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Date:   Thu, 26 Nov 2020 13:40:11 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vincent Guittot <vincent.guittot@...aro.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, Nov 26, 2020 at 11:17:48AM +0100, Vincent Guittot wrote:

> > Something like so then?
> 
> yes. it seems ok
> 
> >
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c

> > @@ -6982,20 +6982,29 @@ static void check_preempt_wakeup(struct
> >  #ifdef CONFIG_SMP
> >  static struct task_struct *pick_task_fair(struct rq *rq)
> >  {
> >         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;
> >
> > +               /* 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;
> >
> > +                       if (unlikely(check_cfs_rq_runtime(cfs_rq)))
> > +                               goto again;

Head-ache though; pick_task() was supposed to be stateless, but now
we're modifying a remote runqueue... I suppose it still works, because
irrespective of which task we end up picking (even idle), we'll schedule
the remote CPU, which would've resulted in the same (and possibly
triggered a reschedule if we'd not done it here).

There's a wrinkle through, other than in schedule(), where we dequeue()
and keep running with the current task while we release rq->lock, this
has preemption enabled as well.

This means that if we do this, the remote CPU could preempt, but the
task is then no longer on the runqueue.

I _think_ it all still works, but yuck!

> > +               }
> >
> > +               se = pick_next_entity(cfs_rq, curr);
> >                 cfs_rq = group_cfs_rq(se);
> >         } while (cfs_rq);
> >

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