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

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Date:   Mon, 24 Feb 2020 17:50:08 -0800
From:   Josh Don <joshdon@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>,
        Ingo Molnar <mingo@...hat.com>,
        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

Bumping in case this fell through the cracks of the patch queue

On Thu, Dec 26, 2019 at 7:05 AM Vincent Guittot
<vincent.guittot@...aro.org> wrote:
>
> 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
> >