cfq: Optimization for close cooperating queue searching

It doesn't make any sense to try to find out a close cooperating
queue if current cfqq is the only one in the group.

Signed-off-by: Gui Jianfeng <guijianfeng@cn.fujitsu.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

block/cfq-iosched.c

@@ -1749,6 +1749,12 @@ static struct cfq_queue *cfq_close_cooperator(struct cfq_data *cfqd,
 	if (CFQQ_SEEKY(cur_cfqq))
 		return NULL;
 
+	/*
+	 * Don't search priority tree if it's the only queue in the group.
+	 */
+	if (cur_cfqq->cfqg->nr_cfqq == 1)
+		return NULL;
+
 	/*
 	 * We should notice if some of the queues are cooperating, eg
 	 * working closely on the same area of the disk. In that case,