Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
MIME-Version: 1.0
References: <1582183784-13502-1-git-send-email-qiwuchen55@gmail.com>
 <CAKfTPtDzb9XD5wrMhcvGn+dz27nh58taDrdp36YHKNusp739Og@mail.gmail.com>
 <20200220100915.GA14721@cqw-OptiPlex-7050> <CAKfTPtCm-Vn1YAC8j-XFLritQxQ-B7d=pqO9U6=c2vCuTNUpsg@mail.gmail.com>
In-Reply-To: <CAKfTPtCm-Vn1YAC8j-XFLritQxQ-B7d=pqO9U6=c2vCuTNUpsg@mail.gmail.com>
From:   Vincent Guittot <vincent.guittot@linaro.org>
Date:   Thu, 20 Feb 2020 13:15:20 +0100
Message-ID: <CAKfTPtBS=HhBvp2ps1SZXc--WoXO_ZOY=+5o7RJ9vDgi9eLAqA@mail.gmail.com>
Subject: Re: [PATCH] sched/fair: add !se->on_rq check before dequeue entity
To:     chenqiwu <qiwuchen55@gmail.com>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        chenqiwu <chenqiwu@xiaomi.com>
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Thu, 20 Feb 2020 at 11:31, Vincent Guittot
<vincent.guittot@linaro.org> wrote:
>
> On Thu, 20 Feb 2020 at 11:09, chenqiwu <qiwuchen55@gmail.com> wrote:
> >
> > On Thu, Feb 20, 2020 at 10:38:02AM +0100, Vincent Guittot wrote:
> > > On Thu, 20 Feb 2020 at 08:29, <qiwuchen55@gmail.com> wrote:
> > > >
> > > > From: chenqiwu <chenqiwu@xiaomi.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