runtime/pprof: increase systemstack calls in TestLabelSystemstack

TestLabelSystemstack needs to collect samples within runtime.systemstack
to complete the test.

The current approach uses fmt.Fprintf, which gets into systemstack
through the allocator and GC, but also does lots of other work. In my
measurements, approximately 2% of samples contain runtime.systemstack.

The new approach uses debug.SetGCPercent, which uses systemstack for
most of its work, including contention on mheap_.lock, which extends
usage even more. In my measurements, approximately 99% of samples
contain runtime.systemstack.

Fixes #50050

Change-Id: I59e5bb756341b716a12e13d2e3fe0adadd7fe956
Reviewed-on: https://go-review.googlesource.com/c/go/+/370375
Reviewed-by: Bryan Mills <bcmills@google.com>
Trust: Michael Pratt <mpratt@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

1 files changed, 8 insertions, 3 deletions

diff --git a/src/runtime/pprof/pprof_test.go b/src/runtime/pprof/pprof_test.go
index 2e6165ff88..b3a8927dc7 100644
--- a/src/runtime/pprof/pprof_test.go
+++ b/src/runtime/pprof/pprof_test.go
@@ -1425,6 +1425,11 @@ func TestLabelRace(t *testing.T) {
 // TestLabelSystemstack makes sure CPU profiler samples of goroutines running
 // on systemstack include the correct pprof labels. See issue #48577
 func TestLabelSystemstack(t *testing.T) {
+	// Grab and re-set the initial value before continuing to ensure
+	// GOGC doesn't actually change following the test.
+	gogc := debug.SetGCPercent(100)
+	debug.SetGCPercent(gogc)
+
 	matches := matchAndAvoidStacks(stackContainsLabeled, []string{"runtime.systemstack;key=value"}, avoidFunctions())
 	p := testCPUProfile(t, matches, func(dur time.Duration) {
 		Do(context.Background(), Labels("key", "value"), func(context.Context) {
@@ -1434,7 +1439,7 @@ func TestLabelSystemstack(t *testing.T) {
 				wg.Add(1)
 				go func() {
 					defer wg.Done()
-					labelHog(stop)
+					labelHog(stop, gogc)
 				}()
 			}
 
@@ -1467,13 +1472,13 @@ func TestLabelSystemstack(t *testing.T) {
 
 // labelHog is designed to burn CPU time in a way that a high number of CPU
 // samples end up running on systemstack.
-func labelHog(stop chan struct{}) {
+func labelHog(stop chan struct{}, gogc int) {
 	for i := 0; ; i++ {
 		select {
 		case <-stop:
 			return
 		default:
-			fmt.Fprintf(io.Discard, "%d", i)
+			debug.SetGCPercent(gogc)
 		}
 	}
 }