Subject: Re: [PATCH v1 1/3] virtio-net: Using single MSIX IRQ for TX/RX Q pair

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 28 Oct 2015 09:21:34 +0200

On Wed, Oct 28, 2015 at 11:13:39AM +0800, Jason Wang wrote:
> 
> 
> On 10/27/2015 04:38 PM, Michael S. Tsirkin wrote:
> > On Mon, Oct 26, 2015 at 10:52:47AM -0700, Ravi Kerur wrote:
> >> Ported earlier patch from Jason Wang (dated 12/26/2014).
> >>
> >> This patch tries to reduce the number of MSIX irqs required for
> >> virtio-net by sharing a MSIX irq for each TX/RX queue pair through
> >> channels. If transport support channel, about half of the MSIX irqs
> >> were reduced.
> >>
> >> Signed-off-by: Ravi Kerur <[email-masked]>
> > Why bother BTW? 
> 
> The reason is we want to save the number of interrupt vectors used.
> Booting a guest with 256 queues with current driver will result all
> tx/rx queues shares a single vector. This is suboptimal.

With a single CPU? But what configures so many queues? Why do it?

> With this
> series, half could be saved.

At cost of e.g. inability to balance the interrupts.

> And more complex policy could be applied on
> top (e.g limit the number of vectors used by driver).

If that's the motivation, I'd like to see a draft of that more complex
policy first.

> > Looks like this is adding a bunch of overhead
> > on data path - to what end?
> 
> I agree some benchmark is needed for this.
> 
> > Maybe you have a huge number of these devices ... but in that case, how
> > about sharing the config interrupt instead?
> > That's only possible if host supports VIRTIO_1
> > (so we can detect config interrupt by reading the ISR).
> >
> >
> >
> >> ---
> >>  drivers/net/virtio_net.c | 29 ++++++++++++++++++++++++++++-
> >>  1 file changed, 28 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> >> index d8838ded..d705cce 100644
> >> --- a/drivers/net/virtio_net.c
> >> +++ b/drivers/net/virtio_net.c
> >> @@ -72,6 +72,9 @@ struct send_queue {
> >>  
> >>  	/* Name of the send queue: output.$index */
> >>  	char name[40];
> >> +
> >> +	/* Name of the channel, shared with irq. */
> >> +	char channel_name[40];
> >>  };
> >>  
> >>  /* Internal representation of a receive virtqueue */
> >> @@ -1529,6 +1532,8 @@ static int virtnet_find_vqs(struct virtnet_info *vi)
> >>  	int ret = -ENOMEM;
> >>  	int i, total_vqs;
> >>  	const char **names;
> >> +	const char **channel_names;
> >> +	unsigned *channels;
> >>  
> >>  	/* We expect 1 RX virtqueue followed by 1 TX virtqueue, followed by
> >>  	 * possible N-1 RX/TX queue pairs used in multiqueue mode, followed by
> >> @@ -1548,6 +1553,17 @@ static int virtnet_find_vqs(struct virtnet_info *vi)
> >>  	if (!names)
> >>  		goto err_names;
> >>  
> >> +	channel_names = kmalloc_array(vi->max_queue_pairs,
> >> +				      sizeof(*channel_names),
> >> +				      GFP_KERNEL);
> >> +	if (!channel_names)
> >> +		goto err_channel_names;
> >> +
> >> +	channels = kmalloc_array(total_vqs, sizeof(*channels),
> >> +				 GFP_KERNEL);
> >> +	if (!channels)
> >> +		goto err_channels;
> >> +
> >>  	/* Parameters for control virtqueue, if any */
> >>  	if (vi->has_cvq) {
> >>  		callbacks[total_vqs - 1] = NULL;
> >> @@ -1562,10 +1578,15 @@ static int virtnet_find_vqs(struct virtnet_info *vi)
> >>  		sprintf(vi->sq[i].name, "output.%d", i);
> >>  		names[rxq2vq(i)] = vi->rq[i].name;
> >>  		names[txq2vq(i)] = vi->sq[i].name;
> >> +		sprintf(vi->sq[i].channel_name, "txrx.%d", i);
> >> +		channel_names[i] = vi->sq[i].channel_name;
> >> +		channels[rxq2vq(i)] = i;
> >> +		channels[txq2vq(i)] = i;
> >>  	}
> >>  
> >>  	ret = vi->vdev->config->find_vqs(vi->vdev, total_vqs, vqs, callbacks,
> >> -					 names);
> >> +					 names, channels, channel_names,
> >> +					 vi->max_queue_pairs);
> >>  	if (ret)
> >>  		goto err_find;
> >>  
> >> @@ -1580,6 +1601,8 @@ static int virtnet_find_vqs(struct virtnet_info *vi)
> >>  		vi->sq[i].vq = vqs[txq2vq(i)];
> >>  	}
> >>  
> >> +	kfree(channels);
> >> +	kfree(channel_names);
> >>  	kfree(names);
> >>  	kfree(callbacks);
> >>  	kfree(vqs);
> >> @@ -1587,6 +1610,10 @@ static int virtnet_find_vqs(struct virtnet_info *vi)
> >>  	return 0;
> >>  
> >>  err_find:
> >> +	kfree(channels);
> >> +err_channels:
> >> +	kfree(channel_names);
> >> +err_channel_names:
> >>  	kfree(names);
> >>  err_names:
> >>  	kfree(callbacks);
> >> -- 
> >> 1.9.1