linux-kernel - Re: [PATCH v2] sched/fair: fix unthrottle_cfs_rq for leaf_cfs

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Message-ID: <CAKfTPtB5C=fZtk1hKYk1JO05Wm93jBiE9zF3V4w6VUBU-QyP_w@mail.gmail.com>
Date:   Wed, 13 May 2020 15:06:33 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Phil Auld <pauld@...hat.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>,
        Tao Zhou <ouwen210@...mail.com>,
        Pavan Kondeti <pkondeti@...eaurora.org>
Subject: Re: [PATCH v2] sched/fair: fix unthrottle_cfs_rq for leaf_cfs_rq list

On Wed, 13 May 2020 at 14:56, Phil Auld <pauld@...hat.com> wrote:
>
> Hi Vincent,
>
> On Wed, May 13, 2020 at 02:34:22PM +0200 Vincent Guittot wrote:
> > Although not exactly identical, unthrottle_cfs_rq() and enqueue_task_fair()
> > are quite close and follow the same sequence for enqueuing an entity in the
> > cfs hierarchy. Modify unthrottle_cfs_rq() to use the same pattern as
> > enqueue_task_fair(). This fixes a problem already faced with the latter and
> > add an optimization in the last for_each_sched_entity loop.
> >
> > Reported-by Tao Zhou <zohooouoto@...o.com.cn>
> > Reviewed-by: Phil Auld <pauld@...hat.com>
> > Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> > ---
> >
> > v2 changes:
> > - Remove useless if statement
> >
> >  kernel/sched/fair.c | 41 ++++++++++++++++++++++++++++++-----------
> >  1 file changed, 30 insertions(+), 11 deletions(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 4e12ba882663..a0c690d57430 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -4816,26 +4816,44 @@ void unthrottle_cfs_rq(struct cfs_rq *cfs_rq)
> >       idle_task_delta = cfs_rq->idle_h_nr_running;
> >       for_each_sched_entity(se) {
> >               if (se->on_rq)
> > -                     enqueue = 0;
>
> Can probably drop the now-unused enqueue variable too.

argh, yes, it dropped during rebase.

preparing a v3

>
>
> Cheers,
> Phil
>
>
>
> > +                     break;
> > +             cfs_rq = cfs_rq_of(se);
> > +             enqueue_entity(cfs_rq, se, ENQUEUE_WAKEUP);
> >
> > +             cfs_rq->h_nr_running += task_delta;
> > +             cfs_rq->idle_h_nr_running += idle_task_delta;
> > +
> > +             /* end evaluation on encountering a throttled cfs_rq */
> > +             if (cfs_rq_throttled(cfs_rq))
> > +                     goto unthrottle_throttle;
> > +     }
> > +
> > +     for_each_sched_entity(se) {
> >               cfs_rq = cfs_rq_of(se);
> > -             if (enqueue) {
> > -                     enqueue_entity(cfs_rq, se, ENQUEUE_WAKEUP);
> > -             } else {
> > -                     update_load_avg(cfs_rq, se, 0);
> > -                     se_update_runnable(se);
> > -             }
> > +
> > +             update_load_avg(cfs_rq, se, UPDATE_TG);
> > +             se_update_runnable(se);
> >
> >               cfs_rq->h_nr_running += task_delta;
> >               cfs_rq->idle_h_nr_running += idle_task_delta;
> >
> > +
> > +             /* end evaluation on encountering a throttled cfs_rq */
> >               if (cfs_rq_throttled(cfs_rq))
> > -                     break;
> > +                     goto unthrottle_throttle;
> > +
> > +             /*
> > +              * One parent has been throttled and cfs_rq removed from the
> > +              * list. Add it back to not break the leaf list.
> > +              */
> > +             if (throttled_hierarchy(cfs_rq))
> > +                     list_add_leaf_cfs_rq(cfs_rq);
> >       }
> >
> > -     if (!se)
> > -             add_nr_running(rq, task_delta);
> > +     /* At this point se is NULL and we are at root level*/
> > +     add_nr_running(rq, task_delta);
> >
> > +unthrottle_throttle:
> >       /*
> >        * The cfs_rq_throttled() breaks in the above iteration can result in
> >        * incomplete leaf list maintenance, resulting in triggering the
> > @@ -4844,7 +4862,8 @@ void unthrottle_cfs_rq(struct cfs_rq *cfs_rq)
> >       for_each_sched_entity(se) {
> >               cfs_rq = cfs_rq_of(se);
> >
> > -             list_add_leaf_cfs_rq(cfs_rq);
> > +             if (list_add_leaf_cfs_rq(cfs_rq))
> > +                     break;
> >       }
> >
> >       assert_list_leaf_cfs_rq(rq);
> > --
> > 2.17.1
> >
>
> --
>