linux-kernel - [PATCH] sched/fair: stabilize asym cpu capacity system idle cpu selection

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Message-Id: <20230711081359.868862-1-vincent.guittot@linaro.org>
Date:   Tue, 11 Jul 2023 10:13:59 +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, vschneid@...hat.com,
        linux-kernel@...r.kernel.org, jiahao.os@...edance.com
Cc:     Vincent Guittot <vincent.guittot@...aro.org>
Subject: [PATCH] sched/fair: stabilize asym cpu capacity system idle cpu selection

select_idle_capacity() not only looks for an idle cpu that fits for the
waking task but also for cpu with highest bandwidth when no cpu fits.
Start the loop with target cpu so it will be selected 1st when no cpu fits
but several cpus shared the same bandwidth. Starting with target cpu
prevents the task to migrate between cpus with same bandwidth at every
wakeup when no cpu fits.

Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
---
 kernel/sched/fair.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index a80a73909dc2..0ee116f992f4 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7065,7 +7065,7 @@ select_idle_capacity(struct task_struct *p, struct sched_domain *sd, int target)
 	util_min = uclamp_eff_value(p, UCLAMP_MIN);
 	util_max = uclamp_eff_value(p, UCLAMP_MAX);
 
-	for_each_cpu_wrap(cpu, cpus, target + 1) {
+	for_each_cpu_wrap(cpu, cpus, target) {
 		unsigned long cpu_cap = capacity_of(cpu);
 
 		if (!available_idle_cpu(cpu) && !sched_idle_cpu(cpu))
-- 
2.34.1