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nbd: add pre_defined_connections module parameter for pre-created devices

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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>
Reviewed-by: Yu Kuai <yukuai@fygo.io>
Link: https://patch.msgid.link/20260805122930.57647-9-yangerkun@huawei.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Yang Erkun 2026-08-05 20:29:30 +08:00 committed by Jens Axboe
parent a9d414b4a1
commit 326d49039c
1 changed files with 15 additions and 1 deletions
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16
drivers/block/nbd.c
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@ -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);
@ -2712,6 +2713,12 @@ static int __init nbd_init(void)
if (nbds_max > 1UL << (MINORBITS - part_shift))
return -EINVAL;
/* An excessively large value will be adjusted in blk_mq_alloc_tag_set */
if (pre_defined_connections < 1) {
pr_err("pre_defined_connections must be >= 1\n");
return -EINVAL;
}
if (register_blkdev(NBD_MAJOR, "nbd"))
return -EIO;
@ -2728,8 +2735,12 @@ static int __init nbd_init(void)
}
nbd_dbg_init();
/*
* 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;
}
@ -2790,3 +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)");