block: avoid unnecessary plug list flush

get_request_wait() could sleep and flush the plug list.  If the list is
already flushed, don't flush again.

Signed-off-by: Shaohua Li <shaohua.li@intel.com>
Reviewed-by: Namhyung Kim <namhyung@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-core.c

@@ -1379,15 +1379,17 @@ get_rq:
 		 */
 		if (list_empty(&plug->list))
 			trace_block_plug(q);
-		else if (!plug->should_sort) {
-			struct request *__rq;
+		else {
+			if (!plug->should_sort) {
+				struct request *__rq;
 
-			__rq = list_entry_rq(plug->list.prev);
-			if (__rq->q != q)
-				plug->should_sort = 1;
+				__rq = list_entry_rq(plug->list.prev);
+				if (__rq->q != q)
+					plug->should_sort = 1;
+			}
+			if (request_count >= BLK_MAX_REQUEST_COUNT)
+				blk_flush_plug_list(plug, false);
 		}
-		if (request_count >= BLK_MAX_REQUEST_COUNT)
-			blk_flush_plug_list(plug, false);
 		list_add_tail(&req->queuelist, &plug->list);
 		drive_stat_acct(req, 1);
 	} else {