[tip: sched/core] sched/fair: Skip update_blocked_averages if we are defering load balance

[tip-bot2 for Vincent Guittot]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     9d783c8dd112ad4b619e74e4bf57d2be0b400693
Gitweb:        https://git.kernel.org/tip/9d783c8dd112ad4b619e74e4bf57d2be0b400693
Author:        Vincent Guittot <vincent.guittot@xxxxxxxxxx>
AuthorDate:    Tue, 19 Oct 2021 14:35:34 +02:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Sun, 31 Oct 2021 11:11:37 +01:00

sched/fair: Skip update_blocked_averages if we are defering load balance

In newidle_balance(), the scheduler skips load balance to the new idle cpu
when the 1st sd of this_rq is:

   this_rq->avg_idle < sd->max_newidle_lb_cost

Doing a costly call to update_blocked_averages() will not be useful and
simply adds overhead when this condition is true.

Check the condition early in newidle_balance() to skip
update_blocked_averages() when possible.

Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
Acked-by: Mel Gorman <mgorman@xxxxxxx>
Link: https://lore.kernel.org/r/20211019123537.17146-3-vincent.guittot@xxxxxxxxxx
---
 kernel/sched/fair.c |  9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index c014567..c4c3686 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -10873,17 +10873,20 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
 	 */
 	rq_unpin_lock(this_rq, rf);
 
+	rcu_read_lock();
+	sd = rcu_dereference_check_sched_domain(this_rq->sd);
+
 	if (this_rq->avg_idle < sysctl_sched_migration_cost ||
-	    !READ_ONCE(this_rq->rd->overload)) {
+	    !READ_ONCE(this_rq->rd->overload) ||
+	    (sd && this_rq->avg_idle < sd->max_newidle_lb_cost)) {
 
-		rcu_read_lock();
-		sd = rcu_dereference_check_sched_domain(this_rq->sd);
 		if (sd)
 			update_next_balance(sd, &next_balance);
 		rcu_read_unlock();
 
 		goto out;
 	}
+	rcu_read_unlock();
 
 	raw_spin_rq_unlock(this_rq);