Re: [PATCH v7 8/8] nbd: add pre_defined_connections module parameter for pre-created devices

Hi,

在 2026/8/5 14:58, Yang Erkun 写道:
> 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 <[email protected]>
> ---
>   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 b316129b9a17..f8f51df31ada 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;

nbd_init() will fail for invalid module parameter like max_part, nbds_max. So I
think it's better to fail for new parameter as well.

> +	/* Set to the intended connection count so nbd_start_device() can skip
> +	 * the queue-freezing blk_mq_update_nr_hw_queues() call.
> +	 */

Please follow the common comment style in this file.

/*
  * xxx
  */

>   	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)");

-- 
Thanks,
Kuai