runtime: elide timer re-check if P has no timers

In golang.org/cl/264477, I missed this new block after rebasing past
golang.org/cl/232298. These fields must be zero if there are no timers.

Updates #28808
Updates #18237

Change-Id: I2d9e1cbf326497c833daa26b11aed9a1e12c2270
Reviewed-on: https://go-review.googlesource.com/c/go/+/266367
Run-TryBot: Michael Pratt <[email protected]>
Reviewed-by: Austin Clements <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Trust: Michael Pratt <[email protected]>

src/runtime/proc.go

@@ -2606,9 +2606,10 @@ stop:
 	// safe-points. We don't need to snapshot the contents because
 	// everything up to cap(allp) is immutable.
 	allpSnapshot := allp
-	// Also snapshot idlepMask. Value changes are OK, but we can't allow
+	// Also snapshot masks. Value changes are OK, but we can't allow
 	// len to change out from under us.
 	idlepMaskSnapshot := idlepMask
+	timerpMaskSnapshot := timerpMask
 
 	// return P and block
 	lock(&sched.lock)
@@ -2670,10 +2671,12 @@ stop:
 	// transitioning from spinning to non-spinning. Note that we cannot use
 	// checkTimers here because it calls adjusttimers which may need to allocate
 	// memory, and that isn't allowed when we don't have an active P.
-	for _, _p_ := range allpSnapshot {
-		w := nobarrierWakeTime(_p_)
-		if w != 0 && (pollUntil == 0 || w < pollUntil) {
-			pollUntil = w
+	for id, _p_ := range allpSnapshot {
+		if timerpMaskSnapshot.read(uint32(id)) {
+			w := nobarrierWakeTime(_p_)
+			if w != 0 && (pollUntil == 0 || w < pollUntil) {
+				pollUntil = w
+			}
 		}
 	}
 	if pollUntil != 0 {