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>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/block/blk-core.c b/block/blk-core.c
index f43c8a5840ae488131dbf395d5eaf96e2fea876d..6403c12f8aad8e594c65b75ede2a589b1093adf7 100644 (file)
--- a/block/blk-core.c
+++ b/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 {