// Copyright 2017 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.
package runtime
import (
"runtime/internal/atomic"
"unsafe"
)
// A profBuf is a lock-free buffer for profiling events,
// safe for concurrent use by one reader and one writer.
// The writer may be a signal handler running without a user g.
// The reader is assumed to be a user g.
//
// Each logged event corresponds to a fixed size header, a list of
// uintptrs (typically a stack), and exactly one unsafe.Pointer tag.
// The header and uintptrs are stored in the circular buffer data and the
// tag is stored in a circular buffer tags, running in parallel.
// In the circular buffer data, each event takes 2+hdrsize+len(stk)
// words: the value 2+hdrsize+len(stk), then the time of the event, then
// hdrsize words giving the fixed-size header, and then len(stk) words
// for the stack.
//
// The current effective offsets into the tags and data circular buffers
// for reading and writing are stored in the high 30 and low 32 bits of r and w.
// The bottom bits of the high 32 are additional flag bits in w, unused in r.
// "Effective" offsets means the total number of reads or writes, mod 2^length.
// The offset in the buffer is the effective offset mod the length of the buffer.
// To make wraparound mod 2^length match wraparound mod length of the buffer,
// the length of the buffer must be a power of two.
//
// If the reader catches up to the writer, a flag passed to read controls
// whether the read blocks until more data is available. A read returns a
// pointer to the buffer data itself; the caller is assumed to be done with
// that data at the next read. The read offset rNext tracks the next offset to
// be returned by read. By definition, r ≤ rNext ≤ w (before wraparound),
// and rNext is only used by the reader, so it can be accessed without atomics.
//
// If the writer gets ahead of the reader, so that the buffer fills,
// future writes are discarded and replaced in the output stream by an
// overflow entry, which has size 2+hdrsize+1, time set to the time of
// the first discarded write, a header of all zeroed words, and a "stack"
// containing one word, the number of discarded writes.
//
// Between the time the buffer fills and the buffer becomes empty enough
// to hold more data, the overflow entry is stored as a pending overflow
// entry in the fields overflow and overflowTime. The pending overflow
// entry can be turned into a real record by either the writer or the
// reader. If the writer is called to write a new record and finds that
// the output buffer has room for both the pending overflow entry and the
// new record, the writer emits the pending overflow entry and the new
// record into the buffer. If the reader is called to read data and finds
// that the output buffer is empty but that there is a pending overflow
// entry, the reader will return a synthesized record for the pending
// overflow entry.
//
// Only the writer can create or add to a pending overflow entry, but
// either the reader or the writer can clear the pending overflow entry.
// A pending overflow entry is indicated by the low 32 bits of 'overflow'
// holding the number of discarded writes, and overflowTime holding the
// time of the first discarded write. The high 32 bits of 'overflow'
// increment each time the low 32 bits transition from zero to non-zero
// or vice versa. This sequence number avoids ABA problems in the use of
// compare-and-swap to coordinate between reader and writer.
// The overflowTime is only written when the low 32 bits of overflow are
// zero, that is, only when there is no pending overflow entry, in
// preparation for creating a new one. The reader can therefore fetch and
// clear the entry atomically using
//
// for {
// overflow = load(&b.overflow)
// if uint32(overflow) == 0 {
// // no pending entry
// break
// }
// time = load(&b.overflowTime)
// if cas(&b.overflow, overflow, ((overflow>>32)+1)<<32) {
// // pending entry cleared
// break
// }
// }
// if uint32(overflow) > 0 {
// emit entry for uint32(overflow), time
// }
//
type profBuf struct {
// accessed atomically
r, w profAtomic
overflow uint64
overflowTime uint64
eof uint32
// immutable (excluding slice content)
hdrsize uintptr
data []uint64
tags []unsafe.Pointer
// owned by reader
rNext profIndex
overflowBuf []uint64 // for use by reader to return overflow record
wait note
}
// A profAtomic is the atomically-accessed word holding a profIndex.
type profAtomic uint64
// A profIndex is the packet tag and data counts and flags bits, described above.
type profIndex uint64
const (
profReaderSleeping profIndex = 1 << 32 // reader is sleeping and must be woken up
profWriteExtra profIndex = 1 << 33 // overflow or eof waiting
)
func (x *profAtomic) load() profIndex {
return profIndex(atomic.Load64((*uint64)(x)))
}
func (x *profAtomic) store(new profIndex) {
atomic.Store64((*uint64)(x), uint64(new))
}
func (x *profAtomic) cas(old, new profIndex) bool {
return atomic.Cas64((*uint64)(x), uint64(old), uint64(new))
}
func (x profIndex) dataCount() uint32 {
return uint32(x)
}
func (x profIndex) tagCount() uint32 {
return uint32(x >> 34)
}
// countSub subtracts two counts obtained from profIndex.dataCount or profIndex.tagCount,
// assuming that they are no more than 2^29 apart (guaranteed since they are never more than
// len(data) or len(tags) apart, respectively).
