mpc.allocs++
        mpc.alloc_bytes += size
        unlock(&proflock)

        // Setprofilebucket locks a bunch of other mutexes, so we call it outside of proflock.
        // This reduces potential contention and chances of deadlocks.
        // Since the object must be alive during call to mProf_Malloc,
        // it's fine to do this non-atomically.
        systemstack(func() {
                setprofilebucket(p, b)
        })
}

// Called when freeing a profiled block.
func mProf_Free(b *bucket, size uintptr) {
        lock(&proflock)
        c := mProf.cycle
        mp := b.mp()
        mpc := &mp.future[(c+1)%uint32(len(mp.future))]
        mpc.frees++
        mpc.free_bytes += size
        unlock(&proflock)
}

var blockprofilerate uint64 // in CPU ticks

// SetBlockProfileRate controls the fraction of goroutine blocking events
// that are reported in the blocking profile. The profiler aims to sample
// an average of one blocking event per rate nanoseconds spent blocked.
//
// To include every blocking event in the profile, pass rate = 1.
// To turn off profiling entirely, pass rate <= 0.
func SetBlockProfileRate(rate int) {
        var r int64
        if rate <= 0 {
                r = 0 // disable profiling
        } else if rate == 1 {
                r = 1 // profile everything
        } else {
                // convert ns to cycles, use float64 to prevent overflow during multiplication
                r = int64(float64(rate) * float64(tickspersecond()) / (1000 * 1000 * 1000))
                if r == 0 {
                        r = 1
                }
        }

        atomic.Store64(&blockprofilerate, uint64(r))
}

func blockevent(cycles int64, skip int) {
        if cycles <= 0 {
                cycles = 1
        }
        if blocksampled(cycles) {
                saveblockevent(cycles, skip+1, blockProfile)
        }
}

func blocksampled(cycles int64) bool {
        rate := int64(atomic.Load64(&blockprofilerate))
        if rate <= 0 || (rate > cycles && int64(fastrand())%rate > cycles) {
                return false
        }
        return true
}

func saveblockevent(cycles int64, skip int, which bucketType) {
        gp := getg()
        var nstk int
        var stk [maxStack]location
        if gp.m.curg == nil || gp.m.curg == gp {
                nstk = callers(skip, stk[:])
        } else {
                // FIXME: This should get a traceback of gp.m.curg.
                // nstk = gcallers(gp.m.curg, skip, stk[:])
                nstk = callers(skip, stk[:])
        }
        lock(&proflock)
        b := stkbucket(which, 0, stk[:nstk], true)
        b.bp().count++
        b.bp().cycles += cycles
        unlock(&proflock)
}

var mutexprofilerate uint64 // fraction sampled

// SetMutexProfileFraction controls the fraction of mutex contention events
// that are reported in the mutex profile. On average 1/rate events are
// reported. The previous rate is returned.
//
// To turn off profiling entirely, pass rate 0.
// To just read the current rate, pass rate -1.
// (For n>1 the details of sampling may change.)
func SetMutexProfileFraction(rate int) int {
        if rate < 0 {
                return int(mutexprofilerate)
        }
        old := mutexprofilerate
        atomic.Store64(&mutexprofilerate, uint64(rate))
        return int(old)
}

//go:linkname mutexevent sync.event
func mutexevent(cycles int64, skip int) {
        if cycles < 0 {
                cycles = 0
        }
        rate := int64(atomic.Load64(&mutexprofilerate))
        // TODO(pjw): measure impact of always calling fastrand vs using something
        // like malloc.go:nextSample()
        if rate > 0 && int64(fastrand())%rate == 0 {
                saveblockevent(cycles, skip+1, mutexProfile)
        }
}

// Go interface to profile data.

