[PATCH v4 7/7] nbd: add nr_hw_queues module parameter for pre-created devices

The function blk_mq_update_nr_hw_queues in nbd_start_device may causes a
queue freeze. The previous commit addressed this issue for newly created
nbd device via setting the expected nr_hw_queues in nbd_dev_add.
However, when reusing an old inactive nbd device, the queue freeze can
still occur when old nbd->tag_set->nr_hw_queues 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, we can add a
module parameter to allow changing the default nr_hw_queues. This way,
users who know their expected number of connections can 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 <[email protected]>
---
 drivers/block/nbd.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index 2bb98e41b99b..1d8cdc5713eb 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 nr_hw_queues = 1;
 static int part_shift;
 
 static int nbd_dev_dbg_init(struct nbd_device *nbd);
@@ -2733,8 +2734,10 @@ static int __init nbd_init(void)
 	}
 	nbd_dbg_init();
 
+	if (nr_hw_queues < 1)
+		nr_hw_queues = 1;
 	for (i = 0; i < nbds_max; i++)
-		nbd_dev_add(i, 1, 1);
+		nbd_dev_add(i, 1, nr_hw_queues);
 	return 0;
 }
 
@@ -2795,3 +2798,6 @@ 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(nr_hw_queues, int, 0444);
+MODULE_PARM_DESC(nr_hw_queues,
+"number of hardware queues for devices pre-created at module load (default: 1). ");
-- 
2.52.0