Re: [PATCH RESEND v9 08/10] sched: replace capacity_factor by usage

Vincent Guittot
2015-02-20T13:31:31Z

On 20 February 2015 at 12:14, Peter Zijlstra <peterz@infradead.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.