On Fri, Nov 09, 2018 at 10:30:50AM +0800, Jason Wang wrote:
>
> On 2018/11/8 下午11:56, 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 上午9:38, 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.
>
>
> Do you mean tx-timer? If yes, we can still enable it for packed ring
Yes we can but I doubt anyone does.
> and the
> timer will finally fired and we can go.
on tx ring full we probably don't want to wait for timer.
But I think we can just prevent qemu from using tx timer
with virtio 1.
>
> > As a work-around Rusty put in this hack to kick on ring full
> > even with notifications disabled.
>
>
> From the commit log it looks more like a performance workaround instead of a
> bug fix.
it's a quality of implementation issue, yes.
>
> > 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 agree.
>
> 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;
> > > > > }
> > > > >
> > > > >
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