linux-kernel - Re: [PATCH RESEND v9 08/10] sched: replace capacity

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Date:	Fri, 20 Feb 2015 12:14:04 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Vincent Guittot <vincent.guittot@...aro.org>
Cc:	mingo@...nel.org, linux-kernel@...r.kernel.org,
	preeti@...ux.vnet.ibm.com, Morten.Rasmussen@....com,
	kamalesh@...ux.vnet.ibm.com, riel@...hat.com, efault@....de,
	nicolas.pitre@...aro.org, dietmar.eggemann@....com,
	linaro-kernel@...ts.linaro.org
Subject: Re: [PATCH RESEND v9 08/10] sched: replace capacity_factor by usage

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.
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