Re: [PATCH 1/1] virtio-blk: add num_io_queues module parameter

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Aug 30, 2021 at 03:00:23PM +0300, Max Gurtovoy wrote:
> Sometimes a user would like to control the amount of IO queues to be
> created for a block device. For example, for limiting the memory
> footprint of virtio-blk devices.
> 
> Signed-off-by: Max Gurtovoy <mgurtovoy@nvidia.com>


Hmm. It's already limited by # of CPUs... Why not just limit
from the hypervisor side? What's the actual use-case here?

> ---
>  drivers/block/virtio_blk.c | 26 +++++++++++++++++++++++++-
>  1 file changed, 25 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> index e574fbf5e6df..77e8468e8593 100644
> --- a/drivers/block/virtio_blk.c
> +++ b/drivers/block/virtio_blk.c
> @@ -24,6 +24,28 @@
>  /* The maximum number of sg elements that fit into a virtqueue */
>  #define VIRTIO_BLK_MAX_SG_ELEMS 32768
>  
> +static int virtblk_queue_count_set(const char *val,
> +		const struct kernel_param *kp)
> +{
> +	unsigned int n;
> +	int ret;
> +
> +	ret = kstrtouint(val, 10, &n);
> +	if (ret != 0 || n > nr_cpu_ids)
> +		return -EINVAL;
> +	return param_set_uint(val, kp);
> +}
> +
> +static const struct kernel_param_ops queue_count_ops = {
> +	.set = virtblk_queue_count_set,
> +	.get = param_get_uint,
> +};
> +
> +static unsigned int num_io_queues;
> +module_param_cb(num_io_queues, &queue_count_ops, &num_io_queues, 0644);
> +MODULE_PARM_DESC(num_io_queues,
> +		 "Number of IO virt queues to use for blk device.");
> +
>  static int major;
>  static DEFINE_IDA(vd_index_ida);
>  
> @@ -501,7 +523,9 @@ static int init_vq(struct virtio_blk *vblk)
>  	if (err)
>  		num_vqs = 1;
>  
> -	num_vqs = min_t(unsigned int, nr_cpu_ids, num_vqs);
> +	num_vqs = min_t(unsigned int,
> +			min_not_zero(num_io_queues, nr_cpu_ids),
> +			num_vqs);
>  
>  	vblk->vqs = kmalloc_array(num_vqs, sizeof(*vblk->vqs), GFP_KERNEL);
>  	if (!vblk->vqs)
> -- 
> 2.18.1