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

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

Ouch, my bad again, sorry :).

I've forwarded the patch to stable@, please let me know if I need to do
something else.

Maxime, please let me know if I need to send a new version with the "Fixed:
" tag :).

Thanks!

On Tue, Feb 4, 2020 at 2:49 PM Kevin Traynor <ktraynor@redhat.com> wrote:

> On 04/02/2020 09:23, Eugenio Perez Martin wrote:
> > 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).
> >
>
> Right, in that case the issue is present on 19.11 stable, so it's worth
> adding the tags to get it fixed in 19.11 stable.
>
> > Do I need to send another patch version with corrected commit message?
> >
>
> Probably Maxime can do it on applying if you ask nicely :-)
>
> > 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;
> >>>  }
> >>>
> >>>
> >>
> >>
> >
>
>