[PATCH] cfq-iosched: use metadata read flag

Give meta data reads preference over regular reads, as the process
often needs to get that out of the way to do the io it was actually
interested in.

Signed-off-by: Jens Axboe <axboe@suse.de>

block/cfq-iosched.c

@@ -141,6 +141,8 @@ struct cfq_queue {
 	int queued[2];
 	/* currently allocated requests */
 	int allocated[2];
+	/* pending metadata requests */
+	int meta_pending;
 	/* fifo list of requests in sort_list */
 	struct list_head fifo;
 
@@ -248,6 +250,10 @@ cfq_choose_req(struct cfq_data *cfqd, struct request *rq1, struct request *rq2)
 		return rq1;
 	else if (rq_is_sync(rq2) && !rq_is_sync(rq1))
 		return rq2;
+	if (rq_is_meta(rq1) && !rq_is_meta(rq2))
+		return rq1;
+	else if (rq_is_meta(rq2) && !rq_is_meta(rq1))
+		return rq2;
 
 	s1 = rq1->sector;
 	s2 = rq2->sector;
@@ -510,6 +516,11 @@ static void cfq_remove_request(struct request *rq)
 
 	list_del_init(&rq->queuelist);
 	cfq_del_rq_rb(rq);
+
+	if (rq_is_meta(rq)) {
+		WARN_ON(!cfqq->meta_pending);
+		cfqq->meta_pending--;
+	}
 }
 
 static int
@@ -1527,8 +1538,18 @@ cfq_should_preempt(struct cfq_data *cfqd, struct cfq_queue *new_cfqq,
 	 */
 	if (new_cfqq->slice_left < cfqd->cfq_slice_idle)
 		return 0;
+	/*
+	 * if the new request is sync, but the currently running queue is
+	 * not, let the sync request have priority.
+	 */
 	if (rq_is_sync(rq) && !cfq_cfqq_sync(cfqq))
 		return 1;
+	/*
+	 * So both queues are sync. Let the new request get disk time if
+	 * it's a metadata request and the current queue is doing regular IO.
+	 */
+	if (rq_is_meta(rq) && !cfqq->meta_pending)
+		return 1;
 
 	return 0;
 }
@@ -1564,6 +1585,9 @@ cfq_rq_enqueued(struct cfq_data *cfqd, struct cfq_queue *cfqq,
 {
 	struct cfq_io_context *cic = RQ_CIC(rq);
 
+	if (rq_is_meta(rq))
+		cfqq->meta_pending++;
+
 	/*
 	 * check if this request is a better next-serve candidate)) {
 	 */