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

Peter Zijlstra
2015-02-20T11:14:20Z

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?

> [1] https://lkml.org/lkml/2014/8/12/295

Patch references are like:
9a5d9ba6a363 ("sched/fair: Allow calculate_imbalance() to move idle cpus")

> /*
> + * 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?

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.

> +/*
> + * 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()?

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

> /*
> * 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.