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-rw-r--r--src/runtime/profbuf.go561
1 files changed, 561 insertions, 0 deletions
diff --git a/src/runtime/profbuf.go b/src/runtime/profbuf.go
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+++ b/src/runtime/profbuf.go
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+// 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
+}