linux-kernel - [PATCH 4/6] sched: Optimize migration_cpu

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Date:   Wed, 24 Feb 2021 13:24:43 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     Valentin Schneider <valentin.schneider@....com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Mel Gorman <mgorman@...e.de>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        linux-kernel@...r.kernel.org, peterz@...radead.org,
        Andi Kleen <andi@...stfloor.org>
Subject: [PATCH 4/6] sched: Optimize migration_cpu_stop()

When the purpose of migration_cpu_stop() is to migrate the task to
'any' valid CPU, don't migrate the task when it's already running on a
valid CPU.

Fixes: 6d337eab041d ("sched: Fix migrate_disable() vs set_cpus_allowed_ptr()")
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 kernel/sched/core.c |   13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1936,14 +1936,25 @@ static int migration_cpu_stop(void *data
 			complete = true;
 		}
 
-		if (dest_cpu < 0)
+		if (dest_cpu < 0) {
+			if (cpumask_test_cpu(task_cpu(p), &p->cpus_mask))
+				goto out;
+
 			dest_cpu = cpumask_any_distribute(&p->cpus_mask);
+		}
 
 		if (task_on_rq_queued(p))
 			rq = __migrate_task(rq, &rf, p, dest_cpu);
 		else
 			p->wake_cpu = dest_cpu;
 
+		/*
+		 * XXX __migrate_task() can fail, at which point we might end
+		 * up running on a dodgy CPU, AFAICT this can only happen
+		 * during CPU hotplug, at which point we'll get pushed out
+		 * anyway, so it's probably not a big deal.
+		 */
+
 	} else if (pending) {
 		/*
 		 * This happens when we get migrated between migrate_enable()'s