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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
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