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

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Message-ID: <20201029173409.GT2628@hirez.programming.kicks-ass.net>
Date:   Thu, 29 Oct 2020 18:34:09 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Valentin Schneider <valentin.schneider@....com>
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 Thu, Oct 29, 2020 at 04:27:09PM +0000, Valentin Schneider wrote:
> 
> 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?

In general, no, in this particular case, yes. Specifically you need
migrate_disable() + schedule() in order to get nr_pinned incremented.

We just happen to run at the tail end of schedule(), so yeah, here it
works.

> So having a "simple"
> 
>   if (is_migration_disabled(push_task))
>         return;
> 
> would help simpletons like me trying to read through this.

Can do I suppose, although I'm no sure what, if anything that helps,
because then we needs yet another comment explaining things.

I ended up with the below. Is that an improvement?

---
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 3d7d5b7b9c99..c9c69511ece4 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7226,11 +7226,19 @@ static void balance_push(struct rq *rq)
 	lockdep_assert_held(&rq->lock);
 	SCHED_WARN_ON(rq->cpu != smp_processor_id());
 
+	/*
+	 * When migrate_disable(), we'll also have nr_pinned incremented due to
+	 * this being the tail end of schedule(). Therefore we do not need to wake
+	 * the hotplug_wait and go straight to jail^Wexit.
+	 */
+	if (is_migration_disabled(push_task))
+		return;
+
 	/*
 	 * 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) || is_migration_disabled(push_task)) {
+	if (is_per_cpu_kthread(push_task)) {
 		/*
 		 * If this is the idle task on the outgoing CPU try to wake
 		 * up the hotplug control thread which might wait for the