// A StackRecord describes a single execution stack.
type StackRecord struct {
        Stack0 [32]uintptr // stack trace for this record; ends at first 0 entry
}

// Stack returns the stack trace associated with the record,
// a prefix of r.Stack0.
func (r *StackRecord) Stack() []uintptr {
        for i, v := range r.Stack0 {
                if v == 0 {
                        return r.Stack0[0:i]
                }
        }
        return r.Stack0[0:]
}

// MemProfileRate controls the fraction of memory allocations
// that are recorded and reported in the memory profile.
// The profiler aims to sample an average of
// one allocation per MemProfileRate bytes allocated.
//
// To include every allocated block in the profile, set MemProfileRate to 1.
// To turn off profiling entirely, set MemProfileRate to 0.
//
// The tools that process the memory profiles assume that the
// profile rate is constant across the lifetime of the program
// and equal to the current value. Programs that change the
// memory profiling rate should do so just once, as early as
// possible in the execution of the program (for example,
// at the beginning of main).
var MemProfileRate int = 512 * 1024

// A MemProfileRecord describes the live objects allocated
// by a particular call sequence (stack trace).
type MemProfileRecord struct {
        AllocBytes, FreeBytes     int64       // number of bytes allocated, freed
        AllocObjects, FreeObjects int64       // number of objects allocated, freed
        Stack0                    [32]uintptr // stack trace for this record; ends at first 0 entry
}

// InUseBytes returns the number of bytes in use (AllocBytes - FreeBytes).
func (r *MemProfileRecord) InUseBytes() int64 { return r.AllocBytes - r.FreeBytes }

// InUseObjects returns the number of objects in use (AllocObjects - FreeObjects).
func (r *MemProfileRecord) InUseObjects() int64 {
        return r.AllocObjects - r.FreeObjects
}

// Stack returns the stack trace associated with the record,
// a prefix of r.Stack0.
func (r *MemProfileRecord) Stack() []uintptr {
        for i, v := range r.Stack0 {
                if v == 0 {
                        return r.Stack0[0:i]
                }
        }
        return r.Stack0[0:]
}

// MemProfile returns a profile of memory allocated and freed per allocation
// site.
//
// MemProfile returns n, the number of records in the current memory profile.
// If len(p) >= n, MemProfile copies the profile into p and returns n, true.
// If len(p) < n, MemProfile does not change p and returns n, false.
//
// If inuseZero is true, the profile includes allocation records
// where r.AllocBytes > 0 but r.AllocBytes == r.FreeBytes.
// These are sites where memory was allocated, but it has all
// been released back to the runtime.
//
// The returned profile may be up to two garbage collection cycles old.
// This is to avoid skewing the profile toward allocations; because
// allocations happen in real time but frees are delayed until the garbage
// collector performs sweeping, the profile only accounts for allocations
// that have had a chance to be freed by the garbage collector.
//
// Most clients should use the runtime/pprof package or
// the testing package's -test.memprofile flag instead
// of calling MemProfile directly.
func MemProfile(p []MemProfileRecord, inuseZero bool) (n int, ok bool) {
        lock(&proflock)
        // If we're between mProf_NextCycle and mProf_Flush, take care
        // of flushing to the active profile so we only have to look
        // at the active profile below.
        mProf_FlushLocked()
        clear := true
        for b := mbuckets; b != nil; b = b.allnext {
                mp := b.mp()
                if inuseZero || mp.active.alloc_bytes != mp.active.free_bytes {
                        n++
                }
                if mp.active.allocs != 0 || mp.active.frees != 0 {
                        clear = false
                }
        }
        if clear {
                // Absolutely no data, suggesting that a garbage collection
                // has not yet happened. In order to allow profiling when
                // garbage collection is disabled from the beginning of execution,
                // accumulate all of the cycles, and recount buckets.
                n = 0
                for b := mbuckets; b != nil; b = b.allnext {
                        mp := b.mp()
                        for c := range mp.future {
                                mp.active.add(&mp.future[c])
                                mp.future[c] = memRecordCycle{}
                        }
                        if inuseZero || mp.active.alloc_bytes != mp.active.free_bytes {
                                n++
                        }
                }
        }
        if n <= len(p) {
                ok = true
                idx := 0
                for b := mbuckets; b != nil; b = b.allnext {
                        mp := b.mp()
                        if inuseZero || mp.active.alloc_bytes != mp.active.free_bytes {
                                record(&p[idx], b)
                                idx++
                        }
                }
        }
        unlock(&proflock)
        return
}

