linux-kernel - Re: [PATCH] sched/fair: fix runnable

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Date:   Thu, 27 Feb 2020 14:12:09 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Tao Zhou <zhout@...aldi.net>
Cc:     Dietmar Eggemann <dietmar.eggemann@....com>,
        Ben Segall <bsegall@...gle.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mel Gorman <mgorman@...e.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Phil Auld <pauld@...hat.com>, Parth Shah <parth@...ux.ibm.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Hillf Danton <hdanton@...a.com>
Subject: Re: [PATCH] sched/fair: fix runnable_avg for throttled cfs

On Thu, 27 Feb 2020 at 14:10, Tao Zhou <zhout@...aldi.net> wrote:
>
> Hi Dietmar,
>
> On Thu, Feb 27, 2020 at 11:20:05AM +0000, Dietmar Eggemann wrote:
> > On 26.02.20 21:01, Vincent Guittot wrote:
> > > On Wed, 26 Feb 2020 at 20:04, <bsegall@...gle.com> wrote:
> > >>
> > >> Vincent Guittot <vincent.guittot@...aro.org> writes:
> > >>
> > >>> When a cfs_rq is throttled, its group entity is dequeued and its running
> > >>> tasks are removed. We must update runnable_avg with current h_nr_running
> > >>> and update group_se->runnable_weight with new h_nr_running at each level
> >
> >                                               ^^^
> >
> > Shouldn't his be 'curren' rather 'new' h_nr_running for
> > group_se->runnable_weight? IMHO, you want to cache the current value
> > before you add/subtract task_delta.
>
> /me think Vincent is right. h_nr_running is updated in the previous
> level or out. The next level will use current h_nr_running to update
> runnable_avg and use the new group cfs_rq's h_nr_running which was
> updated in the previous level or out to update se runnable_weight.

Yes that's what I want to say

>
> update_h_nr_running (E.G. in enqueue_task_fair)
>
> throttle_cfs_rq()
>   for() {
>     update_load_avg()
>       se_runnable() --> current h_nr_running
>     update_se_runnable() --> use group cfs_rq h_nr_running
>                              updated in the prev level or out
>     update_h_nr_running
>   }
>
> Thanks,
> Tao
>
> > >>> of the hierarchy.
> > >>
> > >> You'll also need to do this for task enqueue/dequeue inside of a
> > >> throttled hierarchy, I'm pretty sure.
> > >
> > > AFAICT, this is already done with patch "sched/pelt: Add a new
> > > runnable average signal" when task is enqueued/dequeued inside a
> > > throttled hierarchy
> > >
> > >>
> > >>>
> > >>> Fixes: 9f68395333ad ("sched/pelt: Add a new runnable average signal")
> > >>> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> > >>> ---
> > >>> This patch applies on top of tip/sched/core
> > >>>
> > >>>  kernel/sched/fair.c | 10 ++++++++++
> > >>>  1 file changed, 10 insertions(+)
> > >>>
> > >>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > >>> index fcc968669aea..6d46974e9be7 100644
> > >>> --- a/kernel/sched/fair.c
> > >>> +++ b/kernel/sched/fair.c
> > >>> @@ -4703,6 +4703,11 @@ static void throttle_cfs_rq(struct cfs_rq *cfs_rq)
> > >>>
> > >>>               if (dequeue)
> > >>>                       dequeue_entity(qcfs_rq, se, DEQUEUE_SLEEP);
> > >>> +             else {
> > >>> +                     update_load_avg(qcfs_rq, se, 0);
> > >>> +                     se_update_runnable(se);
> > >>> +             }
> > >>> +
> > >>>               qcfs_rq->h_nr_running -= task_delta;
> > >>>               qcfs_rq->idle_h_nr_running -= idle_task_delta;
> > >>>
> > >>> @@ -4772,6 +4777,11 @@ void unthrottle_cfs_rq(struct cfs_rq *cfs_rq)
> > >>>               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);
> > >>> +             }
> > >>> +
> > >>>               cfs_rq->h_nr_running += task_delta;
> > >>>               cfs_rq->idle_h_nr_running += idle_task_delta;