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

block/blk-core.c

@@ -1302,11 +1302,11 @@ static int __make_request(struct request_queue *q, struct bio *bio)
 			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);