linux-kernel - Re: [PATCH v4 19/19] sched: Comment affine_move

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Message-ID: <jhj8sbpov3y.mognet@arm.com>
Date:   Thu, 29 Oct 2020 16:27:45 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     tglx@...utronix.de, mingo@...nel.org, linux-kernel@...r.kernel.org,
        bigeasy@...utronix.de, qais.yousef@....com, swood@...hat.com,
        juri.lelli@...hat.com, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, bristot@...hat.com, vincent.donnefort@....com,
        tj@...nel.org, ouwen210@...mail.com
Subject: Re: [PATCH v4 19/19] sched: Comment affine_move_task()


On 23/10/20 11:12, Peter Zijlstra wrote:
> + * (1) In the cases covered above. There is one more where the completion is
> + * signaled within affine_move_task() itself: when a subsequent affinity request
> + * cancels the need for an active migration. Consider:
> + *
> + *     Initial conditions: P0->cpus_mask = [0, 1]
> + *
> + *     P0@...0            P1                             P2
> + *
> + *     migrate_disable();
> + *     <preempted>
> + *                        set_cpus_allowed_ptr(P0, [1]);
> + *                          <blocks>
> + *                                                       set_cpus_allowed_ptr(P0, [0, 1]);
> + *                                                         <signal completion>
> + *                          <awakes>
> + *
> + * Note that the above is safe vs a concurrent migrate_enable(), as any
> + * pending affinity completion is preceded an uninstallion of
> + * p->migration_pending done with p->pi_lock held.

I too must have been thinking too much about ponies lately.

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 0a3f9fd3b061..d8c85f180b09 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2147,7 +2147,7 @@ void do_set_cpus_allowed(struct task_struct *p, const struct cpumask *new_mask)
  *                          <awakes>
  *
  * Note that the above is safe vs a concurrent migrate_enable(), as any
- * pending affinity completion is preceded an uninstallion of
+ * pending affinity completion is preceded by an uninstallation of
  * p->migration_pending done with p->pi_lock held.
  */
 static int affine_move_task(struct rq *rq, struct task_struct *p, struct rq_flags *rf,