// Write b's data to r.
func record(r *MemProfileRecord, b *bucket) {
        mp := b.mp()
        r.AllocBytes = int64(mp.active.alloc_bytes)
        r.FreeBytes = int64(mp.active.free_bytes)
        r.AllocObjects = int64(mp.active.allocs)
        r.FreeObjects = int64(mp.active.frees)
        for i, loc := range b.stk() {
                if i >= len(r.Stack0) {
                        break
                }
                r.Stack0[i] = loc.pc
        }
        for i := int(b.nstk); i < len(r.Stack0); i++ {
                r.Stack0[i] = 0
        }
}

func iterate_memprof(fn func(*bucket, uintptr, *location, uintptr, uintptr, uintptr)) {
        lock(&proflock)
        for b := mbuckets; b != nil; b = b.allnext {
                mp := b.mp()
                fn(b, b.nstk, &b.stk()[0], b.size, mp.active.allocs, mp.active.frees)
        }
        unlock(&proflock)
}

// BlockProfileRecord describes blocking events originated
// at a particular call sequence (stack trace).
type BlockProfileRecord struct {
        Count  int64
        Cycles int64
        StackRecord
}

// BlockProfile returns n, the number of records in the current blocking profile.
// If len(p) >= n, BlockProfile copies the profile into p and returns n, true.
// If len(p) < n, BlockProfile does not change p and returns n, false.
//
// Most clients should use the runtime/pprof package or
// the testing package's -test.blockprofile flag instead
// of calling BlockProfile directly.
func BlockProfile(p []BlockProfileRecord) (n int, ok bool) {
        lock(&proflock)
        for b := bbuckets; b != nil; b = b.allnext {
                n++
        }
        if n <= len(p) {
                ok = true
                for b := bbuckets; b != nil; b = b.allnext {
                        bp := b.bp()
                        r := &p[0]
                        r.Count = bp.count
                        r.Cycles = bp.cycles
                        i := 0
                        var loc location
                        for i, loc = range b.stk() {
                                if i >= len(r.Stack0) {
                                        break
                                }
                                r.Stack0[i] = loc.pc
                        }
                        for ; i < len(r.Stack0); i++ {
                                r.Stack0[i] = 0
                        }
                        p = p[1:]
                }
        }
        unlock(&proflock)
        return
}

// MutexProfile returns n, the number of records in the current mutex profile.
// If len(p) >= n, MutexProfile copies the profile into p and returns n, true.
// Otherwise, MutexProfile does not change p, and returns n, false.
//
// Most clients should use the runtime/pprof package
// instead of calling MutexProfile directly.
func MutexProfile(p []BlockProfileRecord) (n int, ok bool) {
        lock(&proflock)
        for b := xbuckets; b != nil; b = b.allnext {
                n++
        }
        if n <= len(p) {
                ok = true
                for b := xbuckets; b != nil; b = b.allnext {
                        bp := b.bp()
                        r := &p[0]
                        r.Count = int64(bp.count)
                        r.Cycles = bp.cycles
                        i := 0
                        var loc location
                        for i, loc = range b.stk() {
                                if i >= len(r.Stack0) {
                                        break
                                }
                                r.Stack0[i] = loc.pc
                        }
                        for ; i < len(r.Stack0); i++ {
                                r.Stack0[i] = 0
                        }
                        p = p[1:]
                }
        }
        unlock(&proflock)
        return
}

// ThreadCreateProfile returns n, the number of records in the thread creation profile.
// If len(p) >= n, ThreadCreateProfile copies the profile into p and returns n, true.
// If len(p) < n, ThreadCreateProfile does not change p and returns n, false.
//
// Most clients should use the runtime/pprof package instead
// of calling ThreadCreateProfile directly.
func ThreadCreateProfile(p []StackRecord) (n int, ok bool) {
        first := (*m)(atomic.Loadp(unsafe.Pointer(&allm)))
        for mp := first; mp != nil; mp = mp.alllink {
                n++
        }
        if n <= len(p) {
                ok = true
                i := 0
                for mp := first; mp != nil; mp = mp.alllink {
                        for j := range mp.createstack {
                                p[i].Stack0[j] = mp.createstack[j].pc
                        }
                        i++
                }
        }
        return
}