// tagCount wraps at 2^30, while dataCount wraps at 2^32.
// This function works for both.
func countSub(x, y uint32) int {
// x-y is 32-bit signed or 30-bit signed; sign-extend to 32 bits and convert to int.
return int(int32(x-y) << 2 >> 2)
}
// addCountsAndClearFlags returns the packed form of "x + (data, tag) - all flags".
func (x profIndex) addCountsAndClearFlags(data, tag int) profIndex {
return profIndex((uint64(x)>>34+uint64(uint32(tag)<<2>>2))<<34 | uint64(uint32(x)+uint32(data)))
}
// hasOverflow reports whether b has any overflow records pending.
func (b *profBuf) hasOverflow() bool {
return uint32(atomic.Load64(&b.overflow)) > 0
}
// takeOverflow consumes the pending overflow records, returning the overflow count
// and the time of the first overflow.
// When called by the reader, it is racing against incrementOverflow.
func (b *profBuf) takeOverflow() (count uint32, time uint64) {
overflow := atomic.Load64(&b.overflow)
time = atomic.Load64(&b.overflowTime)
for {
count = uint32(overflow)
if count == 0 {
time = 0
break
}
// Increment generation, clear overflow count in low bits.
if atomic.Cas64(&b.overflow, overflow, ((overflow>>32)+1)<<32) {
break
}
overflow = atomic.Load64(&b.overflow)
time = atomic.Load64(&b.overflowTime)
}
return uint32(overflow), time
}
// incrementOverflow records a single overflow at time now.
// It is racing against a possible takeOverflow in the reader.
func (b *profBuf) incrementOverflow(now int64) {
for {
overflow := atomic.Load64(&b.overflow)
// Once we see b.overflow reach 0, it's stable: no one else is changing it underfoot.
// We need to set overflowTime if we're incrementing b.overflow from 0.
if uint32(overflow) == 0 {
// Store overflowTime first so it's always available when overflow != 0.
atomic.Store64(&b.overflowTime, uint64(now))
atomic.Store64(&b.overflow, (((overflow>>32)+1)<<32)+1)
break
}
// Otherwise we're racing to increment against reader
// who wants to set b.overflow to 0.
// Out of paranoia, leave 2³²-1 a sticky overflow value,
// to avoid wrapping around. Extremely unlikely.
if int32(overflow) == -1 {
break
}
if atomic.Cas64(&b.overflow, overflow, overflow+1) {
break
}
}
}
// newProfBuf returns a new profiling buffer with room for
// a header of hdrsize words and a buffer of at least bufwords words.
func newProfBuf(hdrsize, bufwords, tags int) *profBuf {
if min := 2 + hdrsize + 1; bufwords < min {
bufwords = min
}
// Buffer sizes must be power of two, so that we don't have to
// worry about uint32 wraparound changing the effective position
// within the buffers. We store 30 bits of count; limiting to 28
// gives us some room for intermediate calculations.
if bufwords >= 1<<28 || tags >= 1<<28 {
throw("newProfBuf: buffer too large")
}
var i int
for i = 1; i < bufwords; i <<= 1 {
}
bufwords = i
for i = 1; i < tags; i <<= 1 {
}
tags = i
b := new(profBuf)
b.hdrsize = uintptr(hdrsize)
b.data = make([]uint64, bufwords)
b.tags = make([]unsafe.Pointer, tags)
b.overflowBuf = make([]uint64, 2+b.hdrsize+1)
return b
}
// canWriteRecord reports whether the buffer has room
// for a single contiguous record with a stack of length nstk.
func (b *profBuf) canWriteRecord(nstk int) bool {
br := b.r.load()
bw := b.w.load()
// room for tag?
if countSub(br.tagCount(), bw.tagCount())+len(b.tags) < 1 {
return false
}
// room for data?
nd := countSub(br.dataCount(), bw.dataCount()) + len(b.data)
want := 2 + int(b.hdrsize) + nstk
i := int(bw.dataCount() % uint32(len(b.data)))
if i+want > len(b.data) {
// Can't fit in trailing fragment of slice.
// Skip over that and start over at beginning of slice.
nd -= len(b.data) - i
}
return nd >= want
}
// canWriteTwoRecords reports whether the buffer has room
// for two records with stack lengths nstk1, nstk2, in that order.
