linux-kernel - Re: [PATCH v2] sched/fair: Remove the cost of a redundant cpumask_next_wrap in select_idle

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Message-ID: <YZ5UDuCII/KHUb9h@hirez.programming.kicks-ass.net>
Date:   Wed, 24 Nov 2021 16:02:38 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Barry Song <21cnbao@...il.com>
Cc:     Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Barry Song <song.bao.hua@...ilicon.com>
Subject: Re: [PATCH v2] sched/fair: Remove the cost of a redundant
 cpumask_next_wrap in select_idle_cpu

On Thu, Nov 25, 2021 at 01:02:00AM +1300, Barry Song wrote:
> On Thu, Nov 25, 2021 at 12:57 AM Barry Song <21cnbao@...il.com> wrote:

> > Let me make it clearer. if nr=5, the original code will  loop 5 times,
> > but in the 5th loop, it returns directly, so  __select_idle_cpu is
> > only done 4 times.
> >
> > if nr=1, the original code will  loop 1 time, but in the 1st loop,
> > it returns directly, so  __select_idle_cpu is  done 0 times.
> 
> this is also why in the first version of patch, i did this:
>                 span_avg = sd->span_weight * avg_idle;
>                 if (span_avg > 4*avg_cost)
> -                       nr = div_u64(span_avg, avg_cost);
> +                       nr = div_u64(span_avg, avg_cost) - 1;
>                 else
> -                       nr = 4;
> +                       nr = 3;
> 
> because we are actually scanning 3 times or div_u64(span_avg, avg_cost) - 1
> times but not 4 times or div_u64(span_avg, avg_cost) times.

It still is confusing, because > 4*span -> nr = avg/span, very much
implies we want to bottom out at 4.

> this is not confusing at all. the only thing which is confusing is the original
> code.

But yes, it seems a whole lot of confusion stacked together. Let make it
sane and say that we do 'nr' iterations, because clearly that was the
intent :-)