// GoroutineProfile returns n, the number of records in the active goroutine stack profile.
// If len(p) >= n, GoroutineProfile copies the profile into p and returns n, true.
// If len(p) < n, GoroutineProfile does not change p and returns n, false.
//
// Most clients should use the runtime/pprof package instead
// of calling GoroutineProfile directly.
func GoroutineProfile(p []StackRecord) (n int, ok bool) {
        gp := getg()

        isOK := func(gp1 *g) bool {
                // Checking isSystemGoroutine here makes GoroutineProfile
                // consistent with both NumGoroutine and Stack.
                return gp1 != gp && readgstatus(gp1) != _Gdead && !isSystemGoroutine(gp1)
        }

        stopTheWorld("profile")

        n = 1
        for _, gp1 := range allgs {
                if isOK(gp1) {
                        n++
                }
        }

        if n <= len(p) {
                ok = true
                r := p

                // Save current goroutine.
                saveg(gp, &r[0])
                r = r[1:]

                // Save other goroutines.
                for _, gp1 := range allgs {
                        if isOK(gp1) {
                                if len(r) == 0 {
                                        // Should be impossible, but better to return a
                                        // truncated profile than to crash the entire process.
                                        break
                                }
                                saveg(gp1, &r[0])
                                r = r[1:]
                        }
                }
        }

        startTheWorld()

        return n, ok
}

func saveg(gp *g, r *StackRecord) {
        if gp == getg() {
                var locbuf [32]location
                n := callers(1, locbuf[:])
                for i := 0; i < n; i++ {
                        r.Stack0[i] = locbuf[i].pc
                }
                if n < len(r.Stack0) {
                        r.Stack0[n] = 0
                }
        } else {
                // FIXME: Not implemented.
                r.Stack0[0] = 0
        }
}

// Stack formats a stack trace of the calling goroutine into buf
// and returns the number of bytes written to buf.
// If all is true, Stack formats stack traces of all other goroutines
// into buf after the trace for the current goroutine.
func Stack(buf []byte, all bool) int {
        if all {
                stopTheWorld("stack trace")
        }

        n := 0
        if len(buf) > 0 {
                gp := getg()
                // Force traceback=1 to override GOTRACEBACK setting,
                // so that Stack's results are consistent.
                // GOTRACEBACK is only about crash dumps.
                gp.m.traceback = 1
                gp.writebuf = buf[0:0:len(buf)]
                goroutineheader(gp)
                traceback(1)
                if all {
                        tracebackothers(gp)
                }
                gp.m.traceback = 0
                n = len(gp.writebuf)
                gp.writebuf = nil
        }

        if all {
                startTheWorld()
        }
        return n
}

// Tracing of alloc/free/gc.

var tracelock mutex

func tracealloc(p unsafe.Pointer, size uintptr, typ *_type) {
        lock(&tracelock)
        gp := getg()
        gp.m.traceback = 2
        if typ == nil {
                print("tracealloc(", p, ", ", hex(size), ")\n")
        } else {
                print("tracealloc(", p, ", ", hex(size), ", ", *typ.string, ")\n")
        }
        if gp.m.curg == nil || gp == gp.m.curg {
                goroutineheader(gp)
                traceback(1)
        } else {
                goroutineheader(gp.m.curg)
                // FIXME: Can't do traceback of other g.
        }
        print("\n")
        gp.m.traceback = 0
        unlock(&tracelock)
}

func tracefree(p unsafe.Pointer, size uintptr) {
        lock(&tracelock)
        gp := getg()
        gp.m.traceback = 2
        print("tracefree(", p, ", ", hex(size), ")\n")
        goroutineheader(gp)
        traceback(1)
        print("\n")
        gp.m.traceback = 0
        unlock(&tracelock)
}

func tracegc() {
        lock(&tracelock)
        gp := getg()
        gp.m.traceback = 2
        print("tracegc()\n")
        // running on m->g0 stack; show all non-g0 goroutines
        tracebackothers(gp)
        print("end tracegc\n")
        print("\n")
        gp.m.traceback = 0
        unlock(&tracelock)
}