// Each record must be contiguous on its own, but the two
// records need not be contiguous (one can be at the end of the buffer
// and the other can wrap around and start at the beginning of the buffer).
func (b *profBuf) canWriteTwoRecords(nstk1, nstk2 int) bool {
br := b.r.load()
bw := b.w.load()
// room for tag?
if countSub(br.tagCount(), bw.tagCount())+len(b.tags) < 2 {
return false
}
// room for data?
nd := countSub(br.dataCount(), bw.dataCount()) + len(b.data)
// first record
want := 2 + int(b.hdrsize) + nstk1
i := int(bw.dataCount() % uint32(len(b.data)))
if i+want > len(b.data) {
// Can't fit in trailing fragment of slice.
// Skip over that and start over at beginning of slice.
nd -= len(b.data) - i
i = 0
}
i += want
nd -= want
// second record
want = 2 + int(b.hdrsize) + nstk2
if i+want > len(b.data) {
// Can't fit in trailing fragment of slice.
// Skip over that and start over at beginning of slice.
nd -= len(b.data) - i
i = 0
}
return nd >= want
}
// write writes an entry to the profiling buffer b.
// The entry begins with a fixed hdr, which must have
// length b.hdrsize, followed by a variable-sized stack
// and a single tag pointer *tagPtr (or nil if tagPtr is nil).
// No write barriers allowed because this might be called from a signal handler.
func (b *profBuf) write(tagPtr *unsafe.Pointer, now int64, hdr []uint64, stk []uintptr) {
if b == nil {
return
}
if len(hdr) > int(b.hdrsize) {
throw("misuse of profBuf.write")
}
if hasOverflow := b.hasOverflow(); hasOverflow && b.canWriteTwoRecords(1, len(stk)) {
// Room for both an overflow record and the one being written.
// Write the overflow record if the reader hasn't gotten to it yet.
// Only racing against reader, not other writers.
count, time := b.takeOverflow()
if count > 0 {
var stk [1]uintptr
stk[0] = uintptr(count)
b.write(nil, int64(time), nil, stk[:])
}
} else if hasOverflow || !b.canWriteRecord(len(stk)) {
// Pending overflow without room to write overflow and new records
// or no overflow but also no room for new record.
b.incrementOverflow(now)
b.wakeupExtra()
return
}
// There's room: write the record.
br := b.r.load()
bw := b.w.load()
// Profiling tag
//
// The tag is a pointer, but we can't run a write barrier here.
// We have interrupted the OS-level execution of gp, but the
// runtime still sees gp as executing. In effect, we are running
// in place of the real gp. Since gp is the only goroutine that
// can overwrite gp.labels, the value of gp.labels is stable during
// this signal handler: it will still be reachable from gp when
// we finish executing. If a GC is in progress right now, it must
// keep gp.labels alive, because gp.labels is reachable from gp.
// If gp were to overwrite gp.labels, the deletion barrier would
// still shade that pointer, which would preserve it for the
// in-progress GC, so all is well. Any future GC will see the
// value we copied when scanning b.tags (heap-allocated).
// We arrange that the store here is always overwriting a nil,
// so there is no need for a deletion barrier on b.tags[wt].
wt := int(bw.tagCount() % uint32(len(b.tags)))
if tagPtr != nil {
*(*uintptr)(unsafe.Pointer(&b.tags[wt])) = uintptr(unsafe.Pointer(*tagPtr))
}
// Main record.
// It has to fit in a contiguous section of the slice, so if it doesn't fit at the end,
// leave a rewind marker (0) and start over at the beginning of the slice.
wd := int(bw.dataCount() % uint32(len(b.data)))
nd := countSub(br.dataCount(), bw.dataCount()) + len(b.data)
skip := 0
if wd+2+int(b.hdrsize)+len(stk) > len(b.data) {
b.data[wd] = 0
skip = len(b.data) - wd
nd -= skip
wd = 0
}
data := b.data[wd:]
data[0] = uint64(2 + b.hdrsize + uintptr(len(stk))) // length
data[1] = uint64(now) // time stamp
// header, zero-padded
i := uintptr(copy(data[2:2+b.hdrsize], hdr))
for ; i < b.hdrsize; i++ {
data[2+i] = 0
}
for i, pc := range stk {
data[2+b.hdrsize+uintptr(i)] = uint64(pc)
}
for {
// Commit write.
// Racing with reader setting flag bits in b.w, to avoid lost wakeups.
old := b.w.load()
new := old.addCountsAndClearFlags(skip+2+len(stk)+int(b.hdrsize), 1)
if !b.w.cas(old, new) {
continue
}
// If there was a reader, wake it up.
if old&profReaderSleeping != 0 {
notewakeup(&b.wait)
}
break
}
}
// close signals that there will be no more writes on the buffer.
// Once all the data has been read from the buffer, reads will return eof=true.
func (b *profBuf) close() {
if atomic.Load(&b.eof) > 0 {
throw("runtime: profBuf already closed")
}
atomic.Store(&b.eof, 1)
b.wakeupExtra()
}
// wakeupExtra must be called after setting one of the "extra"
// atomic fields b.overflow or b.eof.
