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From:   Vincent Guittot <vincent.guittot@linaro.org>
Date:   Wed, 13 May 2020 15:10:28 +0200
Message-ID: <CAKfTPtBFP5eAV-u02x42U2cQnWA56RP+wbj78rWpzj560OS+-g@mail.gmail.com>
Subject: Re: [PATCH v2] sched/fair: enqueue_task_fair optimization
To:     Phil Auld <pauld@redhat.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>,
        Tao Zhou <ouwen210@hotmail.com>,
        Pavan Kondeti <pkondeti@codeaurora.org>
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On Wed, 13 May 2020 at 14:45, Phil Auld <pauld@redhat.com> wrote:
>
> Hi Vincent,
>
> On Wed, May 13, 2020 at 02:33:35PM +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 and we can skip the test.
> >
>
> s/be NULL/be non-NULL/
>
> I think.

This sentence refers to the move of enqueue_throttle and the fact that
se can't be null when goto enqueue_throttle and we can jump directly
after the if statement, which is now removed in v2 because se is
always NULL if we don't use goto enqueue_throttle.

I haven't change the commit message for the remove of if statement

>
> It's more like if it doesn't jump to the label then se must be NULL for
> the loop to terminate.  The final loop is a NOP if se is NULL. The check
> wasn't protecting that.
>
> Otherwise still
>
> > Reviewed-by: Phil Auld <pauld@redhat.com>
>
> Cheers,
> Phil
>
>
> > Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
> > ---
> >
> > v2 changes:
> > - Remove useless if statement
> >
> >  kernel/sched/fair.c | 39 ++++++++++++++++++++-------------------
> >  1 file changed, 20 insertions(+), 19 deletions(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index a0c690d57430..b51b12d63c39 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -5513,28 +5513,29 @@ 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*/
> > +     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
> >
>
> --
>