linux-kernel - Re: [PATCH v5 11/12] sched: replace capacity

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Date:	Thu, 11 Sep 2014 19:26:48 +0200
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...nel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	LAK <linux-arm-kernel@...ts.infradead.org>,
	Rik van Riel <riel@...hat.com>,
	Morten Rasmussen <Morten.Rasmussen@....com>,
	Mike Galbraith <efault@....de>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>
Subject: Re: [PATCH v5 11/12] sched: replace capacity_factor by utilization

On 11 September 2014 18:15, Peter Zijlstra <peterz@...radead.org> wrote:
> On Tue, Aug 26, 2014 at 01:06:54PM +0200, Vincent Guittot wrote:
>> +static inline int group_has_free_capacity(struct sg_lb_stats *sgs,
>> +                     struct lb_env *env)
>>  {
>> +     if ((sgs->group_capacity_orig * 100) >
>> +                     (sgs->group_utilization * env->sd->imbalance_pct))
>> +             return 1;
>> +
>> +     if (sgs->sum_nr_running < sgs->group_weight)
>> +             return 1;
>>
>> +     return 0;
>> +}
>>
>> +static inline int group_is_overloaded(struct sg_lb_stats *sgs,
>> +                     struct lb_env *env)
>> +{
>> +     if (sgs->sum_nr_running <= sgs->group_weight)
>> +             return 0;
>>
>> +     if ((sgs->group_capacity_orig * 100) <
>> +                     (sgs->group_utilization * env->sd->imbalance_pct))
>> +             return 1;
>>
>> +     return 0;
>>  }
>
> I'm confused about the utilization vs capacity_orig. I see how we should

1st point is that I should compare utilization vs capacity and not
capacity_orig.
I should have replaced capacity_orig by capacity in the functions
above when i move the utilization statistic from
rq->avg.runnable_avg_sum to cfs.usage_load_avg.
rq->avg.runnable_avg_sum was measuring all activity on the cpu whereas
cfs.usage_load_avg integrates only cfs tasks

With this change, we don't need sgs->group_capacity_orig anymore but
only sgs->group_capacity. So sgs->group_capacity_orig can be removed
as it's no more used in the code as sg_capacity_factor has been
removed

> maybe scale things with the capacity when comparing between CPUs/groups,
> but not on the same CPU/group.
>
> I would have expected something simple like:
>
> static inline bool group_has_capacity()
> {
>         /* Is there a spare cycle? */
>         if (sgs->group_utilization < sgs->group_weight * SCHED_LOAD_SCALE)
>                 return true;
>
>         /* Are there less tasks than logical CPUs? */
>         if (sgs->sum_nr_running < sgs->group_weight)
>                 return true;
>
>         return false;
> }
>
> Where group_utilization a pure sum of running_avg.
>
> Now this has a hole when there are RT tasks on the system, in that case
> the utilization will never hit 1, but we could fix that another way. I
> don't think the capacity_orig thing is right.
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