Message-ID: <CAKfTPtAT4g4tqpvDFivPrAarKusE+NDK+32CpfV5cuOoSgzqpQ@mail.gmail.com>
Date:	Fri, 20 Feb 2015 14:31:08 +0100
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>,
	Morten Rasmussen <Morten.Rasmussen@....com>,
	Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
	Rik van Riel <riel@...hat.com>,
	Mike Galbraith <efault@....de>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Linaro Kernel Mailman List <linaro-kernel@...ts.linaro.org>
Subject: Re: [PATCH RESEND v9 08/10] sched: replace capacity_factor by usage

On 20 February 2015 at 12:14, Peter Zijlstra <peterz@...radead.org> wrote:
> On Thu, Jan 15, 2015 at 11:09:28AM +0100, Vincent Guittot wrote:
>
>> Finally, the sched_group->sched_group_capacity->capacity_orig has been removed
>> because it's no more used during load balance.
>
> Maybe do that in a separate patch to avoid cluttering this one?

ok

>
>> [1] https://lkml.org/lkml/2014/8/12/295
>
> Patch references are like:
> 9a5d9ba6a363 ("sched/fair: Allow calculate_imbalance() to move idle cpus")
>

ok, I'm going to update the reference


>>  /*
>> + * Check whether the capacity of the rq has been noticeably reduced by side
>> + * activity. The imbalance_pct is used for the threshold.
>> + * Return true is the capacity is reduced
>>   */
>>  static inline int
>> +check_cpu_capacity(struct rq *rq, struct sched_domain *sd)
>>  {
>> +     return ((rq->cpu_capacity * sd->imbalance_pct) <
>> +                             (rq->cpu_capacity_orig * 100));
>>  }
>
> How about cpu_has_capacity() to be consistent with the below function?

I can change the name for consistency but the 2 function are not
testing the same thing , so i would not create any confusion.
group_has_capacity tests if some capacity is not used whereas
check_cpu_capacity/cpu_has_capacity check if the capacity has been
reduced by side activity but doesn't give information about spare
capacity.

>
> This comment could use whitespace:
>
>>  /*
>> + * group_has_capacity returns true if the group has spare capacity that could
>> + * be used by some tasks.
>
>       We consider that a group has spare capacity if the
>> + * number of task is smaller than the number of CPUs or if the usage is lower
>> + * than the available capacity for CFS tasks.
>
>       For the latter, we use a
>> + * threshold to stabilize the state, to take into account the variance of the
>> + * tasks' load and to return true if the available capacity in meaningful for
>> + * the load balancer.
>
>       As an example, an available capacity of 1% can appear
>> + * but it doesn't make any benefit for the load balance.
>>   */
>> +static inline bool
>> +group_has_capacity(struct lb_env *env, struct sg_lb_stats *sgs)
>>  {
>> +     if ((sgs->group_capacity * 100) >
>> +                     (sgs->group_usage * env->sd->imbalance_pct))
>> +             return true;
>>
>> +     if (sgs->sum_nr_running < sgs->group_weight)
>> +             return true;
>> +
>> +     return false;
>> +}
>
> Would it not make sense to first do the nr_running test, its cheaper
> than the multiplication thing.

good point, i will reorder the test

>
>> +/*
>> + *  group_is_overloaded returns true if the group has more tasks than it can
>> + *  handle.
>
>        We consider that a group is overloaded if the number of tasks is
>> + *  greater than the number of CPUs and the tasks already use all available
>> + *  capacity for CFS tasks.
>
>        For the latter, we use a threshold to stabilize
>> + *  the state, to take into account the variance of tasks' load and to return
>> + *  true if available capacity is no more meaningful for load balancer
>> + */
>> +static inline bool
>> +group_is_overloaded(struct lb_env *env, struct sg_lb_stats *sgs)
>> +{
>> +     if (sgs->sum_nr_running <= sgs->group_weight)
>> +             return false;
>>
>> +     if ((sgs->group_capacity * 100) <
>> +                     (sgs->group_usage * env->sd->imbalance_pct))
>> +             return true;
>>
>> +     return false;
>>  }
>
> Maybe a note on the difference between group_is_overloaded() and
> !group_has_capacity()?

ok, i will add a comment

>
> As to the comment, I think it can be reduced by referring to the comment
> of group_has_capacity().

ok, i 'm going to update it

>
>>               /*
>>                * In case the child domain prefers tasks go to siblings
>> +              * first, lower the sg capacity so that we'll try
>>                * and move all the excess tasks away. We lower the capacity
>>                * of a group only if the local group has the capacity to fit
>> +              * these excess tasks.
>
>                    The extra check prevents the case where
>> +              * you always pull from the heaviest group when it is already
>> +              * under-utilized (possible with a large weight task outweighs
>> +              * the tasks on the system).
>>                */
>>               if (prefer_sibling && sds->local &&
>> +                 group_has_capacity(env, &sds->local_stat) &&
>> +                 (sgs->sum_nr_running > 1)) {
>> +                     sgs->group_no_capacity = 1;
>> +                     sgs->group_type = group_overloaded;
>> +             }
>
> Looks OK otherwise I suppose.
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