rbd: queue_depth map option

nr_requests (/sys/block/rbd<id>/queue/nr_requests) is pretty much
irrelevant in blk-mq case because each driver sets its own max depth
that it can handle and that's the number of tags that gets preallocated
on setup.  Users can't increase queue depth beyond that value via
writing to nr_requests.

For rbd we are happy with the default BLKDEV_MAX_RQ (128) for most
cases but we want to give users the opportunity to increase it.
Introduce a new per-device queue_depth option to do just that:

    $ sudo rbd map -o queue_depth=1024 ...

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Alex Elder <elder@linaro.org>

drivers/block/rbd.c

@@ -728,6 +728,7 @@ static struct rbd_client *rbd_client_find(struct ceph_options *ceph_opts)
  * (Per device) rbd map options
  */
 enum {
+	Opt_queue_depth,
 	Opt_last_int,
 	/* int args above */
 	Opt_last_string,
@@ -738,6 +739,7 @@ enum {
 };
 
 static match_table_t rbd_opts_tokens = {
+	{Opt_queue_depth, "queue_depth=%d"},
 	/* int args above */
 	/* string args above */
 	{Opt_read_only, "read_only"},
@@ -748,9 +750,11 @@ static match_table_t rbd_opts_tokens = {
 };
 
 struct rbd_options {
+	int	queue_depth;
 	bool	read_only;
 };
 
+#define RBD_QUEUE_DEPTH_DEFAULT	BLKDEV_MAX_RQ
 #define RBD_READ_ONLY_DEFAULT	false
 
 static int parse_rbd_opts_token(char *c, void *private)
@@ -774,6 +778,13 @@ static int parse_rbd_opts_token(char *c, void *private)
 	}
 
 	switch (token) {
+	case Opt_queue_depth:
+		if (intval < 1) {
+			pr_err("queue_depth out of range\n");
+			return -EINVAL;
+		}
+		rbd_opts->queue_depth = intval;
+		break;
 	case Opt_read_only:
 		rbd_opts->read_only = true;
 		break;
@@ -3761,10 +3772,9 @@ static int rbd_init_disk(struct rbd_device *rbd_dev)
 
 	memset(&rbd_dev->tag_set, 0, sizeof(rbd_dev->tag_set));
 	rbd_dev->tag_set.ops = &rbd_mq_ops;
-	rbd_dev->tag_set.queue_depth = BLKDEV_MAX_RQ;
+	rbd_dev->tag_set.queue_depth = rbd_dev->opts->queue_depth;
 	rbd_dev->tag_set.numa_node = NUMA_NO_NODE;
-	rbd_dev->tag_set.flags =
-		BLK_MQ_F_SHOULD_MERGE | BLK_MQ_F_SG_MERGE;
+	rbd_dev->tag_set.flags = BLK_MQ_F_SHOULD_MERGE | BLK_MQ_F_SG_MERGE;
 	rbd_dev->tag_set.nr_hw_queues = 1;
 	rbd_dev->tag_set.cmd_size = sizeof(struct work_struct);
 
@@ -4948,6 +4958,7 @@ static int rbd_add_parse_args(const char *buf,
 		goto out_mem;
 
 	rbd_opts->read_only = RBD_READ_ONLY_DEFAULT;
+	rbd_opts->queue_depth = RBD_QUEUE_DEPTH_DEFAULT;
 
 	copts = ceph_parse_options(options, mon_addrs,
 					mon_addrs + mon_addrs_size - 1,