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From:   Vineeth Pillai <vineethrp@gmail.com>
Date:   Mon, 23 Aug 2021 16:25:28 -0400
Message-ID: <CAOBnfPjH=y3Lk7AukLeG4mNcJnf5cgV260=PZCbF9u69-T+Q6Q@mail.gmail.com>
Subject: Re: [PATCH] sched/core: fix pick_next_task 'max' tracking
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Josh Don <joshdon@google.com>, Ingo Molnar <mingo@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        linux-kernel@vger.kernel.org, tao.zhou@linux.dev
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Hi Peter,


> > Here, we should have instead updated 'max' when picking for SMT-1. Note
> > that this code would eventually have righted itself, since the retry
> > loop would re-pick p2, and update 'max' accordingly. However, this patch
> > avoids the extra round-trip.
>
> Going with the observation Tao made; how about we rewrite the whole lot
> to not be mind-bending complicated :-)
>
> How's this? It seems to build and pass the core-sched selftest thingy
> (so it must be perfect, right? :-)
>
Nice, the code is much simpler now :-). A minor suggestion down..

> -       for_each_cpu(i, smt_mask) {
> -               struct rq *rq_i = cpu_rq(i);
> -
> +       /*
> +        * For each thread: do the regular task pick and find the max prio task
> +        * amongst them.
> +        *
> +        * Tie-break prio towards the current CPU
> +        */
> +       for_each_cpu_wrap(i, smt_mask, cpu) {
> +               rq_i = cpu_rq(i);
>                 rq_i->core_pick = NULL;
>
>                 if (i != cpu)
>                         update_rq_clock(rq_i);
> +
> +               for_each_class(class) {
> +                       p = rq_i->core_temp = class->pick_task(rq_i);
I think we can use core_pick to store the pick here and core_temp
might not be required. What do you feel?

> +                       if (p)
> +                               break;
> +               }
> +
> +               if (!max || prio_less(max, p, fi_before))
> +                       max = p;


Thanks,
Vineeth