linux-kernel - Re: [PATCH v4 11/19] sched/core: Make migrate disable and CPU hotplug cooperative

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Date:   Thu, 29 Oct 2020 16:27:09 +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 11/19] sched/core: Make migrate disable and CPU hotplug cooperative


On 23/10/20 11:12, Peter Zijlstra wrote:
> @@ -7006,15 +7024,20 @@ static bool balance_push(struct rq *rq)
>        * Both the cpu-hotplug and stop task are in this case and are
>        * required to complete the hotplug process.
>        */
> -	if (is_per_cpu_kthread(push_task)) {
> +	if (is_per_cpu_kthread(push_task) || is_migration_disabled(push_task)) {

is_migration_disabled(p) implies rq_has_pinned_tasks(task_rq(p)), right?

So having a "simple"

  if (is_migration_disabled(push_task))
        return;

would help simpletons like me trying to read through this.

>               /*
>                * If this is the idle task on the outgoing CPU try to wake
>                * up the hotplug control thread which might wait for the
>                * last task to vanish. The rcuwait_active() check is
>                * accurate here because the waiter is pinned on this CPU
>                * and can't obviously be running in parallel.
> +		 *
> +		 * On RT kernels this also has to check whether there are
> +		 * pinned and scheduled out tasks on the runqueue. They
> +		 * need to leave the migrate disabled section first.
>                */
> -		if (!rq->nr_running && rcuwait_active(&rq->hotplug_wait)) {
> +		if (!rq->nr_running && !rq_has_pinned_tasks(rq) &&
> +		    rcuwait_active(&rq->hotplug_wait)) {
>                       raw_spin_unlock(&rq->lock);
>                       rcuwait_wake_up(&rq->hotplug_wait);
>                       raw_spin_lock(&rq->lock);