Re: [PATCH net-next v2 3/5] virtio_ring: add packed ring support

[Jason Wang <jasowang@redhat.com>]

On 2018/11/8 äå10:14, Michael S. Tsirkin wrote:
> On Thu, Nov 08, 2018 at 04:18:25PM +0800, Jason Wang wrote:
> > On 2018/11/8 äå9:38, Tiwei Bie wrote:
> > > > > +
> > > > > +	if (vq->vq.num_free < descs_used) {
> > > > > +		pr_debug("Can't add buf len %i - avail = %i\n",
> > > > > +			 descs_used, vq->vq.num_free);
> > > > > +		/* FIXME: for historical reasons, we force a notify here if
> > > > > +		 * there are outgoing parts to the buffer.  Presumably the
> > > > > +		 * host should service the ring ASAP. */
> > > > I don't think we have a reason to do this for packed ring.
> > > > No historical baggage there, right?
> > > Based on the original commit log, it seems that the notify here
> > > is just an "optimization". But I don't quite understand what does
> > > the "the heuristics which KVM uses" refer to. If it's safe to drop
> > > this in packed ring, I'd like to do it.
> >
> > According to the commit log, it seems like a workaround of lguest networking
> > backend. I agree to drop it, we should not have such burden.
> >
> > But we should notice that, with this removed, the compare between packed vs
> > split is kind of unfair.
> I don't think this ever triggers to be frank. When would it?


I think it can happen e.g in the path of XDP transmission in 
__virtnet_xdp_xmit_one():


ÂÂÂÂÂÂÂ err = virtqueue_add_outbuf(sq->vq, sq->sg, 1, xdpf, GFP_ATOMIC);
ÂÂÂÂÂÂÂ if (unlikely(err))
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ return -ENOSPC; /* Caller handle free/refcnt */


> > Consider the removal of lguest support recently,
> > maybe we can drop this for split ring as well?
> >
> > Thanks
> If it's helpful, then for sure we can drop it for virtio 1.
> Can you see any perf differences at all? With which device?


I don't test but consider the case of XDP_TX in guest plus vhost_net in 
host. Since vhost_net is half duplex, it's pretty easier to trigger this 
condition.

Thanks


> > > commit 44653eae1407f79dff6f52fcf594ae84cb165ec4
> > > Author: Rusty Russell<rusty@rustcorp.com.au>
> > > Date:   Fri Jul 25 12:06:04 2008 -0500
> > >
> > >       virtio: don't always force a notification when ring is full
> > >       We force notification when the ring is full, even if the host has
> > >       indicated it doesn't want to know.  This seemed like a good idea at
> > >       the time: if we fill the transmit ring, we should tell the host
> > >       immediately.
> > >       Unfortunately this logic also applies to the receiving ring, which is
> > >       refilled constantly.  We should introduce real notification thesholds
> > >       to replace this logic.  Meanwhile, removing the logic altogether breaks
> > >       the heuristics which KVM uses, so we use a hack: only notify if there are
> > >       outgoing parts of the new buffer.
> > >       Here are the number of exits with lguest's crappy network implementation:
> > >       Before:
> > >               network xmit 7859051 recv 236420
> > >       After:
> > >               network xmit 7858610 recv 118136
> > >       Signed-off-by: Rusty Russell<rusty@rustcorp.com.au>
> > >
> > > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > > index 72bf8bc09014..21d9a62767af 100644
> > > --- a/drivers/virtio/virtio_ring.c
> > > +++ b/drivers/virtio/virtio_ring.c
> > > @@ -87,8 +87,11 @@ static int vring_add_buf(struct virtqueue *_vq,
> > >    	if (vq->num_free < out + in) {
> > >    		pr_debug("Can't add buf len %i - avail = %i\n",
> > >    			 out + in, vq->num_free);
> > > -		/* We notify*even if*  VRING_USED_F_NO_NOTIFY is set here. */
> > > -		vq->notify(&vq->vq);
> > > +		/* FIXME: for historical reasons, we force a notify here if
> > > +		 * there are outgoing parts to the buffer.  Presumably the
> > > +		 * host should service the ring ASAP. */
> > > +		if (out)
> > > +			vq->notify(&vq->vq);
> > >    		END_USE(vq);
> > >    		return -ENOSPC;
> > >    	}