5879bf7e38
This change adds a new event, GoStatusStack, which is like GoStatus but also carries a stack ID. The purpose of this event is to emit stacks in more places, in particular for goroutines that may never emit a stack-bearing event in a whole generation. This CL targets one specific case: goroutines that were blocked or in a syscall the entire generation. This particular case is handled at the point that we scribble down the goroutine's status before the generation transition. That way, when we're finishing up the generation and emitting events for any goroutines we scribbled down, we have an accurate stack for those goroutines ready to go, and we emit a GoStatusStack instead of a GoStatus event. There's a small drawback with the way we scribble down the stack though: we immediately register it in the stack table instead of tracking the PCs. This means that if a goroutine does run and emit a trace event in between when we scribbled down its stack and the end of the generation, we will have recorded a stack that never actually gets referenced in the trace. This case should be rare. There are two remaining cases where we could emit stacks for goroutines but we don't. One is goroutines that get unblocked but either never run, or run and never block within a generation. We could take a stack trace at the point of unblocking the goroutine, if we're emitting a GoStatus event for it, but unfortunately we don't own the stack at that point. We could obtain ownership by grabbing its _Gscan bit, but that seems a little risky, since we could hold up the goroutine emitting the event for a while. Something to consider for the future. The other remaining case is a goroutine that was runnable when tracing started and began running, but then ran until the end of the generation without getting preempted or blocking. The main issue here is that although the goroutine will have a GoStatus event, it'll only have a GoStart event for it which doesn't emit a stack trace. This case is rare, but still certainly possible. I believe the only way to resolve it is to emit a GoStatusStack event instead of a GoStatus event for a goroutine that we're emitting GoStart for. This case is a bit easier than the last one because at the point of emitting GoStart, we have ownership of the goroutine's stack. We may consider dealing with these in the future, but for now, this CL captures a fairly large class of goroutines, so is worth it on its own. Fixes #65634. Change-Id: Ief3b6df5848b426e7ee6794e98dc7ef5f37ab2d0 Reviewed-on: https://go-review.googlesource.com/c/go/+/567076 Auto-Submit: Michael Knyszek <mknyszek@google.com> Reviewed-by: David Chase <drchase@google.com> LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> |
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