Re: [PATCH v2] sched: Fast idling of CPU when system is partially loaded

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Jun 16, 2014 at 12:48:47PM -0700, Tim Chen wrote:
> +++ b/kernel/sched/fair.c
> @@ -5863,7 +5863,8 @@ static inline int sg_capacity(struct lb_env *env, struct sched_group *group)
>   */
>  static inline void update_sg_lb_stats(struct lb_env *env,
>  			struct sched_group *group, int load_idx,
> -			int local_group, struct sg_lb_stats *sgs)
> +			int local_group, struct sg_lb_stats *sgs,
> +			bool *overload)
>  {
>  	unsigned long load;
>  	int i;
> @@ -5881,6 +5882,8 @@ static inline void update_sg_lb_stats(struct lb_env *env,
>  
>  		sgs->group_load += load;
>  		sgs->sum_nr_running += rq->nr_running;
> +		if (overload && rq->nr_running > 1)
> +			*overload = true;
>  #ifdef CONFIG_NUMA_BALANCING
>  		sgs->nr_numa_running += rq->nr_numa_running;
>  		sgs->nr_preferred_running += rq->nr_preferred_running;
> @@ -5991,6 +5994,7 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
>  	struct sched_group *sg = env->sd->groups;
>  	struct sg_lb_stats tmp_sgs;
>  	int load_idx, prefer_sibling = 0;
> +	bool overload = false;
>  
>  	if (child && child->flags & SD_PREFER_SIBLING)
>  		prefer_sibling = 1;
> @@ -6011,7 +6015,13 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
>  				update_group_power(env->sd, env->dst_cpu);
>  		}
>  
> -		update_sg_lb_stats(env, sg, load_idx, local_group, sgs);
> +		if (env->sd->parent)
> +			update_sg_lb_stats(env, sg, load_idx, local_group, sgs,
> +						NULL);
> +		else
> +			/* gather overload info if we are at root domain */
> +			update_sg_lb_stats(env, sg, load_idx, local_group, sgs,
> +						&overload);
>  
>  		if (local_group)
>  			goto next_group;
> @@ -6045,6 +6055,13 @@ next_group:
>  
>  	if (env->sd->flags & SD_NUMA)
>  		env->fbq_type = fbq_classify_group(&sds->busiest_stat);
> +
> +	if (!env->sd->parent) {
> +		/* update overload indicator if we are at root domain */
> +		if (env->dst_rq->rd->overload != overload)
> +			env->dst_rq->rd->overload = overload;
> +	}
> +
>  }
>  
>  /**

So I don't get why we can't do the below; I think Jason tried to ask the
same...

Making that overload thing unconditional makes the code simpler and the
cost is about the same; it doesn't matter if we test the pointer or
->nr_running, which we've already loaded anyhow.

Also, with only having a single update_sg_lb_stats() callsite GCC can
more easily inline the lot.

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5886,7 +5886,7 @@ static inline void update_sg_lb_stats(st
 
 		sgs->group_load += load;
 		sgs->sum_nr_running += rq->nr_running;
-		if (overload && rq->nr_running > 1)
+		if (rq->nr_running > 1)
 			*overload = true;
 #ifdef CONFIG_NUMA_BALANCING
 		sgs->nr_numa_running += rq->nr_numa_running;
@@ -6019,13 +6019,7 @@ static inline void update_sd_lb_stats(st
 				update_group_capacity(env->sd, env->dst_cpu);
 		}
 
-		if (env->sd->parent)
-			update_sg_lb_stats(env, sg, load_idx, local_group, sgs,
-						NULL);
-		else
-			/* gather overload info if we are at root domain */
-			update_sg_lb_stats(env, sg, load_idx, local_group, sgs,
-						&overload);
+		update_sg_lb_stats(env, sg, load_idx, local_group, sgs, &overload);
 
 		if (local_group)
 			goto next_group;
@@ -6065,7 +6059,6 @@ static inline void update_sd_lb_stats(st
 		if (env->dst_rq->rd->overload != overload)
 			env->dst_rq->rd->overload = overload;
 	}
-
 }
 
 /**