linux-kernel - Re: [PATCH 2/2 v3] sched: use load

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Date:   Fri, 9 Dec 2016 16:22:42 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     mingo@...nel.org, linux-kernel@...r.kernel.org,
        matt@...eblueprint.co.uk, Morten.Rasmussen@....com,
        dietmar.eggemann@....com, kernellwp@...il.com,
        yuyang.du@...el.comc, umgwanakikbuti@...il.com
Subject: Re: [PATCH 2/2 v3] sched: use load_avg for selecting idlest group

On Thu, Dec 08, 2016 at 05:56:54PM +0100, Vincent Guittot wrote:
> @@ -5449,14 +5456,32 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p,
>  		}
>  
>  		/* Adjust by relative CPU capacity of the group */
> -		avg_load = (avg_load * SCHED_CAPACITY_SCALE) / group->sgc->capacity;
> +		avg_load = (avg_load * SCHED_CAPACITY_SCALE) /
> +					group->sgc->capacity;
> +		runnable_load = (runnable_load * SCHED_CAPACITY_SCALE) /
> +					group->sgc->capacity;
>  
>  		if (local_group) {
> -			this_load = avg_load;
> +			this_runnable_load = runnable_load;
> +			this_avg_load = avg_load;
>  			this_spare = max_spare_cap;
>  		} else {
> -			if (avg_load < min_load) {
> -				min_load = avg_load;
> +			if (min_runnable_load > (runnable_load + imbalance)) {
> +				/*
> +				 * The runnable load is significantly smaller
> +				 *  so we can pick this new cpu
> +				 */
> +				min_runnable_load = runnable_load;
> +				min_avg_load = avg_load;
> +				idlest = group;
> +			} else if ((runnable_load < (min_runnable_load + imbalance)) &&
> +					(100*min_avg_load > imbalance_scale*avg_load)) {
> +				/*
> +				 * The runnable loads are close so we take
> +				 * into account blocked load through avg_load
> +				 *  which is blocked + runnable load
> +				 */
> +				min_avg_load = avg_load;
>  				idlest = group;
>  			}
>  
> @@ -5480,13 +5505,16 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p,
>  		goto skip_spare;
>  
>  	if (this_spare > task_util(p) / 2 &&
> -	    imbalance*this_spare > 100*most_spare)
> +	    imbalance_scale*this_spare > 100*most_spare)
>  		return NULL;
>  	else if (most_spare > task_util(p) / 2)
>  		return most_spare_sg;
>  
>  skip_spare:
> -	if (!idlest || 100*this_load < imbalance*min_load)
> +	if (!idlest ||
> +	    (min_runnable_load > (this_runnable_load + imbalance)) ||
> +	    ((this_runnable_load < (min_runnable_load + imbalance)) &&
> +			(100*this_avg_load < imbalance_scale*min_avg_load)))
>  		return NULL;
>  	return idlest;
>  }

I did the below on top for readability.

---
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5469,17 +5469,16 @@ find_idlest_group(struct sched_domain *s
 			if (min_runnable_load > (runnable_load + imbalance)) {
 				/*
 				 * The runnable load is significantly smaller
-				 *  so we can pick this new cpu
+				 * so we can pick this new cpu
 				 */
 				min_runnable_load = runnable_load;
 				min_avg_load = avg_load;
 				idlest = group;
 			} else if ((runnable_load < (min_runnable_load + imbalance)) &&
-					(100*min_avg_load > imbalance_scale*avg_load)) {
+				   (100*min_avg_load > imbalance_scale*avg_load)) {
 				/*
-				 * The runnable loads are close so we take
-				 * into account blocked load through avg_load
-				 *  which is blocked + runnable load
+				 * The runnable loads are close so take the
+				 * blocked load into account through avg_load.
 				 */
 				min_avg_load = avg_load;
 				idlest = group;
@@ -5509,15 +5508,21 @@ find_idlest_group(struct sched_domain *s
 	if (this_spare > task_util(p) / 2 &&
 	    imbalance_scale*this_spare > 100*most_spare)
 		return NULL;
-	else if (most_spare > task_util(p) / 2)
+
+	if (most_spare > task_util(p) / 2)
 		return most_spare_sg;
 
 skip_spare:
-	if (!idlest ||
-	    (min_runnable_load > (this_runnable_load + imbalance)) ||
-	    ((this_runnable_load < (min_runnable_load + imbalance)) &&
-			(100*this_avg_load < imbalance_scale*min_avg_load)))
+	if (!idlest)
+		return NULL;
+
+	if (min_runnable_load > (this_runnable_load + imbalance))
 		return NULL;
+
+	if ((this_runnable_load < (min_runnable_load + imbalance)) &&
+	     (100*this_avg_load < imbalance_scale*min_avg_load))
+		return NULL;
+
 	return idlest;
 }