linux-kernel - Re: [PATCH] sched/fair : fix reordering of enqueue_task

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Message-ID: <CAKfTPtBDeTg0MEsazrtNRObL9nC2wjZTwo944kjZ7O8H0-n4nQ@mail.gmail.com>
Date:   Wed, 4 Mar 2020 18:50:14 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     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>
Cc:     Phil Auld <pauld@...hat.com>, Parth Shah <parth@...ux.ibm.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Hillf Danton <hdanton@...a.com>, Tao Zhou <zhout@...aldi.net>
Subject: Re: [PATCH] sched/fair : fix reordering of enqueue_task_fair

On Wed, 4 Mar 2020 at 16:17, Vincent Guittot <vincent.guittot@...aro.org> wrote:
>
> Even when a cgroup is throttled, the group se of a child cgroup can still
> be enqueued and its gse->on_rq stays true. When a task is enqueued on such
> child, we still have to update the load_avg and increase h_nr_running of
> the throttled cfs. Nevertheless, the 1st for_each_sched_entity loop is
> skipped because of gse->on_rq == true and the 2nd loop because the cfs is
> throttled whereas we have to update both load_avg with the old
> h_nr_running and increment h_nr_running in such case.
>
> Note that the update of load_avg will effectively happen only once in order
> to sync with the throttled time. Next call for updating load_avg will stop
> early because the clock stays unchanged.
>
> Fixes: 6d4d22468dae ("sched/fair: Reorder enqueue/dequeue_task_fair path")
> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> ---

Please ignore this patch, it's not enough to fix all cases


>  kernel/sched/fair.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index fcc968669aea..22d827fc46c3 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5432,7 +5432,7 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
>                 cfs_rq = cfs_rq_of(se);
>
>                 /* end evaluation on encountering a throttled cfs_rq */
> -               if (cfs_rq_throttled(cfs_rq))
> +               if (!se->on_rq && cfs_rq_throttled(cfs_rq))
>                         goto enqueue_throttle;
>
>                 update_load_avg(cfs_rq, se, UPDATE_TG);
> --
> 2.17.1
>