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

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Message-ID: <5250a0fc-3470-b313-0810-5d7a68c7cf50@arm.com>
Date:   Thu, 9 Mar 2023 10:09:39 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Zhang Qiao <zhangqiao22@...wei.com>,
        Vincent Guittot <vincent.guittot@...aro.org>
Cc:     linux-kernel@...r.kernel.org, mingo@...hat.com,
        peterz@...radead.org, 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 09/03/2023 09:37, Zhang Qiao wrote:
> 
> 在 2023/3/8 20:55, Vincent Guittot 写道:
>> Le mercredi 08 mars 2023 à 09:01:05 (+0100), Vincent Guittot a écrit :
>>> On Tue, 7 Mar 2023 at 14:41, Zhang Qiao <zhangqiao22@...wei.com> wrote:

[...]

>>>> 在 2023/3/7 18:26, Vincent Guittot 写道:
>>>>> On Mon, 6 Mar 2023 at 14:53, Vincent Guittot <vincent.guittot@...aro.org> wrote:
>>>>>>
>>>>>> On Mon, 6 Mar 2023 at 13:57, Zhang Qiao <zhangqiao22@...wei.com> wrote:

[...]

>> +static inline bool migrate_long_sleeper(struct sched_entity *se)
>> +{
>> +       struct cfs_rq *cfs_rq;
>> +       u64 sleep_time;
>> +
>> +       if (se->exec_start == 0)
> 
> How about use `se->avg.last_update_time == 0` here?

IMHO, both checks are not needed here since we're still dealing with the
originating CPU of the migration. Both of them are set to 0 only at the
end of migrate_task_rq_fair().


>> +               return false;
>> +
>> +       cfs_rq = cfs_rq_of(se);
>> +       /*
>> +        * If the entity slept for a long time, don't even try to normalize its
>> +        * vruntime with the base as it may be too far off and might generate
>> +        * wrong decision because of s64 overflow.
>> +        * We estimate its sleep duration with the last update of se's pelt.
>> +        * The last update happened before sleeping. The cfs' pelt is not
>> +        * always updated when cfs is idle but this is not a problem because
>> +        * its min_vruntime is not updated too, so the situation can't get
>> +        * worse.
>> +        */
>> +       sleep_time = cfs_rq_last_update_time(cfs_rq) - se->avg.last_update_time;

Looks like this doesn't work for asymmetric CPU capacity systems since
we specifically 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 (including their last_update_time).

[...]