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

[Kevin Traynor <ktraynor@redhat.com>]

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;
>  }
>  
>