linux-kernel - Re: [PATCH 01/10] sched: Remove one division operation in find_busiest

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Date:	Thu, 22 Aug 2013 01:58:28 -0700
From:	Paul Turner <pjt@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	LKML <linux-kernel@...r.kernel.org>,
	Mike Galbraith <efault@....de>, Alex Shi <alex.shi@...el.com>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Namhyung Kim <namhyung@...nel.org>,
	Lei Wen <leiwen@...vell.com>, Joonsoo Kim <js1304@...il.com>,
	Rik van Riel <riel@...riel.com>
Subject: Re: [PATCH 01/10] sched: Remove one division operation in find_busiest_queue()

On Mon, Aug 19, 2013 at 9:00 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> From: Joonsoo Kim <iamjoonsoo.kim@....com>
>
> Remove one division operation in find_busiest_queue() by using
> crosswise multiplication:
>
>         wl_i / power_i > wl_j / power_j :=
>         wl_i * power_j > wl_j * power_i
>
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@....com>
> [peterz: expanded changelog]
> Signed-off-by: Peter Zijlstra <peterz@...radead.org>
> Link: http://lkml.kernel.org/r/1375778203-31343-2-git-send-email-iamjoonsoo.kim@lge.com
> ---
>  kernel/sched/fair.c |    9 ++++-----
>  1 file changed, 4 insertions(+), 5 deletions(-)
>
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5018,7 +5018,7 @@ static struct rq *find_busiest_queue(str
>                                      struct sched_group *group)
>  {
>         struct rq *busiest = NULL, *rq;
> -       unsigned long max_load = 0;
> +       unsigned long busiest_load = 0, busiest_power = SCHED_POWER_SCALE;

Initializing this to SCHED_POWER_SCALE assigns a meaning that isn't
really there.  How about just 1?

>         int i;
>
>         for_each_cpu(i, sched_group_cpus(group)) {
> @@ -5049,10 +5049,9 @@ static struct rq *find_busiest_queue(str
>                  * the load can be moved away from the cpu that is potentially
>                  * running at a lower capacity.
>                  */
> -               wl = (wl * SCHED_POWER_SCALE) / power;
> -
> -               if (wl > max_load) {
> -                       max_load = wl;

A comment wouldn't hurt here.

> +               if (wl * busiest_power > busiest_load * power) {
> +                       busiest_load = wl;
> +                       busiest_power = power;
>                         busiest = rq;
>                 }
>         }
>
>
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