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@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
Acked-by: Mel Gorman <mgorman@suse.de>
Link: https://lkml.kernel.org/r/20210107103325.30851-2-vincent.guittot@linaro.org

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