linux-kernel - [PATCH 2/2] sched/fair: Skip update_blocked_averages if we are defering load balance

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Date:   Mon,  4 Oct 2021 19:14:51 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, bristot@...hat.com, linux-kernel@...r.kernel.org,
        tim.c.chen@...ux.intel.com
Cc:     Vincent Guittot <vincent.guittot@...aro.org>
Subject: [PATCH 2/2] 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@...aro.org>
Signed-off-by: Tim Chen <tim.c.chen@...ux.intel.com>
---
 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 1f78b2e3b71c..1294b78503d9 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -10841,17 +10841,20 @@ 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);
 
-- 
2.17.1