Message-ID: <6a45bb5c-d851-d48d-44b1-cf533293a6d8@huawei.com>
Date:   Thu, 9 Mar 2023 17:30:06 +0800
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Subject: Re: [PATCH v2] sched/fair: sanitize vruntime of entity being migrated
To:     Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Vincent Guittot <vincent.guittot@linaro.org>
CC:     <linux-kernel@vger.kernel.org>, <mingo@redhat.com>,
        <peterz@infradead.org>, <juri.lelli@redhat.com>,
        <rostedt@goodmis.org>, <bsegall@google.com>, <mgorman@suse.de>,
        <bristot@redhat.com>, <vschneid@redhat.com>, <rkagan@amazon.de>
References: <20230306132418.50389-1-zhangqiao22@huawei.com>
 <CAKfTPtAYpkQVDBR0mcymVgu7aYY5rN1svW713mGJxbewHGJRqQ@mail.gmail.com>
 <CAKfTPtAOFthDtQj=EGbTzwG6ZE7GPpp_3Xg9wVr_8epO+fiFjw@mail.gmail.com>
 <d4d849e3-ea4b-1f84-b287-513fb7bff415@huawei.com>
 <CAKfTPtAVrmPCwuJ55e6TLrVjQnsgDAdg2rGY_0DXcJGzBft15Q@mail.gmail.com>
 <ZAiFxWLSb9HDazSI@vingu-book>
 <07c692fd-fe59-1bd4-a6d0-e84bee6dbb3b@huawei.com>
 <5250a0fc-3470-b313-0810-5d7a68c7cf50@arm.com>
From:   Zhang Qiao <zhangqiao22@huawei.com>
In-Reply-To: <5250a0fc-3470-b313-0810-5d7a68c7cf50@arm.com>
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在 2023/3/9 17:09, Dietmar Eggemann 写道:
> 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@huawei.com> wrote:
> 
> [...]
> 
>>>>> 在 2023/3/7 18:26, Vincent Guittot 写道:
>>>>>> On Mon, 6 Mar 2023 at 14:53, Vincent Guittot <vincent.guittot@linaro.org> wrote:
>>>>>>>
>>>>>>> On Mon, 6 Mar 2023 at 13:57, Zhang Qiao <zhangqiao22@huawei.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().

Yes, if place_entity() don't call migrate_long_sleeper(), the check can remove.

> 
> 
>>> +               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).
> 
> [...]
> 
> .
>