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: sta...@dpdk.org
> Signed-off-by: Eugenio Pérez <epere...@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;
> }
>
>
