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

[Max Gurtovoy <mgurtovoy@nvidia.com>]

On 9/12/2021 12:50 PM, Michael S. Tsirkin wrote:
> On Sun, Sep 12, 2021 at 12:37:26PM +0300, Max Gurtovoy wrote:
>> On 9/12/2021 12:07 PM, Michael S. Tsirkin wrote:
>>> On Sat, Sep 11, 2021 at 03:56:45PM +0300, Max Gurtovoy wrote:
>>>> On 9/10/2021 1:57 AM, Michael S. Tsirkin wrote:
>>>>> On Thu, Sep 09, 2021 at 07:45:42PM +0300, Max Gurtovoy wrote:
>>>>>> On 9/9/2021 7:31 PM, Michael S. Tsirkin wrote:
>>>>>>> On Thu, Sep 09, 2021 at 06:51:56PM +0300, Max Gurtovoy wrote:
>>>>>>>> On 9/9/2021 6:40 PM, Michael S. Tsirkin wrote:
>>>>>>>>> On Thu, Sep 09, 2021 at 06:37:37PM +0300, Max Gurtovoy wrote:
>>>>>>>>>> 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");
>>>>>>>>> What's the default value? We should document that.
>>>>>>>> default value for static global variables is 0.
>>>>>>>>
>>>>>>>> MODULE_PARM_DESC(num_io_request_queues, "Number of request virt queues to
>>>>>>>> use for blk device. Minimum value is 1 queue. Default and Maximum value is
>>>>>>>> equal to the total number of CPUs");
>>>>>>> So it says it's the # of cpus but yoiu inspect it with
>>>>>>> sysfs and it's actually 0. Let's say that's confusing
>>>>>>> at the least. why not just let users set it to 0
>>>>>>> and document that?
>>>>>>>
>>>>>> Setting it by the user to 0 makes no sense.
>>>>>>
>>>>>> We can say "if not set, the value equals to the total number of CPUs".
>>>>> the value is 0. it seems to mean "no limit". the actual # of queues is
>>>>> then te smaller between # of cpus and # of hardware queues.
>>>>> I see no reason not to allow user to set that especially if
>>>>> it was originally the value then user changed it
>>>>> and is trying to change it back.
>>>> I fine with that.
>>>>
>>>> MODULE_PARM_DESC(num_io_request_queues, "Number of request virt queues to use for blk device. 0 value means no limitation");
>>>>
>>> OK and the second distinction is that it's a limit on the
>>> number, not the actual number. It's never more than what's provided
>>> by the hypervisor.
>> MODULE_PARM_DESC(num_io_request_queues, "Maximal number of request virt queues to use for blk device. 0 value means no limitation");
>>
>> is that ok ?
>
> Looks ok. And then do we need to limit this to nr_cpu_ids?
> Setting a value that's too high seems harmless ...

why would you want that ?


>
>>>>>> The actual value of the created queues can be seen in /sys/block/vda/mq/ as
>>>>>> done today.
>>>>>>>>>>>>>>>>> +
>>>>>>>>>>>>>>>>>          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).