Re: [dpdk-dev] [PATCH] vhost: flush shadow tx if there is no more packets

[Eugenio Perez Martin <eperezma@redhat.com>]

Hi Kevin!

Sorry, thanks for noticing it! It fixes commit ("31d6c6a5b vhost: optimize
packed ring dequeue"), what was not present on 18.11 version (I've checked
that v19.08 does not contain the failure).

Do I need to send another patch version with corrected commit message?

Thanks!

On Fri, Jan 31, 2020 at 7:38 PM Kevin Traynor <ktraynor@redhat.com> wrote:

> Hi Eugenio,
>
> On 29/01/2020 19:33, Eugenio Pérez wrote:
> > The current implementation of vhost_net in packed vring tries to fill
> > the shadow vector before send any actual changes to the guest. While
> > this can be beneficial for the throughput, it conflicts with some
> > bufferfloats methods like the linux kernel napi, that stops
> > transmitting packets if there are too much bytes/buffers in the
> > driver.
> >
> > To solve it, we flush the shadow packets at the end of
> > virtio_dev_tx_packed if we have starved the vring, i.e., the next
> > buffer is not available for the device.
> >
> > Since this last check can be expensive because of the atomic, we only
> > check it if we have not obtained the expected (count) packets. If it
> > happens to obtain "count" packets and there is no more available
> > packets the caller needs to keep call virtio_dev_tx_packed again.
> >
>
> It seems to be fixing an issue and should be considered for stable
> branches? You can add the tags needed in the commit message here:
>
> Fixes: <commit that introduced bug/missed this case>
> Cc: stable@dpdk.org
>
> > Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> > ---
> >  lib/librte_vhost/virtio_net.c | 27 ++++++++++++++++++++++++++-
> >  1 file changed, 26 insertions(+), 1 deletion(-)
> >
> > diff --git a/lib/librte_vhost/virtio_net.c
> b/lib/librte_vhost/virtio_net.c
> > index 21c311732..ac2842b2d 100644
> > --- a/lib/librte_vhost/virtio_net.c
> > +++ b/lib/librte_vhost/virtio_net.c
> > @@ -2133,6 +2133,20 @@ virtio_dev_tx_packed_zmbuf(struct virtio_net *dev,
> >       return pkt_idx;
> >  }
> >
> > +static __rte_always_inline bool
> > +next_desc_is_avail(const struct vhost_virtqueue *vq)
> > +{
> > +     bool wrap_counter = vq->avail_wrap_counter;
> > +     uint16_t next_used_idx = vq->last_used_idx + 1;
> > +
> > +     if (next_used_idx >= vq->size) {
> > +             next_used_idx -= vq->size;
> > +             wrap_counter ^= 1;
> > +     }
> > +
> > +     return desc_is_avail(&vq->desc_packed[next_used_idx],
> wrap_counter);
> > +}
> > +
> >  static __rte_noinline uint16_t
> >  virtio_dev_tx_packed(struct virtio_net *dev,
> >                    struct vhost_virtqueue *vq,
> > @@ -2165,9 +2179,20 @@ virtio_dev_tx_packed(struct virtio_net *dev,
> >
> >       } while (remained);
> >
> > -     if (vq->shadow_used_idx)
> > +     if (vq->shadow_used_idx) {
> >               do_data_copy_dequeue(vq);
> >
> > +             if (remained && !next_desc_is_avail(vq)) {
> > +                     /*
> > +                      * The guest may be waiting to TX some buffers to
> > +                      * enqueue more to avoid bufferfloat, so we try to
> > +                      * reduce latency here.
> > +                      */
> > +                     vhost_flush_dequeue_shadow_packed(dev, vq);
> > +                     vhost_vring_call_packed(dev, vq);
> > +             }
> > +     }
> > +
> >       return pkt_idx;
> >  }
> >
> >
>
>