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

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Date:   Fri, 28 Sep 2018 08:43:27 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Dietmar Eggemann <dietmar.eggemann@....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 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.

Regards,
Wanpeng Li