linux-kernel - Re: [PATCH] sched/core: fix pick_next

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Date:   Tue, 24 Aug 2021 10:55:23 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Don <joshdon@...gle.com>
Cc:     Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Vineeth Pillai <vineethrp@...il.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Tao Zhou <tao.zhou@...ux.dev>
Subject: Re: [PATCH] sched/core: fix pick_next_task 'max' tracking

On Mon, Aug 23, 2021 at 04:24:26PM -0700, Josh Don wrote:
> Hi Peter,
> 
> On Mon, Aug 23, 2021 at 4:17 AM Peter Zijlstra <peterz@...radead.org> wrote:
> [snip]
> > +       for_each_cpu(i, smt_mask) {
> > +               rq_i = cpu_rq(i);
> > +               p = rq_i->core_temp;
> >
> > -                       /*
> > -                        * If this sibling doesn't yet have a suitable task to
> > -                        * run; ask for the most eligible task, given the
> > -                        * highest priority task already selected for this
> > -                        * core.
> > -                        */
> > -                       p = pick_task(rq_i, class, max, fi_before);
> > +               if (!cookie_equals(p, cookie)) {
> > +                       p = NULL;
> > +                       if (cookie)
> > +                               p = sched_core_find(rq_i, cookie);
> 
> In the case that 'max' has a zero cookie, shouldn't we search for a
> match on this cpu if the original class pick ('p') had a non-zero
> cookie? We don't enqueue tasks with zero cookie in the core_tree, so I
> forget if there was some other reasoning here.

IIRC we don't keep 0-cookies in the tree. Lemme check.

Yeah, see sched_core_enqueue(), they bail for 0-cookie.

This is indeed sub-optimal, but also having the 0-cookie tasks in the
tree has other issues IIRC.