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From:   Wanpeng Li <kernellwp@gmail.com>
Date:   Thu, 27 Sep 2018 09:19:00 +0800
Message-ID: <CANRm+CzasXnjUtA_Xkm27=YDArc-gcpLmpSHv1cAqxihfocxRw@mail.gmail.com>
Subject: Re: [PATCH] sched/fair: vruntime should normalize when switching from fair
To:     Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Steve Muckle <smuckle@google.com>,
        Miguel de Dios <migueldedios@google.com>,
        Ingo Molnar <mingo@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>, kernel-team@android.com,
        Todd Kjos <tkjos@google.com>, Paul Turner <pjt@google.com>,
        quentin.perret@arm.com, Patrick Bellasi <Patrick.Bellasi@arm.com>,
        Chris.Redpath@arm.com, Morten Rasmussen <Morten.Rasmussen@arm.com>,
        John Dias <joaodias@google.com>,
        Wanpeng Li <wanpengli@tencent.com>
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On Thu, 27 Sep 2018 at 06:38, Dietmar Eggemann <dietmar.eggemann@arm.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@arm.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,

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()?

Regards,
Wanpeng Li