runtime: fix idle time double-counting bug

This change fixes a bug in the accounting of sched.idleTime. In just the
case where the GC CPU limiter needs up-to-date data, sched.idleTime is
incremented in both the P-idle-time and idle-mark-work paths, but it
should only be incremented in the former case.

Fixes #74627.

Change-Id: If41b03da102d47d25bec48ff750a9da27019b71d
Reviewed-on: https://go-review.googlesource.com/c/go/+/687998
Reviewed-by: Cherry Mui <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Michael Pratt <[email protected]>

Author: mknyszek

src/runtime/mgclimit.go

@@ -209,14 +209,12 @@ func (l *gcCPULimiterState) updateLocked(now int64) {
 		for _, pp := range allp {
 			typ, duration := pp.limiterEvent.consume(now)
 			switch typ {
-			case limiterEventIdleMarkWork:
-				fallthrough
 			case limiterEventIdle:
-				idleTime += duration
 				sched.idleTime.Add(duration)
-			case limiterEventMarkAssist:
-				fallthrough
-			case limiterEventScavengeAssist:
+				idleTime += duration
+			case limiterEventIdleMarkWork:
+				idleTime += duration
+			case limiterEventMarkAssist, limiterEventScavengeAssist:
 				assistTime += duration
 			case limiterEventNone:
 				break
@@ -470,14 +468,12 @@ func (e *limiterEvent) stop(typ limiterEventType, now int64) {
 	}
 	// Account for the event.
 	switch typ {
-	case limiterEventIdleMarkWork:
-		gcCPULimiter.addIdleTime(duration)
 	case limiterEventIdle:
-		gcCPULimiter.addIdleTime(duration)
 		sched.idleTime.Add(duration)
-	case limiterEventMarkAssist:
-		fallthrough
-	case limiterEventScavengeAssist:
+		gcCPULimiter.addIdleTime(duration)
+	case limiterEventIdleMarkWork:
+		gcCPULimiter.addIdleTime(duration)
+	case limiterEventMarkAssist, limiterEventScavengeAssist:
 		gcCPULimiter.addAssistTime(duration)
 	default:
 		throw("limiterEvent.stop: invalid limiter event type found")