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

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

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 ...

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

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