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

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Message-ID: <CAKfTPtCm-Vn1YAC8j-XFLritQxQ-B7d=pqO9U6=c2vCuTNUpsg@mail.gmail.com>
Date:   Thu, 20 Feb 2020 11:31:23 +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 11:09, 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