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// Copyright 2011 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.
// CPU profiling.
// Based on algorithms and data structures used in
// The main difference between this code and the google-perftools
// code is that this code is written to allow copying the profile data
// to an arbitrary io.Writer, while the google-perftools code always
// writes to an operating system file.
// The signal handler for the profiling clock tick adds a new stack trace
// to a hash table tracking counts for recent traces. Most clock ticks
// hit in the cache. In the event of a cache miss, an entry must be
// evicted from the hash table, copied to a log that will eventually be
// written as profile data. The google-perftools code flushed the
// log itself during the signal handler. This code cannot do that, because
// the io.Writer might block or need system calls or locks that are not
// safe to use from within the signal handler. Instead, we split the log
// into two halves and let the signal handler fill one half while a goroutine
// is writing out the other half. When the signal handler fills its half, it
// offers to swap with the goroutine. If the writer is not done with its half,
// we lose the stack trace for this clock tick (and record that loss).
// The goroutine interacts with the signal handler by calling getprofile() to
// get the next log piece to write, implicitly handing back the last log
// piece it obtained.
// The state of this dance between the signal handler and the goroutine
// is encoded in the Profile.handoff field. If handoff == 0, then the goroutine
// is not using either log half and is waiting (or will soon be waiting) for
// a new piece by calling notesleep(&p->wait). If the signal handler
// changes handoff from 0 to non-zero, it must call notewakeup(&p->wait)
// to wake the goroutine. The value indicates the number of entries in the
// log half being handed off. The goroutine leaves the non-zero value in
// place until it has finished processing the log half and then flips the number
// back to zero. Setting the high bit in handoff means that the profiling is over,
// and the goroutine is now in charge of flushing the data left in the hash table
// to the log and returning that data.
// The handoff field is manipulated using atomic operations.
// For the most part, the manipulation of handoff is orderly: if handoff == 0
// then the signal handler owns it and can change it to non-zero.
// If handoff != 0 then the goroutine owns it and can change it to zero.
// If that were the end of the story then we would not need to manipulate
// handoff using atomic operations. The operations are needed, however,
// in order to let the log closer set the high bit to indicate "EOF" safely
// in the situation when normally the goroutine "owns" handoff.
#include "runtime.h"
#include "malloc.h"
HashSize = 1<<10,
LogSize = 1<<17,
Assoc = 4,
MaxStack = 64,
typedef struct Profile Profile;
typedef struct Bucket Bucket;
typedef struct Entry Entry;
struct Entry {
uintptr count;
uintptr depth;
uintptr stack[MaxStack];
struct Bucket {
Entry entry[Assoc];
struct Profile {
bool on; // profiling is on
Note wait; // goroutine waits here
uintptr count; // tick count
uintptr evicts; // eviction count
uintptr lost; // lost ticks that need to be logged
uintptr totallost; // total lost ticks
// Active recent stack traces.
Bucket hash[HashSize];
// Log of traces evicted from hash.
// Signal handler has filled log[toggle][:nlog].
// Goroutine is writing log[1-toggle][:handoff].
uintptr log[2][LogSize/2];
uintptr nlog;
int32 toggle;
uint32 handoff;
// Writer state.
// Writer maintains its own toggle to avoid races
// looking at signal handler's toggle.
uint32 wtoggle;
bool wholding; // holding & need to release a log half
bool flushing; // flushing hash table - profile is over
static Lock lk;
static Profile *prof;
static void tick(uintptr*, int32);
static void add(Profile*, uintptr*, int32);
static bool evict(Profile*, Entry*);
static bool flushlog(Profile*);
// LostProfileData is a no-op function used in profiles
// to mark the number of profiling stack traces that were
// discarded due to slow data writers.
static void LostProfileData(void) {
// SetCPUProfileRate sets the CPU profiling rate.
// The user documentation is in debug.go.
runtime·SetCPUProfileRate(int32 hz)
uintptr *p;
uintptr n;
// Clamp hz to something reasonable.
if(hz < 0)
hz = 0;
if(hz > 1000000)
hz = 1000000;
if(hz > 0) {
if(prof == nil) {
prof = runtime·SysAlloc(sizeof *prof);
if(prof == nil) {
runtime·printf("runtime: cpu profiling cannot allocate memory\n");
if(prof->on || prof->handoff != 0) {
runtime·printf("runtime: cannot set cpu profile rate until previous profile has finished.\n");
prof->on = true;
p = prof->log[0];
// pprof binary header format.
*p++ = 0; // count for header
*p++ = 3; // depth for header
*p++ = 0; // version number
*p++ = 1000000 / hz; // period (microseconds)
*p++ = 0;
prof->nlog = p - prof->log[0];
prof->toggle = 0;
prof->wholding = false;
prof->wtoggle = 0;
prof->flushing = false;
runtime·setcpuprofilerate(tick, hz);
} else if(prof->on) {
runtime·setcpuprofilerate(nil, 0);
prof->on = false;
// Now add is not running anymore, and getprofile owns the entire log.
// Set the high bit in prof->handoff to tell getprofile.
for(;;) {
n = prof->handoff;
runtime·printf("runtime: setcpuprofile(off) twice");
if(runtime·cas(&prof->handoff, n, n|0x80000000))
if(n == 0) {
// we did the transition from 0 -> nonzero so we wake getprofile
static void
tick(uintptr *pc, int32 n)
add(prof, pc, n);
// add adds the stack trace to the profile.
