linux-kernel - Re: [PATCH] sched/fair: vruntime should normalize when switching from fair

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Message-ID: <e055dace-be32-e074-ab35-dfa84534d466@arm.com>
Date:   Fri, 28 Sep 2018 19:11:50 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Steve Muckle <smuckle@...gle.com>,
        Miguel de Dios <migueldedios@...gle.com>,
        Ingo Molnar <mingo@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, kernel-team@...roid.com,
        Todd Kjos <tkjos@...gle.com>, Paul Turner <pjt@...gle.com>,
        quentin.perret@....com, Patrick Bellasi <Patrick.Bellasi@....com>,
        Chris.Redpath@....com, Morten Rasmussen <Morten.Rasmussen@....com>,
        John Dias <joaodias@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>
Subject: Re: [PATCH] sched/fair: vruntime should normalize when switching from
 fair

On 09/28/2018 02:43 AM, Wanpeng Li wrote:
> On Thu, 27 Sep 2018 at 21:23, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
>>
>> On 09/27/2018 03:19 AM, Wanpeng Li wrote:
>>> On Thu, 27 Sep 2018 at 06:38, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
>>>>
>>>> Hi,
>>>>
>>>> On 09/26/2018 11:50 AM, Wanpeng Li wrote:
>>>>> Hi Dietmar,
>>>>> On Tue, 28 Aug 2018 at 22:55, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
>>>>>>
>>>>>> On 08/27/2018 12:14 PM, Peter Zijlstra wrote:
>>>>>>> On Fri, Aug 24, 2018 at 02:24:48PM -0700, Steve Muckle wrote:
>>>>>>>> On 08/24/2018 02:47 AM, Peter Zijlstra wrote:
>>>>>>>>>>> On 08/17/2018 11:27 AM, Steve Muckle wrote:
>>>>
>>>> [...]
>>>>
>>>>>>>>>>>> - later, when the prio is deboosted and the task is moved back
>>>>>>>>>>>>         to the fair class, the fair rq's min_vruntime is added to
>>>>>>>>>>>>         the task's vruntime, even though it wasn't subtracted earlier.
>>>>>
>>>>> Could you point out when the fair rq's min_vruntime is added to the
>>>>> task's vruntime in your *later* scenario? attach_task_cfs_rq will not
>>>>> do that the same reason as detach_task_cfs_rq. fair task's
>>>>> sched_remote_wakeup is false which results in vruntime will not be
>>>>> renormalized in enqueue_entity.
>>>>
>>>> The cfs_rq->min_vruntime is still added to the se->vruntime in
>>>> enqueue_task_fair().
>>>
>>> I understand what your patch done,
>>
>> It's not my patch ;-) I just helped to find out under which
>> circumstances this issue can happen.
>>
>>> On your CPU4:
>>> scheduler_ipi()
>>>    -> sched_ttwu_pending()
>>>         -> ttwu_do_activate()    => p->sched_remote_wakeup should be
>>> false, so ENQUEUE_WAKEUP is set, ENQUEUE_MIGRATED is not
>>>              -> ttwu_activate()
>>>                   -> activate_task()
>>>                        -> enqueue_task()
>>>                             -> enqueue_task_fair()
>>>                                  -> enqueue_entity()
>>>                                       bool renorm = !(flags &
>>> ENQUEUE_WAKEUP) || (flags & ENQUEUE_MIGRATE)
>>> so renorm is false in enqueue_entity(), why you mentioned that the
>>> cfs_rq->min_vruntime is still added to the se->vruntime in
>>> enqueue_task_fair()?
>>
>> Maybe this is a misunderstanding on my side but didn't you asked me to
>> '... Could you point out when the fair rq's min_vruntime is added to the
>> task's vruntime in your *later* scenario? ...'
> 
> Yeah, if the calltrace above and my analysis is correct, then the fair
> rq's min_vruntime will not be added to the task's vruntime in your
> *later* scenario, which means that your patch is not necessary.

Ah, ok, both, the ENQUEUE_WAKEUP and the ENQUEUE_MIGRATED are not set in 
the enqueue_entity() call so renorm is 1 (flags is 0xe).