Re: [PATCH net] vhost_net: flush batched heads before trying to busy polling

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

On Tue, May 29, 2018 at 02:18:19PM +0800, Jason Wang wrote:
> After commit e2b3b35eb989 ("vhost_net: batch used ring update in rx"),
> we tend to batch updating used heads. But it doesn't flush batched
> heads before trying to do busy polling, this will cause vhost to wait
> for guest TX which waits for the used RX. Fixing by flush batched
> heads before busy loop.
> 
> 1 byte TCP_RR performance recovers from 13107.83 to 50402.65.
> 
> Fixes: e2b3b35eb989 ("vhost_net: batch used ring update in rx")
> Signed-off-by: Jason Wang <jasowang@redhat.com>

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

> ---
>  drivers/vhost/net.c | 37 ++++++++++++++++++++++++-------------
>  1 file changed, 24 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 986058a..eeaf673 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -105,7 +105,9 @@ struct vhost_net_virtqueue {
>  	/* vhost zerocopy support fields below: */
>  	/* last used idx for outstanding DMA zerocopy buffers */
>  	int upend_idx;
> -	/* first used idx for DMA done zerocopy buffers */
> +	/* For TX, first used idx for DMA done zerocopy buffers
> +	 * For RX, number of batched heads
> +	 */
>  	int done_idx;
>  	/* an array of userspace buffers info */
>  	struct ubuf_info *ubuf_info;
> @@ -626,6 +628,18 @@ static int sk_has_rx_data(struct sock *sk)
>  	return skb_queue_empty(&sk->sk_receive_queue);
>  }
>  
> +static void vhost_rx_signal_used(struct vhost_net_virtqueue *nvq)
> +{
> +	struct vhost_virtqueue *vq = &nvq->vq;
> +	struct vhost_dev *dev = vq->dev;
> +
> +	if (!nvq->done_idx)
> +		return;
> +
> +	vhost_add_used_and_signal_n(dev, vq, vq->heads, nvq->done_idx);
> +	nvq->done_idx = 0;
> +}
> +
>  static int vhost_net_rx_peek_head_len(struct vhost_net *net, struct sock *sk)
>  {
>  	struct vhost_net_virtqueue *rvq = &net->vqs[VHOST_NET_VQ_RX];
> @@ -635,6 +649,8 @@ static int vhost_net_rx_peek_head_len(struct vhost_net *net, struct sock *sk)
>  	int len = peek_head_len(rvq, sk);
>  
>  	if (!len && vq->busyloop_timeout) {
> +		/* Flush batched heads first */
> +		vhost_rx_signal_used(rvq);
>  		/* Both tx vq and rx socket were polled here */
>  		mutex_lock_nested(&vq->mutex, 1);
>  		vhost_disable_notify(&net->dev, vq);
> @@ -762,7 +778,7 @@ static void handle_rx(struct vhost_net *net)
>  	};
>  	size_t total_len = 0;
>  	int err, mergeable;
> -	s16 headcount, nheads = 0;
> +	s16 headcount;
>  	size_t vhost_hlen, sock_hlen;
>  	size_t vhost_len, sock_len;
>  	struct socket *sock;
> @@ -790,8 +806,8 @@ static void handle_rx(struct vhost_net *net)
>  	while ((sock_len = vhost_net_rx_peek_head_len(net, sock->sk))) {
>  		sock_len += sock_hlen;
>  		vhost_len = sock_len + vhost_hlen;
> -		headcount = get_rx_bufs(vq, vq->heads + nheads, vhost_len,
> -					&in, vq_log, &log,
> +		headcount = get_rx_bufs(vq, vq->heads + nvq->done_idx,
> +					vhost_len, &in, vq_log, &log,
>  					likely(mergeable) ? UIO_MAXIOV : 1);
>  		/* On error, stop handling until the next kick. */
>  		if (unlikely(headcount < 0))
> @@ -862,12 +878,9 @@ static void handle_rx(struct vhost_net *net)
>  			vhost_discard_vq_desc(vq, headcount);
>  			goto out;
>  		}
> -		nheads += headcount;
> -		if (nheads > VHOST_RX_BATCH) {
> -			vhost_add_used_and_signal_n(&net->dev, vq, vq->heads,
> -						    nheads);
> -			nheads = 0;
> -		}
> +		nvq->done_idx += headcount;
> +		if (nvq->done_idx > VHOST_RX_BATCH)
> +			vhost_rx_signal_used(nvq);
>  		if (unlikely(vq_log))
>  			vhost_log_write(vq, vq_log, log, vhost_len);
>  		total_len += vhost_len;
> @@ -878,9 +891,7 @@ static void handle_rx(struct vhost_net *net)
>  	}
>  	vhost_net_enable_vq(net, vq);
>  out:
> -	if (nheads)
> -		vhost_add_used_and_signal_n(&net->dev, vq, vq->heads,
> -					    nheads);
> +	vhost_rx_signal_used(nvq);
>  	mutex_unlock(&vq->mutex);
>  }
>  
> -- 
> 2.7.4