Subject: Re: [PATCH net-next V4 10/10] vhost_net: try batch dequing from skb array

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 10 May 2017 15:34:08 +0300

On Wed, May 10, 2017 at 11:36:22AM +0800, Jason Wang wrote:
> We used to dequeue one skb during recvmsg() from skb_array, this could
> be inefficient because of the bad cache utilization and spinlock
> touching for each packet. This patch tries to batch them by calling
> batch dequeuing helpers explicitly on the exported skb array and pass
> the skb back through msg_control for underlayer socket to finish the
> userspace copying.
> 
> Batch dequeuing is also the requirement for more batching improvement
> on rx.
> 
> Tests were done by pktgen on tap with XDP1 in guest on top of batch
> zeroing:
> 
> rx batch | pps
> 
> 256        2.41Mpps (+6.16%)
> 128        2.48Mpps (+8.80%)
> 64         2.38Mpps (+3.96%) <- Default
> 16         2.31Mpps (+1.76%)
> 4          2.31Mpps (+1.76%)
> 1          2.30Mpps (+1.32%)
> 0          2.27Mpps (+7.48%)
> 
> Signed-off-by: Jason Wang <[email-masked]>
> ---
>  drivers/vhost/net.c | 117 +++++++++++++++++++++++++++++++++++++++++++++++++---
>  1 file changed, 111 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 9b51989..fbaecf3 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -28,6 +28,8 @@
>  #include <linux/if_macvlan.h>
>  #include <linux/if_tap.h>
>  #include <linux/if_vlan.h>
> +#include <linux/skb_array.h>
> +#include <linux/skbuff.h>
>  
>  #include <net/sock.h>
>  
> @@ -85,6 +87,13 @@ struct vhost_net_ubuf_ref {
>  	struct vhost_virtqueue *vq;
>  };
>  
> +#define VHOST_RX_BATCH 64
> +struct vhost_net_buf {
> +	struct sk_buff *queue[VHOST_RX_BATCH];
> +	int tail;
> +	int head;
> +};
> +

Do you strictly need to put this inline? This structure is quite big
already. Do you see a measureabe difference if you make it

	struct sk_buff **queue;
	int tail;
	int head;

?

Will also make it easier to play with the size in the future
should someone want to see how does it work e.g. for different
ring sizes.

