block: change force plug flush call order

Do blk_flush_plug_list() first and then add new request aDo blk_flush_plug_list() first and then add new request aDo blk_flush_plug_list() first and then add new request at the tail. New
request can't be merged to existing requests, but later new requests might
be merged with this new one. If blk_flush_plug_list() is done later, the
merge doesn't happen.
Believe it or not, this fixes a 10% regression running sysbench workload.

Signed-off-by: Shaohua Li <shli@kernel.org>
Signed-off-by: Shaohua Li <shaohua.li@intel.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

diff --git a/block/blk-core.c b/block/blk-core.c
index 90e1ffd..67dba69 100644 (file)
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1302,11 +1302,11 @@ get_rq:
                        if (__rq->q != q)
                                plug->should_sort = 1;
                }
-               list_add_tail(&req->queuelist, &plug->list);
-               plug->count++;
-               drive_stat_acct(req, 1);
                if (plug->count >= BLK_MAX_REQUEST_COUNT)
                        blk_flush_plug_list(plug, false);
+               plug->count++;
+               list_add_tail(&req->queuelist, &plug->list);
+               drive_stat_acct(req, 1);
        } else {
                spin_lock_irq(q->queue_lock);
                add_acct_request(q, req, where);