runtime: skip TestCPUMetricsSleep as flaky

This test is fundamentally flaky because of a mismatch between how
internal idle time is calculated and how the test expects it to be
calculated. It's unclear how to resolve this mismatch, given that it's
perfectly valid for a goroutine to remain asleep while background
goroutines (e.g. the scavenger) run. In practice, we might be able to
set some generous lower-bound, but until we can confirm that on the
affected platforms, skip the test as flaky unconditionally.

For #60276.
For #60376.

Change-Id: Iffd5c4be10cf8ae8a6c285b61fcc9173235fbb2a
Reviewed-on: https://go-review.googlesource.com/c/go/+/497876
Run-TryBot: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/runtime/metrics_test.go

@@ -5,6 +5,7 @@
 package runtime_test
 
 import (
+	"internal/testenv"
 	"reflect"
 	"runtime"
 	"runtime/debug"
@@ -645,6 +646,18 @@ func TestCPUMetricsSleep(t *testing.T) {
 		// test is basically meaningless on this platform.
 		t.Skip("wasip1 currently busy-waits in idle time; test not applicable")
 	}
+
+	// Unconditionally skip this test as flaky.
+	//
+	// There's a fundamental issue with this test, which is that there's no
+	// guarantee the application will go idle; background goroutines and
+	// time spent in the scheduler going to sleep can always erode idle time
+	// sufficiently such that minimum idle time (or maximum user time) stays
+	// within some threshold.
+	//
+	// Leave this as skipped while we figure out a better way to check this.
+	testenv.SkipFlaky(t, 60376)
+
 	names := []string{
 		"/cpu/classes/idle:cpu-seconds",
 
@@ -681,8 +694,11 @@ func TestCPUMetricsSleep(t *testing.T) {
 	metrics.Read(m2)
 
 	// If the bug we expect is happening, then the Sleep CPU time will be accounted for
-	// as user time rather than idle time. Because we're doing this on one core, the
-	// maximum amount of time that can be attributed to user time is the time spent asleep.
+	// as user time rather than idle time.
+	//
+	// TODO(mknyszek): This number here is wrong. Background goroutines and just slow
+	// platforms spending a non-trivial amount of time in the scheduler doing things
+	// could easily erode idle time beyond this minimum.
 	minIdleCPUSeconds := dur.Seconds() * float64(runtime.GOMAXPROCS(-1))
 
 	if dt := m2[0].Value.Float64() - m1[0].Value.Float64(); dt < minIdleCPUSeconds {