linux-kernel - [PATCH] sched/fair: Don't migrate with src_cpu == dst

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Message-Id: <20201203060449.3352126-1-swood@redhat.com>
Date:   Thu,  3 Dec 2020 00:04:49 -0600
From:   Scott Wood <swood@...hat.com>
To:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Dietmar Eggemann <dietmar.eggemann@....com>,
        Mel Gorman <mgorman@...e.de>, linux-kernel@...r.kernel.org,
        linux-rt-users@...r.kernel.org,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Scott Wood <swood@...hat.com>
Subject: [PATCH] sched/fair: Don't migrate with src_cpu == dst_cpu

Besides being a waste of time to try to move tasks to where they already
are, this avoids triggering the WARN_ON_ONCE(is_migration_disabled(p))
in set_task_cpu().

Signed-off-by: Scott Wood <swood@...hat.com>
---
Patch is against tip/master.  Assertion was seen by running rteval on the
RT tree.

 kernel/sched/fair.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index e7e21ac479a2..f443626164d4 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7574,7 +7574,8 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
 
 		/* Prevent to re-select dst_cpu via env's CPUs: */
 		for_each_cpu_and(cpu, env->dst_grpmask, env->cpus) {
-			if (cpumask_test_cpu(cpu, p->cpus_ptr)) {
+			if (cpu != env->src_cpu &&
+			    cpumask_test_cpu(cpu, p->cpus_ptr)) {
 				env->flags |= LBF_DST_PINNED;
 				env->new_dst_cpu = cpu;
 				break;
-- 
2.27.0