>  struct vhost_net_virtqueue {
>  	struct vhost_virtqueue vq;
>  	size_t vhost_hlen;
> @@ -99,6 +108,8 @@ struct vhost_net_virtqueue {
>  	/* Reference counting for outstanding ubufs.
>  	 * Protected by vq mutex. Writers must also take device mutex. */
>  	struct vhost_net_ubuf_ref *ubufs;
> +	struct skb_array *rx_array;
> +	struct vhost_net_buf rxq;
>  };
>  
>  struct vhost_net {
> @@ -117,6 +128,71 @@ struct vhost_net {
>  
>  static unsigned vhost_net_zcopy_mask __read_mostly;
>  
> +static void *vhost_net_buf_get_ptr(struct vhost_net_buf *rxq)
> +{
> +	if (rxq->tail != rxq->head)
> +		return rxq->queue[rxq->head];
> +	else
> +		return NULL;
> +}
> +
> +static int vhost_net_buf_get_size(struct vhost_net_buf *rxq)
> +{
> +	return rxq->tail - rxq->head;
> +}
> +
> +static int vhost_net_buf_is_empty(struct vhost_net_buf *rxq)
> +{
> +	return rxq->tail == rxq->head;
> +}
> +
> +static void *vhost_net_buf_consume(struct vhost_net_buf *rxq)
> +{
> +	void *ret = vhost_net_buf_get_ptr(rxq);
> +	++rxq->head;
> +	return ret;
> +}
> +
> +static int vhost_net_buf_produce(struct vhost_net_virtqueue *nvq)
> +{
> +	struct vhost_net_buf *rxq = &nvq->rxq;
> +
> +	rxq->head = 0;
> +	rxq->tail = skb_array_consume_batched(nvq->rx_array, rxq->queue,
> +					      VHOST_RX_BATCH);
> +	return rxq->tail;
> +}
> +
> +static void vhost_net_buf_unproduce(struct vhost_net_virtqueue *nvq)
> +{
> +	struct vhost_net_buf *rxq = &nvq->rxq;
> +
> +	if (nvq->rx_array && !vhost_net_buf_is_empty(rxq)) {
> +		skb_array_unconsume(nvq->rx_array, rxq->queue + rxq->head,
> +				    vhost_net_buf_get_size(rxq));
> +		rxq->head = rxq->tail = 0;
> +	}
> +}
> +
> +static int vhost_net_buf_peek(struct vhost_net_virtqueue *nvq)
> +{
> +	struct vhost_net_buf *rxq = &nvq->rxq;
> +
> +	if (!vhost_net_buf_is_empty(rxq))
> +		goto out;
> +
> +	if (!vhost_net_buf_produce(nvq))
> +		return 0;
> +
> +out:
> +	return __skb_array_len_with_tag(vhost_net_buf_get_ptr(rxq));
> +}
> +
> +static void vhost_net_buf_init(struct vhost_net_buf *rxq)
> +{
> +	rxq->head = rxq->tail = 0;
> +}
> +
>  static void vhost_net_enable_zcopy(int vq)
>  {
>  	vhost_net_zcopy_mask |= 0x1 << vq;
> @@ -201,6 +277,7 @@ static void vhost_net_vq_reset(struct vhost_net *n)
>  		n->vqs[i].ubufs = NULL;
>  		n->vqs[i].vhost_hlen = 0;
>  		n->vqs[i].sock_hlen = 0;
> +		vhost_net_buf_init(&n->vqs[i].rxq);
>  	}
>  
>  }
> @@ -503,15 +580,14 @@ static void handle_tx(struct vhost_net *net)
>  	mutex_unlock(&vq->mutex);
>  }
>  
> -static int peek_head_len(struct sock *sk)
> +static int peek_head_len(struct vhost_net_virtqueue *rvq, struct sock *sk)
>  {
> -	struct socket *sock = sk->sk_socket;
>  	struct sk_buff *head;
>  	int len = 0;
>  	unsigned long flags;
>  
> -	if (sock->ops->peek_len)
> -		return sock->ops->peek_len(sock);
> +	if (rvq->rx_array)
> +		return vhost_net_buf_peek(rvq);
>  
>  	spin_lock_irqsave(&sk->sk_receive_queue.lock, flags);
>  	head = skb_peek(&sk->sk_receive_queue);
> @@ -537,10 +613,11 @@ static int sk_has_rx_data(struct sock *sk)
>  
>  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];
>  	struct vhost_net_virtqueue *nvq = &net->vqs[VHOST_NET_VQ_TX];
>  	struct vhost_virtqueue *vq = &nvq->vq;
>  	unsigned long uninitialized_var(endtime);
> -	int len = peek_head_len(sk);
> +	int len = peek_head_len(rvq, sk);
>  
>  	if (!len && vq->busyloop_timeout) {
>  		/* Both tx vq and rx socket were polled here */
> @@ -561,7 +638,7 @@ static int vhost_net_rx_peek_head_len(struct vhost_net *net, struct sock *sk)
>  			vhost_poll_queue(&vq->poll);
>  		mutex_unlock(&vq->mutex);
>  
> -		len = peek_head_len(sk);
> +		len = peek_head_len(rvq, sk);
>  	}
>  
>  	return len;
> @@ -699,6 +776,8 @@ static void handle_rx(struct vhost_net *net)
>  		/* On error, stop handling until the next kick. */
>  		if (unlikely(headcount < 0))
>  			goto out;
> +		if (nvq->rx_array)
> +			msg.msg_control = vhost_net_buf_consume(&nvq->rxq);
>  		/* On overrun, truncate and discard */
>  		if (unlikely(headcount > UIO_MAXIOV)) {
>  			iov_iter_init(&msg.msg_iter, READ, vq->iov, 1, 1);
> @@ -841,6 +920,7 @@ static int vhost_net_open(struct inode *inode, struct file *f)
>  		n->vqs[i].done_idx = 0;
>  		n->vqs[i].vhost_hlen = 0;
>  		n->vqs[i].sock_hlen = 0;
> +		vhost_net_buf_init(&n->vqs[i].rxq);
>  	}
>  	vhost_dev_init(dev, vqs, VHOST_NET_VQ_MAX);
>  
> @@ -856,11 +936,14 @@ static struct socket *vhost_net_stop_vq(struct vhost_net *n,
>  					struct vhost_virtqueue *vq)
>  {
>  	struct socket *sock;
> +	struct vhost_net_virtqueue *nvq =
> +		container_of(vq, struct vhost_net_virtqueue, vq);
>  
>  	mutex_lock(&vq->mutex);
>  	sock = vq->private_data;
>  	vhost_net_disable_vq(n, vq);
>  	vq->private_data = NULL;
> +	vhost_net_buf_unproduce(nvq);
>  	mutex_unlock(&vq->mutex);
>  	return sock;
>  }
> @@ -953,6 +1036,25 @@ static struct socket *get_raw_socket(int fd)
>  	return ERR_PTR(r);
>  }
>  
> +static struct skb_array *get_tap_skb_array(int fd)
> +{
> +	struct skb_array *array;
> +	struct file *file = fget(fd);
> +
> +	if (!file)
> +		return NULL;
> +	array = tun_get_skb_array(file);
> +	if (!IS_ERR(array))
> +		goto out;
> +	array = tap_get_skb_array(file);
> +	if (!IS_ERR(array))
> +		goto out;
> +	array = NULL;
> +out:
> +	fput(file);
> +	return array;
> +}
> +
>  static struct socket *get_tap_socket(int fd)
>  {
>  	struct file *file = fget(fd);
> @@ -1029,6 +1131,9 @@ static long vhost_net_set_backend(struct vhost_net *n, unsigned index, int fd)
>  
>  		vhost_net_disable_vq(n, vq);
>  		vq->private_data = sock;
> +		vhost_net_buf_unproduce(nvq);
> +		if (index == VHOST_NET_VQ_RX)
> +			nvq->rx_array = get_tap_skb_array(fd);
>  		r = vhost_vq_init_access(vq);
>  		if (r)
>  			goto err_used;
> -- 
> 2.7.4