linux-kernel - Re: [PATCH v2] sched/fair: Do not set skip buddy up the sched hierarchy

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Date:   Fri, 6 Dec 2019 08:57:11 +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 v2] sched/fair: Do not set skip buddy up the sched hierarchy

Hi Josh,

On Wed, 4 Dec 2019 at 21:06, Josh Don <joshdon@...gle.com> wrote:
>
> From: Venkatesh Pallipadi <venki@...gle.com>
>
> Setting skip buddy all the way up the hierarchy does not play well
> with intra-cgroup yield. One typical usecase of yield is when a
> thread in a cgroup wants to yield CPU to another thread within the
> same cgroup. For such a case, setting the skip buddy all the way 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.
>
> Signed-off-by: Josh Don <joshdon@...gle.com>

There is a mismatch between the author Venkatesh Pallipadi and the
signoff Josh Don
If Venkatesh is the original author and you have then done some
modifications, your both signed-off should be there

Apart from that, the change makes sense to me

> ---
> v2: Only clear skip buddy on the current cfs_rq
>
>  kernel/sched/fair.c | 18 +++++++++++-------
>  1 file changed, 11 insertions(+), 7 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 08a233e97a01..0b7a1958ad52 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 +6549,15 @@ 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;
> +       /*
> +        * One typical usecase of yield is when a thread in a cgroup
> +        * wants to yield CPU to another thread within the same cgroup.
> +        * For such a case, setting the skip buddy all the way up the
> +        * hierarchy is counter-productive, as that results in CPU being
> +        * yielded to a task in some other cgroup. So, only set skip
> +        * for the task requesting it.
> +        */
> +       cfs_rq_of(se)->skip = se;
>  }
>
>  /*
> --
> 2.24.0.393.g34dc348eaf-goog
>