// It records the change in b.w and wakes up the reader if needed.
func (b *profBuf) wakeupExtra() {
for {
old := b.w.load()
new := old | profWriteExtra
if !b.w.cas(old, new) {
continue
}
if old&profReaderSleeping != 0 {
notewakeup(&b.wait)
}
break
}
}
// profBufReadMode specifies whether to block when no data is available to read.
type profBufReadMode int
const (
profBufBlocking profBufReadMode = iota
profBufNonBlocking
)
var overflowTag [1]unsafe.Pointer // always nil
func (b *profBuf) read(mode profBufReadMode) (data []uint64, tags []unsafe.Pointer, eof bool) {
if b == nil {
return nil, nil, true
}
br := b.rNext
// Commit previous read, returning that part of the ring to the writer.
// First clear tags that have now been read, both to avoid holding
// up the memory they point at for longer than necessary
// and so that b.write can assume it is always overwriting
// nil tag entries (see comment in b.write).
rPrev := b.r.load()
if rPrev != br {
ntag := countSub(br.tagCount(), rPrev.tagCount())
ti := int(rPrev.tagCount() % uint32(len(b.tags)))
for i := 0; i < ntag; i++ {
b.tags[ti] = nil
if ti++; ti == len(b.tags) {
ti = 0
}
}
b.r.store(br)
}
Read:
bw := b.w.load()
numData := countSub(bw.dataCount(), br.dataCount())
if numData == 0 {
if b.hasOverflow() {
// No data to read, but there is overflow to report.
// Racing with writer flushing b.overflow into a real record.
count, time := b.takeOverflow()
if count == 0 {
// Lost the race, go around again.
goto Read
}
// Won the race, report overflow.
dst := b.overflowBuf
dst[0] = uint64(2 + b.hdrsize + 1)
dst[1] = uint64(time)
for i := uintptr(0); i < b.hdrsize; i++ {
dst[2+i] = 0
}
dst[2+b.hdrsize] = uint64(count)
return dst[:2+b.hdrsize+1], overflowTag[:1], false
}
if atomic.Load(&b.eof) > 0 {
// No data, no overflow, EOF set: done.
return nil, nil, true
}
if bw&profWriteExtra != 0 {
// Writer claims to have published extra information (overflow or eof).
// Attempt to clear notification and then check again.
// If we fail to clear the notification it means b.w changed,
// so we still need to check again.
b.w.cas(bw, bw&^profWriteExtra)
goto Read
}
// Nothing to read right now.
// Return or sleep according to mode.
if mode == profBufNonBlocking {
return nil, nil, false
}
if !b.w.cas(bw, bw|profReaderSleeping) {
goto Read
}
// Committed to sleeping.
notetsleepg(&b.wait, -1)
noteclear(&b.wait)
goto Read
}
data = b.data[br.dataCount()%uint32(len(b.data)):]
if len(data) > numData {
data = data[:numData]
} else {
numData -= len(data) // available in case of wraparound
}
skip := 0
if data[0] == 0 {
// Wraparound record. Go back to the beginning of the ring.
skip = len(data)
data = b.data
if len(data) > numData {
data = data[:numData]
}
}
ntag := countSub(bw.tagCount(), br.tagCount())
if ntag == 0 {
throw("runtime: malformed profBuf buffer - tag and data out of sync")
}
tags = b.tags[br.tagCount()%uint32(len(b.tags)):]
if len(tags) > ntag {
tags = tags[:ntag]
}
// Count out whole data records until either data or tags is done.
// They are always in sync in the buffer, but due to an end-of-slice
// wraparound we might need to stop early and return the rest
// in the next call.
di := 0
ti := 0
for di < len(data) && data[di] != 0 && ti < len(tags) {
if uintptr(di)+uintptr(data[di]) > uintptr(len(data)) {
throw("runtime: malformed profBuf buffer - invalid size")
}
di += int(data[di])
ti++
}
// Remember how much we returned, to commit read on next call.
b.rNext = br.addCountsAndClearFlags(skip+di, ti)
if raceenabled {
// Match racereleasemerge in runtime_setProfLabel,
// so that the setting of the labels in runtime_setProfLabel
// is treated as happening before any use of the labels
// by our caller. The synchronization on labelSync itself is a fiction
// for the race detector. The actual synchronization is handled
// by the fact that the signal handler only reads from the current
// goroutine and uses atomics to write the updated queue indices,
// and then the read-out from the signal handler buffer uses
// atomics to read those queue indices.
raceacquire(unsafe.Pointer(&labelSync))
}
return data[:di], tags[:ti], false
}