linux-kernel - Re: [PATCH] sched/fair: add !se->on

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Date:   Thu, 20 Feb 2020 13:15:20 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     chenqiwu <qiwuchen55@...il.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        chenqiwu <chenqiwu@...omi.com>
Subject: Re: [PATCH] sched/fair: add !se->on_rq check before dequeue entity

On Thu, 20 Feb 2020 at 11:31, Vincent Guittot
<vincent.guittot@...aro.org> wrote:
>
> On Thu, 20 Feb 2020 at 11:09, chenqiwu <qiwuchen55@...il.com> wrote:
> >
> > On Thu, Feb 20, 2020 at 10:38:02AM +0100, Vincent Guittot wrote:
> > > On Thu, 20 Feb 2020 at 08:29, <qiwuchen55@...il.com> wrote:
> > > >
> > > > From: chenqiwu <chenqiwu@...omi.com>
> > > >
> > > > We igonre checking for !se->on_rq condition before dequeue one
> > > > entity from cfs rq. It must be required in case the entity has
> > > > been dequeued.
> > >
> > > Do you have a use case that triggers this situation ?
> > >
> > > This is the only way to reach this situation seems to be dequeuing a
> > > task on a throttled cfs_rq
> > >
> > Sorry, I have no use case triggers this situation. It's just found by
> > reading code.
> > I agree the situation you mentioned above may have a racy with
> > dequeue_task_fair() in the following code path:
> > __schedule
> >         pick_next_task_fair
> >                 put_prev_entity
> >                         check_cfs_rq_runtime
> >                                 throttle_cfs_rq
> >                                         dequeue_entity
> >
> > So this check is worth to be added for dequeue_task_fair().
>
> In fact the check is already done thanks to the: if (cfs_rq_throttled(cfs_rq))
> AFAICT, there is no other way to enqueue a task on a cfs_rq for which
> the group entity is not enqueued

Hmm i have been too quick in my reply. I wanted to say:
AFAICT, there is no other way to dequeue a task from a cfs_rq for
which the group entity is not enqueued