runtime, cmd/trace: track goroutines blocked on GC assists

Currently when a goroutine blocks on a GC assist, it emits a generic
EvGoBlock event. Since assist blocking events and, in particular, the
length of the blocked assist queue, are important for diagnosing GC
behavior, this commit adds a new EvGoBlockGC event for blocking on a
GC assist. The trace viewer uses this event to report a "waiting on
GC" count in the "Goroutines" row. This makes sense because, unlike
other blocked goroutines, these goroutines do have work to do, so
being blocked on a GC assist is quite similar to being in the
"runnable" state, which we also report in the trace viewer.

Change-Id: Ic21a326992606b121ea3d3d00110d8d1fdc7a5ef
Reviewed-on: https://go-review.googlesource.com/30704
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>

1 files changed, 2 insertions, 1 deletions

diff --git a/src/runtime/trace.go b/src/runtime/trace.go
index 0bb529ea9d..a8f4ab60d3 100644
--- a/src/runtime/trace.go
+++ b/src/runtime/trace.go
@@ -61,7 +61,8 @@ const (
 	traceEvGoUnblockLocal = 39 // goroutine is unblocked on the same P as the last event [timestamp, goroutine id, stack]
 	traceEvGoSysExitLocal = 40 // syscall exit on the same P as the last event [timestamp, goroutine id, real timestamp]
 	traceEvGoStartLabel   = 41 // goroutine starts running with label [timestamp, goroutine id, seq, label string id]
-	traceEvCount          = 42
+	traceEvGoBlockGC      = 42 // goroutine blocks on GC assist [timestamp, stack]
+	traceEvCount          = 43
 )
 
 const (