sched: Check if we got a shallowest_idle_cpu before searching for least_loaded_cpu

Idle cpu is idler than non-idle cpu, so we needn't search for least_loaded_cpu
after we have found an idle cpu.
Signed-off-by: NYao Dongdong <yaodongdong@huawei.com>
Reviewed-by: NSrikar Dronamraju <srikar@linux.vnet.ibm.com>
Signed-off-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/1414469286-6023-1-git-send-email-yaodongdong@huawei.comSigned-off-by: NIngo Molnar <mingo@kernel.org>

kernel/sched/fair.c

@@ -4641,7 +4641,7 @@ find_idlest_cpu(struct sched_group *group, struct task_struct *p, int this_cpu)
 				latest_idle_timestamp = rq->idle_stamp;
 				shallowest_idle_cpu = i;
 			}
-		} else {
+		} else if (shallowest_idle_cpu == -1) {
 			load = weighted_cpuload(i);
 			if (load < min_load || (load == min_load && i == this_cpu)) {
 				min_load = load;