Re: [PATCH v2] vhost: batch used descs chains write-back with packed ring

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Dec 19, 2018 at 10:29:52AM +0100, Maxime Coquelin wrote:
> Instead of writing back descriptors chains in order, let's
> write the first chain flags last in order to improve batching.
> 
> With Kernel's pktgen benchmark, ~3% performance gain is measured.
> 
> Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>

Acked-by: Michael S. Tsirkin <mst@redhat.com>

> ---
> 
> V2:
> Revert back to initial implementation to have a write
> barrier before every descs flags store, but still
> store first desc flags last. (Missing barrier reported
> by Ilya)
> 
> 
>  lib/librte_vhost/virtio_net.c | 19 ++++++++++++++++---
>  1 file changed, 16 insertions(+), 3 deletions(-)
> 
> diff --git a/lib/librte_vhost/virtio_net.c b/lib/librte_vhost/virtio_net.c
> index 8c657a101..de436af79 100644
> --- a/lib/librte_vhost/virtio_net.c
> +++ b/lib/librte_vhost/virtio_net.c
> @@ -97,6 +97,8 @@ flush_shadow_used_ring_packed(struct virtio_net *dev,
>  {
>  	int i;
>  	uint16_t used_idx = vq->last_used_idx;
> +	uint16_t head_idx = vq->last_used_idx;
> +	uint16_t head_flags = 0;
>  
>  	/* Split loop in two to save memory barriers */
>  	for (i = 0; i < vq->shadow_used_idx; i++) {
> @@ -126,12 +128,17 @@ flush_shadow_used_ring_packed(struct virtio_net *dev,
>  			flags &= ~VRING_DESC_F_AVAIL;
>  		}
>  
> -		vq->desc_packed[vq->last_used_idx].flags = flags;
> +		if (i > 0) {
> +			vq->desc_packed[vq->last_used_idx].flags = flags;
>  
> -		vhost_log_cache_used_vring(dev, vq,
> +			vhost_log_cache_used_vring(dev, vq,
>  					vq->last_used_idx *
>  					sizeof(struct vring_packed_desc),
>  					sizeof(struct vring_packed_desc));
> +		} else {
> +			head_idx = vq->last_used_idx;
> +			head_flags = flags;
> +		}
>  
>  		vq->last_used_idx += vq->shadow_used_packed[i].count;
>  		if (vq->last_used_idx >= vq->size) {
> @@ -140,7 +147,13 @@ flush_shadow_used_ring_packed(struct virtio_net *dev,
>  		}
>  	}
>  
> -	rte_smp_wmb();
> +	vq->desc_packed[head_idx].flags = head_flags;
> +
> +	vhost_log_cache_used_vring(dev, vq,
> +				vq->last_used_idx *
> +				sizeof(struct vring_packed_desc),
> +				sizeof(struct vring_packed_desc));
> +
>  	vq->shadow_used_idx = 0;
>  	vhost_log_cache_sync(dev, vq);
>  }
> -- 
> 2.17.2