Subject: [PATCH v7 6/7] sched: replace capacity_factor by usage

From: [email-masked] (Peter Zijlstra)
Date: Thu, 9 Oct 2014 16:16:07 +0200

On Tue, Oct 07, 2014 at 02:13:36PM +0200, Vincent Guittot wrote:
> @@ -6214,17 +6178,21 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
>  
>  		/*
>  		 * In case the child domain prefers tasks go to siblings
> +		 * first, lower the sg capacity to one 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, i.e. group_capacity > 0. 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)) {
> +			if (sgs->sum_nr_running > 1)
> +				sgs->group_no_capacity = 1;
> +			sgs->group_capacity = min(sgs->group_capacity,
> +						SCHED_CAPACITY_SCALE);
> +		}
>  
>  		if (update_sd_pick_busiest(env, sds, sg, sgs)) {
>  			sds->busiest = sg;

> @@ -6490,8 +6460,8 @@ static struct sched_group *find_busiest_group(struct lb_env *env)
>  		goto force_balance;
>  
>  	/* SD_BALANCE_NEWIDLE trumps SMP nice when underutilized */
> -	if (env->idle == CPU_NEWLY_IDLE && local->group_has_free_capacity &&
> -	    !busiest->group_has_free_capacity)
> +	if (env->idle == CPU_NEWLY_IDLE && group_has_capacity(env, local) &&
> +	    busiest->group_no_capacity)
>  		goto force_balance;
>  
>  	/*

This is two calls to group_has_capacity() on the local group. Why not
compute once in update_sd_lb_stats()?