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

[Max Gurtovoy <mgurtovoy@nvidia.com>]

On 9/9/2021 4:42 PM, Michael S. Tsirkin wrote:
> On Mon, Sep 06, 2021 at 02:59:40PM +0300, Max Gurtovoy wrote:
>> On 9/6/2021 2:20 PM, Michael S. Tsirkin wrote:
>>> On Mon, Sep 06, 2021 at 01:31:32AM +0300, Max Gurtovoy wrote:
>>>> 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);
>>>>>>> +}
>>> Hmm which tree is this for?
>> I've mentioned in the note that it apply on branch for-5.15/drivers. But now
>> you can apply it on linus/master as well.
>>
>>
>>>>>>> +
>>>>>>> +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");
>>>
>>> better:
>>>
>>> +MODULE_PARM_DESC(num_io_request_queues,
>>> +                "Limit number of IO request virt queues to use for each device. 0 for now limit");
>> You proposed it and I replied on it bellow.
>
> Can't say I understand 100% what you are saying. Want to send
> a description that does make sense to you but
> also reflects reality? 0 is the default so
> "should > 0" besides being ungrammatical does not seem t"
> reflect what it does ...

if you "modprobe virtio_blk" the previous behavior will happen.

You can't "modprobe virtio_blk num_io_request_queues=0" since the 
minimal value is 1.

So your description is not reflecting the code.

We can do:

MODULE_PARM_DESC(num_io_request_queues, "Number of request virt queues to use for blk device. Minimum value is 1 queue");

>
>>>
>>>>>>> +
>>>>>>>     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).