linux-kernel - [PATCH 3/4] sched/fair: minimize concurrent LBs between domain level

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Date:   Mon, 14 Sep 2020 12:03:39 +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, linux-kernel@...r.kernel.org
Cc:     valentin.schneider@....com,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: [PATCH 3/4] sched/fair: minimize concurrent LBs between domain level

sched domains tend to trigger simultaneously the load balance loop but
the larger domains often need more time to collect statistics. This
slowness makes the larger domain trying to detach tasks from a rq whereas
tasks already migrated somewhere else at a sub-domain level. This is not
a real problem for idle LB because the period of smaller domains will
increase with its CPUs being busy and this will let time for higher ones
to pulled tasks. But this becomes a problem when all CPUs are already busy
because all domains stay synced when they trigger their LB.

A simple way to minimize simultaneous LB of all domains is to decrement the
the busy interval by 1 jiffies. Because of the busy_factor, the interval of
larger domain will not be a multiple of smaller ones anymore.

Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
---
 kernel/sched/fair.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 765be8273292..7d7eefd8e2d4 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9780,6 +9780,9 @@ get_sd_balance_interval(struct sched_domain *sd, int cpu_busy)

 	/* scale ms to jiffies */
 	interval = msecs_to_jiffies(interval);
+	if (cpu_busy)
+		interval -= 1;
+
 	interval = clamp(interval, 1UL, max_load_balance_interval);

 	return interval;
-- 
2.17.1