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From:   Vincent Guittot <vincent.guittot@linaro.org>
Date:   Mon, 4 Nov 2019 15:54:50 +0100
Message-ID: <CAKfTPtDeMevgn0JFxjzUWhp4dS10T2MQSA9P82nOGX73xUjXoA@mail.gmail.com>
Subject: Re: [PATCH] sched/fair: Do not set skip buddy up the sched hierarchy
To:     Josh Don <joshdon@google.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>, Paul Turner <pjt@google.com>,
        Mel Gorman <mgorman@suse.de>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Venkatesh Pallipadi <venki@google.com>
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On Thu, 31 Oct 2019 at 19:45, Josh Don <joshdon@google.com> wrote:
>
> From: Venkatesh Pallipadi <venki@google.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@google.com>
> ---
>  kernel/sched/fair.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 682a754ea3e1..52ab06585d7f 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6647,8 +6647,15 @@ 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;
>  }

You should also update __clear_buddies_skip  to only clear this skip

>
>  /*
> --
> 2.23.0.700.g56cf767bdb-goog
>