runtime: remove futile wakeups from trace

Channels and sync.Mutex'es allow another goroutine to acquire resource
ahead of an unblocked goroutine. This is good for performance, but
leads to futile wakeups (the unblocked goroutine needs to block again).
Futile wakeups caused user confusion during the very first evaluation
of tracing functionality on a real server (a goroutine as if acquires a mutex
in a loop, while there is no loop in user code).

This change detects futile wakeups on channels and emits a special event
to denote the fact. Later parser finds entire wakeup sequences
(unblock->start->block) and removes them.

sync.Mutex will be supported in a separate change.

Change-Id: Iaaaee9d5c0921afc62b449a97447445030ac19d3
Reviewed-on: https://go-review.googlesource.com/7380
Reviewed-by: Keith Randall <khr@golang.org>

1 files changed, 3 insertions, 1 deletions

diff --git a/src/runtime/select.go b/src/runtime/select.go
index 73fcb439f1..98ac5a3d61 100644
--- a/src/runtime/select.go
+++ b/src/runtime/select.go
@@ -308,6 +308,7 @@ func selectgoImpl(sel *hselect) (uintptr, uint16) {
 		k      *scase
 		sglist *sudog
 		sgnext *sudog
+		futile byte
 	)
 
 loop:
@@ -392,7 +393,7 @@ loop:
 
 	// wait for someone to wake us up
 	gp.param = nil
-	gopark(selparkcommit, unsafe.Pointer(sel), "select", traceEvGoBlockSelect, 2)
+	gopark(selparkcommit, unsafe.Pointer(sel), "select", traceEvGoBlockSelect|futile, 2)
 
 	// someone woke us up
 	sellock(sel)
@@ -435,6 +436,7 @@ loop:
 	}
 
 	if cas == nil {
+		futile = traceFutileWakeup
 		goto loop
 	}