runtime: prevent timer starvation in cores scheduler

src/runtime/scheduler_cores.go

@@ -195,18 +195,6 @@ func run() {
 
 func scheduler(_ bool) {
 	for mainExited.Load() == 0 {
-		// Check for ready-to-run tasks.
-		if runnable := runqueue.Pop(); runnable != nil {
-			// Resume it now.
-			setCurrentTask(runnable)
-			runnable.RunState = task.RunStateRunning
-			schedulerLock.Unlock() // unlock before resuming, Pause() will lock again
-			runnable.Resume()
-			setCurrentTask(nil)
-
-			continue
-		}
-
 		var now timeUnit
 		if sleepQueue != nil || timerQueue != nil {
 			now = ticks()
@@ -253,6 +241,18 @@ func scheduler(_ bool) {
 			}
 		}
 
+		// Check for ready-to-run tasks.
+		if runnable := runqueue.Pop(); runnable != nil {
+			// Resume it now.
+			setCurrentTask(runnable)
+			runnable.RunState = task.RunStateRunning
+			schedulerLock.Unlock() // unlock before resuming, Pause() will lock again
+			runnable.Resume()
+			setCurrentTask(nil)
+
+			continue
+		}
+
 		// At this point, there are no runnable tasks anymore.
 		// If another core is using the clock, let it handle the sleep queue.
 		if hasSleepingCore() {