sched,fair: skip newidle_balance if a wakeup is pending

The try_to_wake_up function has an optimization where it can queue
a task for wakeup on its previous CPU, if the task is still in the
middle of going to sleep inside schedule().

Once schedule() re-enables IRQs, the task will be woken up with an
IPI, and placed back on the runqueue.

If we have such a wakeup pending, there is no need to search other
CPUs for runnable tasks. Just skip (or bail out early from) newidle
balancing, and run the just woken up task.

For a memcache like workload test, this reduces total CPU use by
about 2%, proportionally split between user and system time,
and p99 and p95 application response time by 2-3% on average.
The schedstats run_delay number shows a similar improvement.

Signed-off-by: Rik van Riel <riel@surriel.com>

kernel/sched/fair.c

@@ -7200,6 +7200,14 @@ done: __maybe_unused;
 	if (!rf)
 		return NULL;
 
+	/*
+	 * We have a woken up task pending here. No need to search for ones
+	 * elsewhere. This task will be enqueued the moment we unblock irqs
+	 * upon exiting the scheduler.
+	 */
+	if (rq->ttwu_pending)
+		return NULL;
+
 	new_tasks = newidle_balance(rq, rf);
 
 	/*
@@ -10661,7 +10669,8 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
 		 * Stop searching for tasks to pull if there are
 		 * now runnable tasks on this rq.
 		 */
-		if (pulled_task || this_rq->nr_running > 0)
+		if (pulled_task || this_rq->nr_running > 0 ||
+						this_rq->ttwu_pending)
 			break;
 	}
 	rcu_read_unlock();