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

From: Tiwei Bie
Date: Thu Nov 08 2018 - 20:51:50 EST


On Thu, Nov 08, 2018 at 10:56:02AM -0500, Michael S. Tsirkin wrote:
> On Thu, Nov 08, 2018 at 07:51:48PM +0800, Tiwei Bie 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.
> >
> > Do you know why removing this notify in Tx will break "the
> > heuristics which KVM uses"? Or what does "the heuristics
> > which KVM uses" refer to?
>
> Yes. QEMU has a mode where it disables notifications and processes TX
> ring periodically from a timer. It's off by default but used to be on
> by default a long time ago. If ring becomes full this causes traffic
> stalls. As a work-around Rusty put in this hack to kick on ring full
> even with notifications disabled. It's easy enough to make sure QEMU
> does not combine devices with packed ring support with the timer hack.
> And I am guessing it's safe enough to also block that option completely
> e.g. when virtio 1.0 is enabled.

I see. Thanks!

>
> >
> > > 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. Consider the removal of lguest support recently,
> > > maybe we can drop this for split ring as well?
> > >
> > > Thanks
> > >
> > >
> > > >
> > > > commit 44653eae1407f79dff6f52fcf594ae84cb165ec4
> > > > Author: Rusty Russell<rusty@xxxxxxxxxxxxxxx>
> > > > 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@xxxxxxxxxxxxxxx>
> > > >
> > > > 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;
> > > > }
> > > >
> > > >