Re: [PATCH v2 1/1] virtio-blk: add num_io_queues module parameter

[Max Gurtovoy <mgurtovoy@nvidia.com>]

On 9/5/2021 7:02 PM, Michael S. Tsirkin wrote:
> On Thu, Sep 02, 2021 at 02:45:52PM +0100, Stefan Hajnoczi wrote:
>> On Tue, Aug 31, 2021 at 04:50:35PM +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>
>>> ---
>>>
>>> changes from v1:
>>>   - use param_set_uint_minmax (from Christoph)
>>>   - added "Should > 0" to module description
>>>
>>> Note: This commit apply on top of Jens's branch for-5.15/drivers
>>> ---
>>>   drivers/block/virtio_blk.c | 20 +++++++++++++++++++-
>>>   1 file changed, 19 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
>>> index 4b49df2dfd23..9332fc4e9b31 100644
>>> --- a/drivers/block/virtio_blk.c
>>> +++ b/drivers/block/virtio_blk.c
>>> @@ -24,6 +24,22 @@
>>>   /* 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)
>>> +{
>>> +	return param_set_uint_minmax(val, kp, 1, nr_cpu_ids);
>>> +}
>>> +
>>> +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. Should > 0");
>>> +
>>>   static int major;
>>>   static DEFINE_IDA(vd_index_ida);
>>>   
>>> @@ -501,7 +517,9 @@ 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);
>> If you respin, please consider calling them request queues. That's the
>> terminology from the VIRTIO spec and it's nice to keep it consistent.
>> But the purpose of num_io_queues is clear, so:
>>
>> Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
> I did this:
> +static unsigned int num_io_request_queues;
> +module_param_cb(num_io_request_queues, &queue_count_ops, &num_io_request_queues, 0644);
> +MODULE_PARM_DESC(num_io_request_queues,
> +                "Limit number of IO request virt queues to use for each device. 0 for now limit");

The parameter is writable and can be changed and then new devices might 
be probed with new value.

It can't be zero in the code. we can change param_set_uint_minmax args 
and say that 0 says nr_cpus.

I'm ok with the renaming but I prefer to stick to the description we 
gave in V3 of this patch (and maybe enable value of 0 as mentioned above).