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runtime: replace traceBuf slice with index

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Currently traceBuf keeps track of where it is in the trace buffer by
also maintaining a slice that points in to this buffer with an initial
length of 0 and a cap of the length of the array. All writes to this
buffer are done by appending to the slice (as long as the bounds
checks are right, it will never overflow and the append won't allocate
a new slice).

Each of these appends generates a write barrier. As long as we never
overflow the buffer, this write barrier won't fire, but this wreaks
havoc with eliminating write barriers from the tracing code. If we
were to overflow the buffer, this would both allocate and invoke a
write barrier, both things that are dicey at best to do in many of the
contexts tracing happens. It also wastes space in the traceBuf and
leads to more complex code and more complex generated code.

Replace this slice trick with keeping track of a simple array
position.

Updates #10600.

Change-Id: I0a63eecec1992e195449f414ed47653f66318d0e
Reviewed-on: https://go-review.googlesource.com/16814
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
This commit is contained in:
Austin Clements 2015-11-10 14:58:51 -05:00
parent 2be1ed80c5
commit f5c42cf88e
1 changed files with 42 additions and 32 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

74
src/runtime/trace.go
View File

@ -140,7 +140,7 @@ type traceBufHeader struct {
link traceBufPtr // in trace.empty/full
lastSeq uint64 // sequence number of last event
lastTicks uint64 // when we wrote the last event
buf []byte // trace data, always points to traceBuf.arr
pos int // next write offset in arr
stk [traceStackSize]uintptr // scratch buffer for traceback
}
@ -253,7 +253,7 @@ func StopTrace() {
p.tracebuf = 0
}
}
if trace.buf != 0 && len(trace.buf.ptr().buf) != 0 {
if trace.buf != 0 && trace.buf.ptr().pos != 0 {
buf := trace.buf
trace.buf = 0
traceFullQueue(buf)
@ -361,7 +361,7 @@ func ReadTrace() []byte {
trace.reading = buf
trace.lockOwner = nil
unlock(&trace.lock)
return buf.ptr().buf
return buf.ptr().arr[:buf.ptr().pos]
}
// Write footer with timer frequency.
if !trace.footerWritten {
@ -477,7 +477,7 @@ func traceEvent(ev byte, skip int, args ...uint64) {
}
buf := (*bufp).ptr()
const maxSize = 2 + 5*traceBytesPerNumber // event type, length, sequence, timestamp, stack id and two add params
if buf == nil || cap(buf.buf)-len(buf.buf) < maxSize {
if buf == nil || len(buf.arr)-buf.pos < maxSize {
buf = traceFlush(traceBufPtrOf(buf)).ptr()
(*bufp).set(buf)
}
@ -486,13 +486,11 @@ func traceEvent(ev byte, skip int, args ...uint64) {
seqDiff := seq - buf.lastSeq
ticks := uint64(ticksraw) / traceTickDiv
tickDiff := ticks - buf.lastTicks
if len(buf.buf) == 0 {
data := buf.buf
data = append(data, traceEvBatch|1<<traceArgCountShift)
data = traceAppend(data, uint64(pid))
data = traceAppend(data, seq)
data = traceAppend(data, ticks)
buf.buf = data
if buf.pos == 0 {
buf.byte(traceEvBatch | 1<<traceArgCountShift)
buf.varint(uint64(pid))
buf.varint(seq)
buf.varint(ticks)
seqDiff = 0
tickDiff = 0
}
@ -507,21 +505,21 @@ func traceEvent(ev byte, skip int, args ...uint64) {
if narg > 3 {
narg = 3
}
data := buf.buf
data = append(data, ev|narg<<traceArgCountShift)
startPos := buf.pos
buf.byte(ev | narg<<traceArgCountShift)
var lenp *byte
if narg == 3 {
// Reserve the byte for length assuming that length < 128.
data = append(data, 0)
lenp = &data[len(data)-1]
buf.varint(0)
lenp = &buf.arr[buf.pos-1]
}
data = traceAppend(data, seqDiff)
data = traceAppend(data, tickDiff)
buf.varint(seqDiff)
buf.varint(tickDiff)
for _, a := range args {
data = traceAppend(data, a)
buf.varint(a)
}
if skip == 0 {
data = append(data, 0)
buf.varint(0)
} else if skip > 0 {
_g_ := getg()
gp := mp.curg
@ -539,9 +537,9 @@ func traceEvent(ev byte, skip int, args ...uint64) {
nstk-- // skip runtime.main
}
id := trace.stackTab.put(buf.stk[:nstk])
data = traceAppend(data, uint64(id))
buf.varint(uint64(id))
}
evSize := len(data) - len(buf.buf)
evSize := buf.pos - startPos
if evSize > maxSize {
throw("invalid length of trace event")
}
@ -549,7 +547,6 @@ func traceEvent(ev byte, skip int, args ...uint64) {
// Fill in actual length.
*lenp = byte(evSize - 2)
}
buf.buf = data
traceReleaseBuffer(pid)
}
@ -579,9 +576,6 @@ func traceFlush(buf traceBufPtr) traceBufPtr {
lock(&trace.lock)
}
if buf != 0 {
if buf := buf.ptr(); &buf.buf[0] != &buf.arr[0] {
throw("trace buffer overflow")
}
traceFullQueue(buf)
}
if trace.empty != 0 {
@ -595,7 +589,7 @@ func traceFlush(buf traceBufPtr) traceBufPtr {
}
bufp := buf.ptr()
bufp.link.set(nil)
bufp.buf = bufp.arr[:0]
bufp.pos = 0
bufp.lastTicks = 0
if dolock {
unlock(&trace.lock)
@ -612,6 +606,24 @@ func traceAppend(buf []byte, v uint64) []byte {
return buf
}
// 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
}
// 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 {
@ -704,7 +716,7 @@ func (tab *traceStackTable) dump() {
stk := stk.ptr()
for ; stk != nil; stk = stk.link.ptr() {
maxSize := 1 + (3+stk.n)*traceBytesPerNumber
if cap(buf.buf)-len(buf.buf) < maxSize {
if len(buf.arr)-buf.pos < maxSize {
buf = traceFlush(traceBufPtrOf(buf)).ptr()
}
// Form the event in the temp buffer, we need to know the actual length.
@ -715,11 +727,9 @@ func (tab *traceStackTable) dump() {
tmpbuf = traceAppend(tmpbuf, uint64(pc))
}
// Now copy to the buffer.
data := buf.buf
data = append(data, traceEvStack|3<<traceArgCountShift)
data = traceAppend(data, uint64(len(tmpbuf)))
data = append(data, tmpbuf...)
buf.buf = data
buf.byte(traceEvStack | 3<<traceArgCountShift)
buf.varint(uint64(len(tmpbuf)))
buf.pos += copy(buf.arr[buf.pos:], tmpbuf)
}
}