linux-kernel - Re: [PATCH v3] sched/fair: enqueue_task

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Date:   Wed, 13 May 2020 18:02:47 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Tao Zhou <ouwen210@...mail.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>,
        Phil Auld <pauld@...hat.com>,
        Pavan Kondeti <pkondeti@...eaurora.org>
Subject: Re: [PATCH v3] sched/fair: enqueue_task_fair optimization

On Wed, 13 May 2020 at 17:51, Tao Zhou <ouwen210@...mail.com> wrote:
>
> Hi Vincent,
>
> Sorry for the duplicate.
>
> On Wed, May 13, 2020 at 03:55:02PM +0200, Vincent Guittot wrote:
> > enqueue_task_fair jumps to enqueue_throttle label when cfs_rq_of(se) is
> > throttled which means that se can't be NULL in such case and we can move
> > the label after the if (!se) statement. Futhermore, the latter can be
> > removed because se is always NULL when reaching this point.
> >
> > Reviewed-by: Phil Auld <pauld@...hat.com>
> > Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> > ---
> >
> > v3 changes:
> >   - updated commit message
> >   - removed an extra }
> >
> >  kernel/sched/fair.c | 39 +++++++++++++++++++--------------------
> >  1 file changed, 19 insertions(+), 20 deletions(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 9a58874ef104..4e586863827b 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -5512,28 +5512,27 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
> >                         list_add_leaf_cfs_rq(cfs_rq);
> >       }
> >
> > -enqueue_throttle:
> > -     if (!se) {
> > -             add_nr_running(rq, 1);
> > -             /*
> > -              * Since new tasks are assigned an initial util_avg equal to
> > -              * half of the spare capacity of their CPU, tiny tasks have the
> > -              * ability to cross the overutilized threshold, which will
> > -              * result in the load balancer ruining all the task placement
> > -              * done by EAS. As a way to mitigate that effect, do not account
> > -              * for the first enqueue operation of new tasks during the
> > -              * overutilized flag detection.
> > -              *
> > -              * A better way of solving this problem would be to wait for
> > -              * the PELT signals of tasks to converge before taking them
> > -              * into account, but that is not straightforward to implement,
> > -              * and the following generally works well enough in practice.
> > -              */
> > -             if (flags & ENQUEUE_WAKEUP)
> > -                     update_overutilized_status(rq);
> > +     /* At this point se is NULL and we are at root level*/
>
> The same as another patch, lack one blank at the end of above comment.
>
> When apply v3 of 'sched/fair: Fix enqueue_task_fair warning some more'
> We need to edit to align to, I don't know why. When I tried to pull some
> thing to a function not done yet.

I forgot to mention that this patch has been done on top of Phil's one
20200512135222.GC2201@...ien.usersys.redhat.com and apply on top of it

>
> Thanks,
> Tao
>
> > +     add_nr_running(rq, 1);
> >
> > -     }
> > +     /*
> > +      * Since new tasks are assigned an initial util_avg equal to
> > +      * half of the spare capacity of their CPU, tiny tasks have the
> > +      * ability to cross the overutilized threshold, which will
> > +      * result in the load balancer ruining all the task placement
> > +      * done by EAS. As a way to mitigate that effect, do not account
> > +      * for the first enqueue operation of new tasks during the
> > +      * overutilized flag detection.
> > +      *
> > +      * A better way of solving this problem would be to wait for
> > +      * the PELT signals of tasks to converge before taking them
> > +      * into account, but that is not straightforward to implement,
> > +      * and the following generally works well enough in practice.
> > +      */
> > +     if (flags & ENQUEUE_WAKEUP)
> > +             update_overutilized_status(rq);
> >
> > +enqueue_throttle:
> >       if (cfs_bandwidth_used()) {
> >               /*
> >                * When bandwidth control is enabled; the cfs_rq_throttled()
> > --
> > 2.17.1