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runtime/pprof: document possibility of empty stacks 

I spent quite a while determining the cause of empty stacks in
profiles and reasoning out why this is okay. There isn't a great place
to record this knowledge, but a documentation comment on
appendLocsForStack is better than nothing.

Updates #51550.

Change-Id: I2eefc6ea31f1af885885c3d96199319f45edb4ce
Reviewed-on: https://go-review.googlesource.com/c/go/+/460695
Reviewed-by: Felix Geisendörfer <felix.geisendoerfer@datadoghq.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>
This commit is contained in:
Austin Clements 2023-01-05 09:08:25 -05:00
parent d9f23cfe78
commit 0bbd67e52f
1 changed files with 4 additions and 0 deletions
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4
src/runtime/pprof/proto.go
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@ -395,6 +395,10 @@ func (b *profileBuilder) build() {
// location ID slice, locs. The addresses in the stack are return PCs or 1 + the PC of // location ID slice, locs. The addresses in the stack are return PCs or 1 + the PC of
// an inline marker as the runtime traceback function returns. // an inline marker as the runtime traceback function returns.
// //
// It may return an empty slice even if locs is non-empty, for example if locs consists
// solely of runtime.goexit. We still count these empty stacks in profiles in order to
// get the right cumulative sample count.
//
// It may emit to b.pb, so there must be no message encoding in progress. // It may emit to b.pb, so there must be no message encoding in progress.
func (b *profileBuilder) appendLocsForStack(locs []uint64, stk []uintptr) (newLocs []uint64) { func (b *profileBuilder) appendLocsForStack(locs []uint64, stk []uintptr) (newLocs []uint64) {
b.deck.reset() b.deck.reset()