linux-kernel - Re: [PATCH 2/2] sched: Plug race between SCA, hotplug and migration_cpu

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Date:   Thu, 27 May 2021 11:12:22 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     linux-kernel@...r.kernel.org, Will Deacon <will@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Qais Yousef <qais.yousef@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Quentin Perret <qperret@...gle.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        kernel-team@...roid.com
Subject: Re: [PATCH 2/2] sched: Plug race between SCA, hotplug and
 migration_cpu_stop()

On Wed, May 26, 2021 at 09:57:51PM +0100, Valentin Schneider wrote:

> -			rq = __migrate_task(rq, &rf, p, arg->dest_cpu);

Suggests we ought to at the very least include something like the below.

/me continues reading...

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2246,28 +2246,6 @@ struct set_affinity_pending {
 	struct migration_arg	arg;
 };
 
-/*
- * Move (not current) task off this CPU, onto the destination CPU. We're doing
- * this because either it can't run here any more (set_cpus_allowed()
- * away from this CPU, or CPU going down), or because we're
- * attempting to rebalance this task on exec (sched_exec).
- *
- * So we race with normal scheduler movements, but that's OK, as long
- * as the task is no longer on this CPU.
- */
-static struct rq *__migrate_task(struct rq *rq, struct rq_flags *rf,
-				 struct task_struct *p, int dest_cpu)
-{
-	/* Affinity changed (again). */
-	if (!is_cpu_allowed(p, dest_cpu))
-		return rq;
-
-	update_rq_clock(rq);
-	rq = move_queued_task(rq, rf, p, dest_cpu);
-
-	return rq;
-}
-
 static int select_fallback_rq(int cpu, struct task_struct *p);
 
 /*
@@ -2292,7 +2270,7 @@ static int migration_cpu_stop(void *data
 	local_irq_save(rf.flags);
 	/*
 	 * We need to explicitly wake pending tasks before running
-	 * __migrate_task() such that we will not miss enforcing cpus_ptr
+	 * move_queued_task() such that we will not miss enforcing cpus_ptr
 	 * during wakeups, see set_cpus_allowed_ptr()'s TASK_WAKING test.
 	 */
 	flush_smp_call_function_from_idle();
@@ -8463,7 +8441,7 @@ static int __balance_push_cpu_stop(void
 
 	if (task_rq(p) == rq && task_on_rq_queued(p)) {
 		cpu = select_fallback_rq(rq->cpu, p);
-		rq = __migrate_task(rq, &rf, p, cpu);
+		rq = move_queued_task(rq, &rf, p, cpu);
 	}
 
 	rq_unlock(rq, &rf);