Re: [PATCH v5 8/8] nbd: add nr_hw_queues module parameter for pre-created devices

Hi,

=E5=9C=A8 2026/7/30 16:20, Yang Erkun =E5=86=99=E9=81=93:
> 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 6369a409e10c..2194c7d7b2ea 100644
> --- a/drivers/block/nbd.c
> +++ b/drivers/block/nbd.c
> @@ -166,6 +166,7 @@ static struct dentry *nbd_dbg_dir;
>  =20
>   static unsigned int nbds_max =3D 16;
>   static int max_part =3D 16;
> +static int nr_hw_queues =3D 1;
>   static int part_shift;
>  =20
>   static int nbd_dev_dbg_init(struct nbd_device *nbd);
> @@ -2740,8 +2741,10 @@ static int __init nbd_init(void)
>   	}
>   	nbd_dbg_init();
>  =20
> +	if (nr_hw_queues < 1)
> +		nr_hw_queues =3D 1;
>   	for (i =3D 0; i < nbds_max; i++)
> -		nbd_dev_add(i, 1, 1);
> +		nbd_dev_add(i, 1, nr_hw_queues);

I still feel the module param name nr_hw_queues really confusing. How about=
 rename it like
pre_defined_connections? And add comments here that user should config the =
exact number of
connections to avoid queue freeze from nbd_start_device().

>   	return 0;
>   }
>  =20
> @@ -2802,3 +2805,6 @@ module_param(nbds_max, int, 0444);
>   MODULE_PARM_DESC(nbds_max, "number of network block devices to initiali=
ze (default: 16)");
>   module_param(max_part, int, 0444);
>   MODULE_PARM_DESC(max_part, "number of partitions per device (default: 1=
6)");
> +module_param(nr_hw_queues, int, 0444);
> +MODULE_PARM_DESC(nr_hw_queues,
> +"number of hardware queues for devices pre-created at module load (defau=
lt: 1). ");

--=20
Thanks,
Kuai