From: Valentin Schneider <valentin.schneider@arm.com>
To: Qais Yousef <qais.yousef@arm.com>
Cc: linux-kernel@vger.kernel.org, tglx@linutronix.de,
mingo@kernel.org, bigeasy@linutronix.de, swood@redhat.com,
peterz@infradead.org, juri.lelli@redhat.com,
vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de,
bristot@redhat.com, vincent.donnefort@arm.com, tj@kernel.org
Subject: Re: [RFC PATCH] sched/core: Fix premature p->migration_pending completion
Date: Wed, 03 Feb 2021 18:59:08 +0000 [thread overview]
Message-ID: <jhjft2d2d1f.mognet@arm.com> (raw)
In-Reply-To: <20210203172344.uzq2iod4g46ffame@e107158-lin>
On 03/02/21 17:23, Qais Yousef wrote:
> On 01/27/21 19:30, Valentin Schneider wrote:
>> Fiddling some more with a TLA+ model of set_cpus_allowed_ptr() & friends
>> unearthed one more outstanding issue. This doesn't even involve
>> migrate_disable(), but rather affinity changes and execution of the stopper
>> racing with each other.
>>
>> My own interpretation of the (lengthy) TLA+ splat (note the potential for
>> errors at each level) is:
>>
>> Initial conditions:
>> victim.cpus_mask = {CPU0, CPU1}
>>
>> CPU0 CPU1 CPU<don't care>
>>
>> switch_to(victim)
>> set_cpus_allowed(victim, {CPU1})
>> kick CPU0 migration_cpu_stop({.dest_cpu = CPU1})
>> switch_to(stopper/0)
>> // e.g. CFS load balance
>> move_queued_task(CPU0, victim, CPU1);
>> switch_to(victim)
>> set_cpus_allowed(victim, {CPU0});
>> task_rq_unlock();
>> migration_cpu_stop(dest_cpu=CPU1)
>
> This migration stop is due to set_cpus_allowed(victim, {CPU1}), right?
>
Right
>> task_rq(p) != rq && pending
>> kick CPU1 migration_cpu_stop({.dest_cpu = CPU1})
>>
>> switch_to(stopper/1)
>> migration_cpu_stop(dest_cpu=CPU1)
>
> And this migration stop is due to set_cpus_allowed(victim, {CPU0}), right?
>
Nein! This is a retriggering of the "current" stopper (triggered by
set_cpus_allowed(victim, {CPU1})), see the tail of that
else if (dest_cpu < 0 || pending)
branch in migration_cpu_stop(), is what I'm trying to hint at with that
task_rq(p) != rq && pending
> If I didn't miss something, then dest_cpu should be CPU0 too, not CPU1 and the
> task should be moved back to CPU0 as expected?
>
> Thanks
>
> --
> Qais Yousef
next prev parent reply other threads:[~2021-02-03 19:00 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-01-27 19:30 [RFC PATCH] sched/core: Fix premature p->migration_pending completion Valentin Schneider
[not found] ` <BN8PR12MB2978A9A4435A01EDC97D27E89ABA9@BN8PR12MB2978.namprd12.prod.outlook.com>
2021-01-28 18:56 ` Valentin Schneider
2021-02-03 17:23 ` Qais Yousef
2021-02-03 18:59 ` Valentin Schneider [this message]
2021-02-04 15:30 ` Qais Yousef
2021-02-05 11:02 ` Valentin Schneider
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