runtime: lower mutex contention test expectations

As of https://go.dev/cl/586796, the runtime/metrics view of internal
mutex contention is sampled at 1 per gTrackingPeriod, rather than either
1 (immediately prior to CL 586796) or the more frequent of
gTrackingPeriod or the mutex profiling rate (Go 1.22). Thus, we no
longer have a real lower bound on the amount of contention that
runtime/metrics will report. Relax the test's expectations again.

For #64253

Change-Id: I94e1d92348a03599a819ec8ac785a0eb3c1ddd73
Reviewed-on: https://go-review.googlesource.com/c/go/+/587515
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Rhys Hiltner <rhys.hiltner@gmail.com>

src/runtime/metrics_test.go

@@ -1036,6 +1036,13 @@ func TestRuntimeLockMetricsAndProfile(t *testing.T) {
 			if metricGrowth == 0 && strictTiming {
 				// If the critical section is very short, systems with low timer
 				// resolution may be unable to measure it via nanotime.
+				//
+				// This is sampled at 1 per gTrackingPeriod, but the explicit
+				// runtime.mutex tests create 200 contention events. Observing
+				// zero of those has a probability of (7/8)^200 = 2.5e-12 which
+				// is acceptably low (though the calculation has a tenuous
+				// dependency on cheaprandn being a good-enough source of
+				// entropy).
 				t.Errorf("no increase in /sync/mutex/wait/total:seconds metric")
 			}
 			// This comparison is possible because the time measurements in support of
@@ -1111,7 +1118,7 @@ func TestRuntimeLockMetricsAndProfile(t *testing.T) {
 	name := t.Name()
 
 	t.Run("runtime.lock", func(t *testing.T) {
-		mus := make([]runtime.Mutex, 100)
+		mus := make([]runtime.Mutex, 200)
 		var needContention atomic.Int64
 		baseDelay := 100 * time.Microsecond // large relative to system noise, for comparison between clocks
 		fastDelayMicros := baseDelay.Microseconds()
@@ -1207,6 +1214,11 @@ func TestRuntimeLockMetricsAndProfile(t *testing.T) {
 			needContention.Store(int64(len(mus) - 1))
 			metricGrowth, profileGrowth, n, _ := testcase(true, stks, workers, fn)(t)
 
+			t.Run("metric", func(t *testing.T) {
+				// The runtime/metrics view is sampled at 1 per gTrackingPeriod,
+				// so we don't have a hard lower bound here.
+				testenv.SkipFlaky(t, 64253)
+
 				if have, want := metricGrowth, baseDelay.Seconds()*float64(len(mus)); have < want {
 					// The test imposes a delay with usleep, verified with calls to
 					// nanotime. Compare against the runtime/metrics package's view
@@ -1214,6 +1226,7 @@ func TestRuntimeLockMetricsAndProfile(t *testing.T) {
 					// on cputicks).
 					t.Errorf("runtime/metrics reported less than the known minimum contention duration (%fs < %fs)", have, want)
 				}
+			})
 			if have, want := n, int64(len(mus)); have != want {
 				t.Errorf("mutex profile reported contention count different from the known true count (%d != %d)", have, want)
 			}