Subject: Re: [net-next rfc v7 3/3] virtio-net: change the number of queues through ethtool

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 3 Dec 2012 13:25:07 +0200

On Mon, Dec 03, 2012 at 02:09:28PM +0800, Jason Wang wrote:
> On Sunday, December 02, 2012 06:09:06 PM Michael S. Tsirkin wrote:
> > On Tue, Nov 27, 2012 at 06:16:00PM +0800, Jason Wang wrote:
> > > This patch implement the {set|get}_channels method of ethool to allow user
> > > to change the number of queues dymaically when the device is running.
> > > This would let the user to configure it on demand.
> > > 
> > > Signed-off-by: Jason Wang <[email-masked]>
> > > ---
> > > 
> > >  drivers/net/virtio_net.c |   41 +++++++++++++++++++++++++++++++++++++++++
> > >  1 files changed, 41 insertions(+), 0 deletions(-)
> > > 
> > > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > > index bcaa6e5..f08ec2a 100644
> > > --- a/drivers/net/virtio_net.c
> > > +++ b/drivers/net/virtio_net.c
> > > @@ -1578,10 +1578,51 @@ static struct virtio_driver virtio_net_driver = {
> > > 
> > >  #endif
> > >  };
> > > 
> > > +/* TODO: Eliminate OOO packets during switching */
> > > +static int virtnet_set_channels(struct net_device *dev,
> > > +				struct ethtool_channels *channels)
> > > +{
> > > +	struct virtnet_info *vi = netdev_priv(dev);
> > > +	u16 queue_pairs = channels->combined_count;

by the way shouldn't this be combined_count / 2?

And below channels->max_combined = vi->max_queue_pairs * 2; ?

> > > +
> > > +	/* We don't support separate rx/tx channels.
> > > +	 * We don't allow setting 'other' channels.
> > > +	 */
> > > +	if (channels->rx_count || channels->tx_count || channels->other_count)
> > > +		return -EINVAL;
> > > +
> > > +	/* Only two modes were support currently */
> > > +	if (queue_pairs != vi->max_queue_pairs && queue_pairs != 1)
> > > +		return -EINVAL;
> > > +
> > 
> > Why the limitation?
> 
> Not sure the value bettwen 1 and max_queue_pairs is useful. But anyway, I can 
> remove this limitation.

I guess a reasonable value would be number of active CPUs,
which can change with CPU hotplug.

> > Also how does userspace discover what the legal values are?
> 
> Userspace only check whether the value is greater than max_queue_pairs.

Exactly. One can query max combined but the fact that configuring
any less is impossible might be surprising.

> > 
> > > +	vi->curr_queue_pairs = queue_pairs;
> > > +	BUG_ON(virtnet_set_queues(vi));
> > > +
> > > +	netif_set_real_num_tx_queues(dev, vi->curr_queue_pairs);
> > > +	netif_set_real_num_rx_queues(dev, vi->curr_queue_pairs);
> > > +
> > > +	return 0;
> > > +}
> > > +
> > > +static void virtnet_get_channels(struct net_device *dev,
> > > +				 struct ethtool_channels *channels)
> > > +{
> > > +	struct virtnet_info *vi = netdev_priv(dev);
> > > +
> > > +	channels->combined_count = vi->curr_queue_pairs;
> > > +	channels->max_combined = vi->max_queue_pairs;
> > > +	channels->max_other = 0;
> > > +	channels->rx_count = 0;
> > > +	channels->tx_count = 0;
> > > +	channels->other_count = 0;
> > > +}
> > > +
> > > 
> > >  static const struct ethtool_ops virtnet_ethtool_ops = {
> > >  
> > >  	.get_drvinfo = virtnet_get_drvinfo,
> > >  	.get_link = ethtool_op_get_link,
> > >  	.get_ringparam = virtnet_get_ringparam,
> > > 
> > > +	.set_channels = virtnet_set_channels,
> > > +	.get_channels = virtnet_get_channels,
> > > 
> > >  };
> > >  
> > >  static int __init init(void)
> > 
> > --
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