linux-kernel - Re: [PATCH v3] sched/fair: Do not set skip buddy up the sched hierarchy

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Date:   Thu, 26 Dec 2019 16:05:46 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Josh Don <joshdon@...gle.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>,
        Paul Turner <pjt@...gle.com>
Subject: Re: [PATCH v3] sched/fair: Do not set skip buddy up the sched hierarchy

On Thu, 19 Dec 2019 at 01:14, Josh Don <joshdon@...gle.com> wrote:
>
> From: Venkatesh Pallipadi <venki@...gle.com>
>
> Setting the skip buddy up the hierarchy effectively means that a thread
> calling yield will also end up skipping the other tasks in its hierarchy
> as well. However, a yielding thread shouldn't end up causing this
> behavior on behalf of its entire hierarchy.
>
> For typical uses of yield, setting the skip buddy up the hierarchy is
> counter-productive, as that results in CPU being yielded to a task in
> some other cgroup.
>
> So, limit the skip effect only to the task requesting it.
>
> Co-developed-by: Josh Don <joshdon@...gle.com>
> Signed-off-by: Josh Don <joshdon@...gle.com>

Reviewed-by: Vincent Guittot <vincent.guittot@...aro.org>

> ---
> v2: Only clear skip buddy on the current cfs_rq
>
> v3: Modify comment describing the justification for this change.
>
>  kernel/sched/fair.c | 15 ++++++++-------
>  1 file changed, 8 insertions(+), 7 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 08a233e97a01..0056b57d52cb 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4051,13 +4051,10 @@ static void __clear_buddies_next(struct sched_entity *se)
>
>  static void __clear_buddies_skip(struct sched_entity *se)
>  {
> -       for_each_sched_entity(se) {
> -               struct cfs_rq *cfs_rq = cfs_rq_of(se);
> -               if (cfs_rq->skip != se)
> -                       break;
> +       struct cfs_rq *cfs_rq = cfs_rq_of(se);
>
> +       if (cfs_rq->skip == se)
>                 cfs_rq->skip = NULL;
> -       }
>  }
>
>  static void clear_buddies(struct cfs_rq *cfs_rq, struct sched_entity *se)
> @@ -6552,8 +6549,12 @@ static void set_next_buddy(struct sched_entity *se)
>
>  static void set_skip_buddy(struct sched_entity *se)
>  {
> -       for_each_sched_entity(se)
> -               cfs_rq_of(se)->skip = se;
> +       /*
> +        * Only set the skip buddy for the task requesting it. Setting the skip
> +        * buddy up the hierarchy would result in skipping all other tasks in
> +        * the hierarchy as well.
> +        */
> +       cfs_rq_of(se)->skip = se;
>  }
>
>  /*
> --
> 2.24.1.735.g03f4e72817-goog
>