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>

1 files changed, 9 insertions, 7 deletions

diff --git a/block/blk-core.c b/block/blk-core.c
index f43c8a5840ae..6403c12f8aad 100644
--- 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 {