linux-kernel - Re: [PATCH v2] sched/fair: sanitize vruntime of entity being migrated

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Date:   Wed, 15 Mar 2023 11:15:01 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Zhang Qiao <zhangqiao22@...wei.com>, linux-kernel@...r.kernel.org,
        mingo@...hat.com, juri.lelli@...hat.com, rostedt@...dmis.org,
        bsegall@...gle.com, mgorman@...e.de, bristot@...hat.com,
        vschneid@...hat.com, rkagan@...zon.de
Subject: Re: [PATCH v2] sched/fair: sanitize vruntime of entity being migrated

On 15/03/2023 09:42, Vincent Guittot wrote:
> On Wed, 15 Mar 2023 at 08:18, Vincent Guittot
> <vincent.guittot@...aro.org> wrote:
>>
>> On Tue, 14 Mar 2023 at 18:16, Peter Zijlstra <peterz@...radead.org> wrote:
>>>
>>> On Tue, Mar 14, 2023 at 02:24:37PM +0100, Vincent Guittot wrote:
>>>
>>>>> @@ -7632,11 +7646,8 @@ static void migrate_task_rq_fair(struct task_struct *p, int new_cpu)
>>>>>          * min_vruntime -- the latter is done by enqueue_entity() when placing
>>>>>          * the task on the new runqueue.
>>>>>          */
>>>>> -       if (READ_ONCE(p->__state) == TASK_WAKING) {
>>>>> -               struct cfs_rq *cfs_rq = cfs_rq_of(se);
>>>>> -
>>>>> +       if (READ_ONCE(p->__state) == TASK_WAKING || reset_vruntime(cfs_rq, se))
>>>>
>>>> That's somehow what was proposed in one of the previous proposals but
>>>> we can't call rq_clock_task(rq_of(cfs_rq)) because rq lock might not
>>>> be hold and rq task clock has not been updated before being used
>>>
>>> Argh indeed. I spend a lot of time ensuring we didn't take the old rq
>>> lock on wakeup -- and then a lot of time cursing about how we don't :-)
>>>
>>> Now, if we could rely on the rq-clock being no more than 1 tick behind
>>> current, this would still be entirely sufficient to catch the long sleep
>>> case.
>>
>> We should also take care when loading rq_clock_task that we are not
>> racing with an update especially for a 32bits system like pelt
>> last_update_time
> 
> We still have this possibility:
> https://lore.kernel.org/lkml/ZAiFxWLSb9HDazSI@vingu-book/
> 
> which uses pelt last_update_time when migrating and keep using
> rq_clock_task in place_entity

Isn't there an issue with this approach on asymmetric CPU capacity systems?

We do a sync_entity_load_avg() in select_task_rq_fair()
(find_energy_efficient_cpu() for EAS and select_idle_sibling() for CAS)
to sync cfs_rq and se.

[...]