linux-kernel - [tip: sched/core] sched/fair: Stabilize asym cpu capacity system idle cpu selection

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Date:   Mon, 17 Jul 2023 12:56:10 -0000
From:   "tip-bot2 for Vincent Guittot" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Vincent Guittot <vincent.guittot@...aro.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: sched/core] sched/fair: Stabilize asym cpu capacity system idle
 cpu selection

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

Commit-ID:     7ee7642c91410fb90cc45e799a3a46e1607ecd79
Gitweb:        https://git.kernel.org/tip/7ee7642c91410fb90cc45e799a3a46e1607ecd79
Author:        Vincent Guittot <vincent.guittot@...aro.org>
AuthorDate:    Tue, 11 Jul 2023 10:13:59 +02:00
Committer:     Peter Zijlstra <peterz@...radead.org>
CommitterDate: Thu, 13 Jul 2023 15:21:53 +02:00

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>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Link: https://lkml.kernel.org/r/20230711081359.868862-1-vincent.guittot@linaro.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 a879883..0cd1cdb 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7096,7 +7096,7 @@ 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))