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

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Date:   Thu, 27 Sep 2018 15:22:47 +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 09/27/2018 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? ...'