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

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Date:	Tue, 27 Aug 2013 02:13:40 -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 26, 2013 at 5:07 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Sat, Aug 24, 2013 at 03:33:59AM -0700, Paul Turner wrote:
>> On Mon, Aug 19, 2013 at 9:01 AM, Peter Zijlstra <peterz@...radead.org> wrote:
>> > +++ 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. ]
>
> You've got me confused again. You're talking about something like the
> below?

Nevermind, I was looking at a tip tree as I reviewed this one.  What I
was suggesting was exactly:
  "[PATCH 01/10] sched: Remove one division operation in find_busiest_queue()"

>
> I suppose the problem with that is that we could keep selecting the
> busiest rq with an unmovable task due to:
>
> move_tasks():
>                 if ((load / 2) > env->imbalance)
>                         goto next;
>
> That said, the condition in fbq() should at least be modified to match
> this. Now the entire capacity crap comes from:
>
>   bdb94aa sched: Try to deal with low capacity
>
> But thinking a little more about it, if power drops that low imbalance
> is likely to be _huge_ and we'd not meet that condition. Now if only I
> wrote a more comprehensive Changelog and explained why that wouldn't be
> the case. /me kicks himself.
>
> ---
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4990,28 +4990,12 @@ static struct sched_group *find_busiest_
>  static struct rq *find_busiest_queue(struct lb_env *env,
>                                      struct sched_group *group)
>  {
> -       struct rq *busiest = NULL, *rq;
>         unsigned long busiest_load = 0, busiest_power = 1;
> +       struct rq *busiest = NULL;
>         int i;
>
>         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);
> -               unsigned long wl;
> -
> -               if (!capacity)
> -                       capacity = fix_small_capacity(env->sd, group);
> -
> -               rq = cpu_rq(i);
> -               wl = weighted_cpuload(i);
> -
> -               /*
> -                * When comparing with imbalance, use weighted_cpuload()
> -                * which is not scaled with the cpu power.
> -                */
> -               if (capacity && rq->nr_running == 1 && wl > env->imbalance)
> -                       continue;
> +               unsigned long wl = weighted_cpuload(i);
>
>                 /*
>                  * For the load comparisons with the other cpu's, consider
> @@ -5027,7 +5011,7 @@ static struct rq *find_busiest_queue(str
>                 if (wl * busiest_power > busiest_load * power) {
>                         busiest_load = wl;
>                         busiest_power = power;
> -                       busiest = rq;
> +                       busiest = cpu_rq(i);
>                 }
>         }
>
>
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