[PATCH v2] sched,fair: skip newidle_balance if a wakeup is pending

[Rik van Riel <riel@surriel.com>]

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>
---
v2:
 - fix !SMP build error and prev-not-CFS case by moving check into newidle_balance
 - fix formatting of if condition
 - audit newidle_balance return value use to make sure we get that right
 - reset idle_stamp when breaking out of the loop due to ->ttwu_pending

 kernel/sched/fair.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 69680158963f..5e26f013e182 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -10594,6 +10594,14 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
 	u64 curr_cost = 0;
 
 	update_misfit_status(NULL, this_rq);
+
+	/*
+	 * There is a task waiting to run. No need to search for one.
+	 * Return 0; the task will be enqueued when switching to idle.
+	 */
+	if (this_rq->ttwu_pending)
+		return 0;
+
 	/*
 	 * We must set idle_stamp _before_ calling idle_balance(), such that we
 	 * measure the duration of idle_balance() as idle time.
@@ -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();
@@ -10688,7 +10697,7 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
 	if (this_rq->nr_running != this_rq->cfs.h_nr_running)
 		pulled_task = -1;
 
-	if (pulled_task)
+	if (pulled_task || this_rq->ttwu_pending)
 		this_rq->idle_stamp = 0;
 
 	rq_repin_lock(this_rq, rf);
-- 
2.25.4