Subject: Re: [PATCH V3 1/2] vhost_net: stop polling socket during rx processing

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 7 Jun 2016 15:23:13 +0300

On Wed, Jun 01, 2016 at 01:56:33AM -0400, Jason Wang wrote:
> We don't stop rx polling socket during rx processing, this will lead
> unnecessary wakeups from under layer net devices (E.g
> sock_def_readable() form tun). Rx will be slowed down in this
> way. This patch avoids this by stop polling socket during rx
> processing. A small drawback is that this introduces some overheads in
> light load case because of the extra start/stop polling, but single
> netperf TCP_RR does not notice any change. In a super heavy load case,
> e.g using pktgen to inject packet to guest, we get about ~8.8%
> improvement on pps:
> 
> before: ~1240000 pkt/s
> after:  ~1350000 pkt/s
> 
> Signed-off-by: Jason Wang <[email-masked]>

I guess this works though I suspect it's even faster to
maintain some state in the vq structure so we don't need
to play with waitq all the time.

For now

Acked-by: Michael S. Tsirkin <[email-masked]>

> ---
>  drivers/vhost/net.c | 64 +++++++++++++++++++++++++++--------------------------
>  1 file changed, 33 insertions(+), 31 deletions(-)
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index f744eeb..1d3e45f 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -301,6 +301,32 @@ static bool vhost_can_busy_poll(struct vhost_dev *dev,
>  	       !vhost_has_work(dev);
>  }
>  
> +static void vhost_net_disable_vq(struct vhost_net *n,
> +				 struct vhost_virtqueue *vq)
> +{
> +	struct vhost_net_virtqueue *nvq =
> +		container_of(vq, struct vhost_net_virtqueue, vq);
> +	struct vhost_poll *poll = n->poll + (nvq - n->vqs);
> +	if (!vq->private_data)
> +		return;
> +	vhost_poll_stop(poll);
> +}
> +
> +static int vhost_net_enable_vq(struct vhost_net *n,
> +				struct vhost_virtqueue *vq)
> +{
> +	struct vhost_net_virtqueue *nvq =
> +		container_of(vq, struct vhost_net_virtqueue, vq);
> +	struct vhost_poll *poll = n->poll + (nvq - n->vqs);
> +	struct socket *sock;
> +
> +	sock = vq->private_data;
> +	if (!sock)
> +		return 0;
> +
> +	return vhost_poll_start(poll, sock->file);
> +}
> +
>  static int vhost_net_tx_get_vq_desc(struct vhost_net *net,
>  				    struct vhost_virtqueue *vq,
>  				    struct iovec iov[], unsigned int iov_size,
> @@ -613,6 +639,7 @@ static void handle_rx(struct vhost_net *net)
>  	if (!sock)
>  		goto out;
>  	vhost_disable_notify(&net->dev, vq);
> +	vhost_net_disable_vq(net, vq);
>  
>  	vhost_hlen = nvq->vhost_hlen;
>  	sock_hlen = nvq->sock_hlen;
> @@ -629,7 +656,7 @@ static void handle_rx(struct vhost_net *net)
>  					likely(mergeable) ? UIO_MAXIOV : 1);
>  		/* On error, stop handling until the next kick. */
>  		if (unlikely(headcount < 0))
> -			break;
> +			goto out;
>  		/* On overrun, truncate and discard */
>  		if (unlikely(headcount > UIO_MAXIOV)) {
>  			iov_iter_init(&msg.msg_iter, READ, vq->iov, 1, 1);
> @@ -648,7 +675,7 @@ static void handle_rx(struct vhost_net *net)
>  			}
>  			/* Nothing new?  Wait for eventfd to tell us
>  			 * they refilled. */
> -			break;
> +			goto out;
>  		}
>  		/* We don't need to be notified again. */
>  		iov_iter_init(&msg.msg_iter, READ, vq->iov, in, vhost_len);
> @@ -676,7 +703,7 @@ static void handle_rx(struct vhost_net *net)
>  					 &fixup) != sizeof(hdr)) {
>  				vq_err(vq, "Unable to write vnet_hdr "
>  				       "at addr %p\n", vq->iov->iov_base);
> -				break;
> +				goto out;
>  			}
>  		} else {
>  			/* Header came from socket; we'll need to patch
> @@ -692,7 +719,7 @@ static void handle_rx(struct vhost_net *net)
>  				 &fixup) != sizeof num_buffers) {
>  			vq_err(vq, "Failed num_buffers write");
>  			vhost_discard_vq_desc(vq, headcount);
> -			break;
> +			goto out;
>  		}
>  		vhost_add_used_and_signal_n(&net->dev, vq, vq->heads,
>  					    headcount);
> @@ -701,9 +728,10 @@ static void handle_rx(struct vhost_net *net)
>  		total_len += vhost_len;
>  		if (unlikely(total_len >= VHOST_NET_WEIGHT)) {
>  			vhost_poll_queue(&vq->poll);
> -			break;
> +			goto out;
>  		}
>  	}
> +	vhost_net_enable_vq(net, vq);
>  out:
>  	mutex_unlock(&vq->mutex);
>  }
> @@ -782,32 +810,6 @@ static int vhost_net_open(struct inode *inode, struct file *f)
>  	return 0;
>  }
>  
> -static void vhost_net_disable_vq(struct vhost_net *n,
> -				 struct vhost_virtqueue *vq)
> -{
> -	struct vhost_net_virtqueue *nvq =
> -		container_of(vq, struct vhost_net_virtqueue, vq);
> -	struct vhost_poll *poll = n->poll + (nvq - n->vqs);
> -	if (!vq->private_data)
> -		return;
> -	vhost_poll_stop(poll);
> -}
> -
> -static int vhost_net_enable_vq(struct vhost_net *n,
> -				struct vhost_virtqueue *vq)
> -{
> -	struct vhost_net_virtqueue *nvq =
> -		container_of(vq, struct vhost_net_virtqueue, vq);
> -	struct vhost_poll *poll = n->poll + (nvq - n->vqs);
> -	struct socket *sock;
> -
> -	sock = vq->private_data;
> -	if (!sock)
> -		return 0;
> -
> -	return vhost_poll_start(poll, sock->file);
> -}
> -
>  static struct socket *vhost_net_stop_vq(struct vhost_net *n,
>  					struct vhost_virtqueue *vq)
>  {
> -- 
> 1.8.3.1