[tip: sched/core] sched/fair: Skip idle cfs_rq

[tip-bot2 for Vincent Guittot]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     fc488ffd4297f661b3e9d7450dcdb9089a53df7c
Gitweb:        https://git.kernel.org/tip/fc488ffd4297f661b3e9d7450dcdb9089a53df7c
Author:        Vincent Guittot <vincent.guittot@xxxxxxxxxx>
AuthorDate:    Thu, 07 Jan 2021 11:33:23 +01:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Thu, 14 Jan 2021 11:20:10 +01:00

sched/fair: Skip idle cfs_rq

Don't waste time checking whether an idle cfs_rq could be the busiest
queue. Furthermore, this can end up selecting a cfs_rq with a high load
but being idle in case of migrate_load.

Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: Valentin Schneider <valentin.schneider@xxxxxxx>
Acked-by: Mel Gorman <mgorman@xxxxxxx>
Link: https://lkml.kernel.org/r/20210107103325.30851-2-vincent.guittot@xxxxxxxxxx
---
 kernel/sched/fair.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 40d3ebf..13de7ae 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9402,8 +9402,11 @@ static struct rq *find_busiest_queue(struct lb_env *env,
 		if (rt > env->fbq_type)
 			continue;
 
-		capacity = capacity_of(i);
 		nr_running = rq->cfs.h_nr_running;
+		if (!nr_running)
+			continue;
+
+		capacity = capacity_of(i);
 
 		/*
 		 * For ASYM_CPUCAPACITY domains, don't pick a CPU that could