// It is called from signal handlers and other limited environments
// and cannot allocate memory or acquire locks that might be
// held at the time of the signal, nor can it use substantial amounts
// of stack. It is allowed to call evict.
static void
add(Profile *p, uintptr *pc, int32 n)
int32 i, j;
uintptr h, x;
Bucket *b;
Entry *e;
if(n > MaxStack)
n = MaxStack;
// Compute hash.
h = 0;
for(i=0; i<n; i++) {
h = h<<8 | (h>>(8*(sizeof(h)-1)));
x = pc[i];
h += x*31 + x*7 + x*3;
// Add to entry count if already present in table.
b = &p->hash[h%HashSize];
for(i=0; i<Assoc; i++) {
e = &b->entry[i];
if(e->depth != n)
for(j=0; j<n; j++)
if(e->stack[j] != pc[j])
goto ContinueAssoc;
// Evict entry with smallest count.
e = &b->entry[0];
for(i=1; i<Assoc; i++)
if(b->entry[i].count < e->count)
e = &b->entry[i];
if(e->count > 0) {
if(!evict(p, e)) {
// Could not evict entry. Record lost stack.
// Reuse the newly evicted entry.
e->depth = n;
e->count = 1;
for(i=0; i<n; i++)
e->stack[i] = pc[i];
// evict copies the given entry's data into the log, so that
// the entry can be reused. evict is called from add, which
// is called from the profiling signal handler, so it must not
// allocate memory or block. It is safe to call flushLog.
// evict returns true if the entry was copied to the log,
// false if there was no room available.
static bool
evict(Profile *p, Entry *e)
int32 i, d, nslot;
uintptr *log, *q;
d = e->depth;
nslot = d+2;
log = p->log[p->toggle];
if(p->nlog+nslot > nelem(p->log[0])) {
return false;
log = p->log[p->toggle];
q = log+p->nlog;
*q++ = e->count;
*q++ = d;
for(i=0; i<d; i++)
*q++ = e->stack[i];
p->nlog = q - log;
e->count = 0;
return true;
// flushlog tries to flush the current log and switch to the other one.
// flushlog is called from evict, called from add, called from the signal handler,
// so it cannot allocate memory or block. It can try to swap logs with
// the writing goroutine, as explained in the comment at the top of this file.
static bool
flushlog(Profile *p)
uintptr *log, *q;
if(!runtime·cas(&p->handoff, 0, p->nlog))
return false;
p->toggle = 1 - p->toggle;
log = p->log[p->toggle];
q = log;
if(p->lost > 0) {
*q++ = p->lost;
*q++ = 1;
*q++ = (uintptr)LostProfileData;
p->nlog = q - log;
return true;
// getprofile blocks until the next block of profiling data is available
// and returns it as a []byte. It is called from the writing goroutine.
getprofile(Profile *p)
uint32 i, j, n;
Slice ret;
Bucket *b;
Entry *e;
ret.array = nil;
ret.len = 0;
ret.cap = 0;
if(p == nil)
return ret;
if(p->wholding) {
// Release previous log to signal handling side.
// Loop because we are racing against setprofile(off).
for(;;) {
n = p->handoff;
if(n == 0) {
runtime·printf("runtime: phase error during cpu profile handoff\n");
return ret;
if(n & 0x80000000) {
p->wtoggle = 1 - p->wtoggle;
p->wholding = false;
p->flushing = true;
goto flush;
if(runtime·cas(&p->handoff, n, 0))
p->wtoggle = 1 - p->wtoggle;
p->wholding = false;
goto flush;
if(!p->on && p->handoff == 0)
return ret;
// Wait for new log.
n = p->handoff;
if(n == 0) {
runtime·printf("runtime: phase error during cpu profile wait\n");
return ret;
if(n == 0x80000000) {
p->flushing = true;
goto flush;
n &= ~0x80000000;
// Return new log to caller.
p->wholding = true;
ret.array = (byte*)p->log[p->wtoggle];
ret.len = n*sizeof(uintptr);
ret.cap = ret.len;
return ret;
// In flush mode.
// Add is no longer being called. We own the log.
// Also, p->handoff is non-zero, so flushlog will return false.
// Evict the hash table into the log and return it.
for(i=0; i<HashSize; i++) {
b = &p->hash[i];
for(j=0; j<Assoc; j++) {
e = &b->entry[j];
if(e->count > 0 && !evict(p, e)) {
// Filled the log. Stop the loop and return what we've got.
goto breakflush;
// Return pending log data.
if(p->nlog > 0) {
// Note that we're using toggle now, not wtoggle,
// because we're working on the log directly.
ret.array = (byte*)p->log[p->toggle];
ret.len = p->nlog*sizeof(uintptr);
ret.cap = ret.len;
p->nlog = 0;
return ret;
// Made it through the table without finding anything to log.
// Finally done. Clean up and return nil.
p->flushing = false;
if(!runtime·cas(&p->handoff, p->handoff, 0))
runtime·printf("runtime: profile flush racing with something\n");
return ret; // set to nil at top of function
// CPUProfile returns the next cpu profile block as a []byte.
// The user documentation is in debug.go.
runtime·CPUProfile(Slice ret)
ret = getprofile(prof);