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

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

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.

> > 
> > > 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).