linux-kernel - Re: [PATCH -tip 02/32] sched: Introduce sched_class::pick

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Date:   Thu, 26 Nov 2020 11:17:48 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        Nishanth Aravamudan <naravamudan@...italocean.com>,
        Julien Desfossez <jdesfossez@...italocean.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Vineeth Pillai <viremana@...ux.microsoft.com>,
        Aaron Lu <aaron.lwe@...il.com>,
        Aubrey Li <aubrey.intel@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Kees Cook <keescook@...omium.org>,
        Greg Kerr <kerrnel@...gle.com>, Phil Auld <pauld@...hat.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>, vineeth@...byteword.org,
        Chen Yu <yu.c.chen@...el.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Agata Gruza <agata.gruza@...el.com>,
        Antonio Gomez Iglesias <antonio.gomez.iglesias@...el.com>,
        graf@...zon.com, konrad.wilk@...cle.com, dfaggioli@...e.com,
        Paul Turner <pjt@...gle.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Patrick Bellasi <derkling@...gle.com>,
        Jiang Biao <benbjiang@...cent.com>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        OWeisse@...ch.edu, Dhaval Giani <dhaval.giani@...cle.com>,
        Junaid Shahid <junaids@...gle.com>, jsbarnes@...gle.com,
        "Hyser,Chris" <chris.hyser@...cle.com>,
        Ben Segall <bsegall@...gle.com>, Josh Don <joshdon@...gle.com>,
        Hao Luo <haoluo@...gle.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Aubrey Li <aubrey.li@...ux.intel.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Tim Chen <tim.c.chen@...el.com>
Subject: Re: [PATCH -tip 02/32] sched: Introduce sched_class::pick_task()

On Thu, 26 Nov 2020 at 10:07, Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Wed, Nov 25, 2020 at 05:28:36PM +0100, Vincent Guittot wrote:
> > On Wed, 18 Nov 2020 at 00:20, Joel Fernandes (Google)
>
> > > +#ifdef CONFIG_SMP
> > > +static struct task_struct *pick_task_fair(struct rq *rq)
> > > +{
> > > +       struct cfs_rq *cfs_rq = &rq->cfs;
> > > +       struct sched_entity *se;
> > > +
> > > +       if (!cfs_rq->nr_running)
> > > +               return NULL;
> > > +
> > > +       do {
> > > +               struct sched_entity *curr = cfs_rq->curr;
> > > +
> > > +               se = pick_next_entity(cfs_rq, NULL);
> >
> > Calling pick_next_entity clears buddies. This is fine without
> > coresched because the se will be the next one. But calling
> > pick_task_fair doesn't mean that the se will be used
>
> Urgh, nice one :/
>
> > > +
> > > +               if (curr) {
> > > +                       if (se && curr->on_rq)
> > > +                               update_curr(cfs_rq);
> > > +
> >
> > Shouldn't you check if cfs_rq is throttled ?
>
> Hmm,... I suppose we do.
>
> > > +                       if (!se || entity_before(curr, se))
> > > +                               se = curr;
> > > +               }
> > > +
> > > +               cfs_rq = group_cfs_rq(se);
> > > +       } while (cfs_rq);
> > > +
> > > +       return task_of(se);
> > > +}
> > > +#endif
>
> Something like so then?

yes. it seems ok

>
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4354,6 +4354,8 @@ check_preempt_tick(struct cfs_rq *cfs_rq
>  static void
>  set_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *se)
>  {
> +       clear_buddies(cfs_rq, se);
> +
>         /* 'current' is not kept within the tree. */
>         if (se->on_rq) {
>                 /*
> @@ -4440,8 +4442,6 @@ pick_next_entity(struct cfs_rq *cfs_rq,
>                 se = cfs_rq->last;
>         }
>
> -       clear_buddies(cfs_rq, se);
> -
>         return se;
>  }
>
> @@ -6982,20 +6982,29 @@ static void check_preempt_wakeup(struct
>  #ifdef CONFIG_SMP
>  static struct task_struct *pick_task_fair(struct rq *rq)
>  {
> -       struct cfs_rq *cfs_rq = &rq->cfs;
>         struct sched_entity *se;
> -
> +       struct cfs_rq *cfs_rq;
> +
> +again:
> +       cfs_rq = &rq->cfs;
>         if (!cfs_rq->nr_running)
>                 return NULL;
>
>         do {
>                 struct sched_entity *curr = cfs_rq->curr;
>
> -               if (curr && curr->on_rq)
> -                       update_curr(cfs_rq);
> +               /* When we pick for a remote RQ, we'll not have done put_prev_entity() */
> +               if (curr) {
> +                       if (curr->on_rq)
> +                               update_curr(cfs_rq);
> +                       else
> +                               curr = NULL;
>
> -               se = pick_next_entity(cfs_rq, curr);
> +                       if (unlikely(check_cfs_rq_runtime(cfs_rq)))
> +                               goto again;
> +               }
>
> +               se = pick_next_entity(cfs_rq, curr);
>                 cfs_rq = group_cfs_rq(se);
>         } while (cfs_rq);
>