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

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Date:   Thu, 12 Dec 2019 09:05:21 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Josh Don <joshdon@...gle.com>, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     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 Fri, 6 Dec 2019 at 23:13, Josh Don <joshdon@...gle.com> wrote:
>
> Hi Vincent,
>
> Thanks for taking a look.
>
> > 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
>
> Venkatesh no longer works at Google, so I don't have a way to get in
> touch with him.  Is my signed-off insufficient for this case?

Maybe you can add a Co-developed-by tag to reflect your additional changes
I guess that as long as you agree with the DCO, it's ok :
https://www.kernel.org/doc/html/v5.4/process/submitting-patches.html#sign-your-work-the-developer-s-certificate-of-origin

Ingo, Peter, what do you think ?


>
>
> On Thu, Dec 5, 2019 at 11:57 PM Vincent Guittot
> <vincent.guittot@...aro.org> wrote:
> >
> > 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
> > >