Re: [RFC PATCH] sched: fix hierarchical order in rq->leaf_cfs_rq_list

[Peter Zijlstra <peterz@infradead.org>]

On Tue, May 24, 2016 at 11:55:10AM +0200, Vincent Guittot wrote:
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 218f8e8..6d3fbf2 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -290,15 +290,31 @@ static inline void list_add_leaf_cfs_rq(struct cfs_rq *cfs_rq)
>  		 * Ensure we either appear before our parent (if already
>  		 * enqueued) or force our parent to appear after us when it is
>  		 * enqueued.  The fact that we always enqueue bottom-up
> +		 * reduces this to two cases and a specila case for the root

'special'

> +		 * cfs_rq.
>  		 */
>  		if (cfs_rq->tg->parent &&
>  		    cfs_rq->tg->parent->cfs_rq[cpu_of(rq_of(cfs_rq))]->on_list) {
> +			/* Add the child just before its parent */
> +			list_add_tail_rcu(&cfs_rq->leaf_cfs_rq_list,
> +				&(cfs_rq->tg->parent->cfs_rq[cpu_of(rq_of(cfs_rq))]->leaf_cfs_rq_list));
> +			rq_of(cfs_rq)->leaf_alone = &rq_of(cfs_rq)->leaf_cfs_rq_list;
> +		} else if (!cfs_rq->tg->parent) {
> +			/*
> +			 * cfs_rq without parent should be
> +			 * at the end of the list
> +			 */
>  			list_add_tail_rcu(&cfs_rq->leaf_cfs_rq_list,
>  				&rq_of(cfs_rq)->leaf_cfs_rq_list);
> +			rq_of(cfs_rq)->leaf_alone = &rq_of(cfs_rq)->leaf_cfs_rq_list;
> +		} else {
> +			/*
> +			 * Our parent has not already been added so make sure
> +			 * that it will be put after us
> +			 */
> +			list_add_rcu(&cfs_rq->leaf_cfs_rq_list,
> +				rq_of(cfs_rq)->leaf_alone);
> +			rq_of(cfs_rq)->leaf_alone = &cfs_rq->leaf_cfs_rq_list;
>  		}
>  
>  		cfs_rq->on_list = 1;

Paul, Ben ?

This used to be critical for update_shares() (now
update_blocked_averages), but IIRC is not critical for that since PELT.

I find its more readable with like so..


Also; I feel the comments can use more love; my head hurts ;-)

---
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -286,35 +286,38 @@ static inline struct cfs_rq *group_cfs_r
 static inline void list_add_leaf_cfs_rq(struct cfs_rq *cfs_rq)
 {
 	if (!cfs_rq->on_list) {
+		struct rq *rq = rq_of(cfs_rq);
+		int cpu = cpu_of(rq);
+
 		/*
 		 * Ensure we either appear before our parent (if already
 		 * enqueued) or force our parent to appear after us when it is
 		 * enqueued.  The fact that we always enqueue bottom-up
-		 * reduces this to two cases and a specila case for the root
+		 * reduces this to two cases and a special case for the root
 		 * cfs_rq.
 		 */
 		if (cfs_rq->tg->parent &&
-		    cfs_rq->tg->parent->cfs_rq[cpu_of(rq_of(cfs_rq))]->on_list) {
+		    cfs_rq->tg->parent->cfs_rq[cpu]->on_list) {
 			/* Add the child just before its parent */
 			list_add_tail_rcu(&cfs_rq->leaf_cfs_rq_list,
-				&(cfs_rq->tg->parent->cfs_rq[cpu_of(rq_of(cfs_rq))]->leaf_cfs_rq_list));
-			rq_of(cfs_rq)->leaf_alone = &rq_of(cfs_rq)->leaf_cfs_rq_list;
+				&(cfs_rq->tg->parent->cfs_rq[cpu]->leaf_cfs_rq_list));
+			rq->leaf_alone = &rq->leaf_cfs_rq_list;
 		} else if (!cfs_rq->tg->parent) {
 			/*
 			 * cfs_rq without parent should be
 			 * at the end of the list
 			 */
 			list_add_tail_rcu(&cfs_rq->leaf_cfs_rq_list,
-				&rq_of(cfs_rq)->leaf_cfs_rq_list);
-			rq_of(cfs_rq)->leaf_alone = &rq_of(cfs_rq)->leaf_cfs_rq_list;
+					  &rq->leaf_cfs_rq_list);
+			rq->leaf_alone = &rq->leaf_cfs_rq_list;
 		} else {
 			/*
 			 * Our parent has not already been added so make sure
 			 * that it will be put after us
 			 */
 			list_add_rcu(&cfs_rq->leaf_cfs_rq_list,
-				rq_of(cfs_rq)->leaf_alone);
-			rq_of(cfs_rq)->leaf_alone = &cfs_rq->leaf_cfs_rq_list;
+				     rq->leaf_alone);
+			rq->leaf_alone = &cfs_rq->leaf_cfs_rq_list;
 		}
 
 		cfs_rq->on_list = 1;