[PATCH v6 8/8] nbd: add pre_defined_connections module parameter for pre-created devices

[Yang Erkun <yangerkun@huawei.com>]

blk_mq_update_nr_hw_queues() in nbd_start_device() may cause a queue
freeze.  The previous commit addressed this for newly created nbd
devices by setting the expected nr_hw_queues in nbd_dev_add().  However,
when reusing an old inactive nbd device, the queue freeze can still
occur if the old nbd->tag_set->nr_hw_queues does not match the new
socket connection count.  Inactive nbd devices can originate from two
sources: loading the nbd module with nbds_max, which sets the default
nr_hw_queues to 1, and the netlink method, which sets nr_hw_queues
according to the expected number of socket connections.  For the first
case, add a module parameter so the default nr_hw_queues can be
changed.  Users who know their expected number of connections can then
prevent queue freezes on pre-created devices via nbds_max.

Before this patchset:
real    0m2.195s
user    0m0.005s
sys     0m0.022s

After this patchset:
real    0m0.090s
user    0m0.004s
sys     0m0.018s

Signed-off-by: Yang Erkun <yangerkun@huawei.com>
---
 drivers/block/nbd.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index 481911c5ac50..eec14617fa15 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -166,6 +166,7 @@ static struct dentry *nbd_dbg_dir;
 
 static unsigned int nbds_max = 16;
 static int max_part = 16;
+static int pre_defined_connections = 1;
 static int part_shift;
 
 static int nbd_dev_dbg_init(struct nbd_device *nbd);
@@ -2733,8 +2734,13 @@ static int __init nbd_init(void)
 	}
 	nbd_dbg_init();
 
+	if (pre_defined_connections < 1)
+		pre_defined_connections = 1;
+	/* Set to the intended connection count so nbd_start_device() can skip
+	 * the queue-freezing blk_mq_update_nr_hw_queues() call.
+	 */
 	for (i = 0; i < nbds_max; i++)
-		nbd_dev_add(i, 1, 1);
+		nbd_dev_add(i, 1, pre_defined_connections);
 	return 0;
 }
 
@@ -2795,3 +2801,6 @@ module_param(nbds_max, int, 0444);
 MODULE_PARM_DESC(nbds_max, "number of network block devices to initialize (default: 16)");
 module_param(max_part, int, 0444);
 MODULE_PARM_DESC(max_part, "number of partitions per device (default: 16)");
+module_param(pre_defined_connections, int, 0444);
+MODULE_PARM_DESC(pre_defined_connections,
+"number of connections for devices pre-created at module load (default: 1)");
-- 
2.52.0