linux-kernel - Re: [PATCH 07/10] sched, fair: Optimize find_busiest

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Date:	Sat, 24 Aug 2013 03:33:59 -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>, Rik van Riel <riel@...riel.com>,
	Joonsoo Kim <js1304@...il.com>
Subject: Re: [PATCH 07/10] sched, fair: Optimize find_busiest_queue()

On Mon, Aug 19, 2013 at 9:01 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> Use for_each_cpu_and() and thereby avoid computing the capacity for
> CPUs we know we're not interested in.
>
> Signed-off-by: Peter Zijlstra <peterz@...radead.org>
> ---
>  kernel/sched/fair.c |    5 +----
>  1 file changed, 1 insertion(+), 4 deletions(-)
>
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4977,7 +4977,7 @@ static struct rq *find_busiest_queue(str
>         unsigned long busiest_load = 0, busiest_power = SCHED_POWER_SCALE;
>         int i;
>
> -       for_each_cpu(i, sched_group_cpus(group)) {
> +       for_each_cpu_and(i, sched_group_cpus(group), env->cpus) {
>                 unsigned long power = power_of(i);
>                 unsigned long capacity = DIV_ROUND_CLOSEST(power,
>                                                            SCHED_POWER_SCALE);
> @@ -4986,9 +4986,6 @@ static struct rq *find_busiest_queue(str
>                 if (!capacity)
>                         capacity = fix_small_capacity(env->sd, group);
>
> -               if (!cpumask_test_cpu(i, env->cpus))
> -                       continue;
> -
>                 rq = cpu_rq(i);
>                 wl = weighted_cpuload(i);

There's no need to actually do the divisions immediately below this also.

e.g.
  unsigned long max_load_power = SCHED_POWER_SCALE;
  ...
  if (wl * max_load_power > max_load * power) {
   max_load = wl;
   max_load_power = power;
   ...

This would actually end up being a little more accurate even.

[ Alternatively without caching max_load_power we could compare wl *
power vs max_load * SCHED_POWER_SCALE. ]

Reviewed-by: Paul Turner <pjt@...gle.com>
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