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

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Date:   Fri, 28 Sep 2018 09:10:04 -0700
From:   Steve Muckle <smuckle@...gle.com>
To:     Wanpeng Li <kernellwp@...il.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        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 05:43 PM, Wanpeng Li wrote:
>>> 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.

In the scenario I observed, the task is not waking - it is running and 
being deboosted from priority inheritance, transitioning from RT to CFS.

Dietmar and I both were able to reproduce the issue with the testcase I 
posted earlier in this thread.

thanks,
Steve