Subject: Re: [PATCH net-next 11/11] vhost_net: batch submitting XDP buffers to underlayer sockets

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 6 Sep 2018 12:46:24 -0400

On Thu, Sep 06, 2018 at 12:05:26PM +0800, Jason Wang wrote:
> This patch implements XDP batching for vhost_net. The idea is first to
> try to do userspace copy and build XDP buff directly in vhost. Instead
> of submitting the packet immediately, vhost_net will batch them in an
> array and submit every 64 (VHOST_NET_BATCH) packets to the under layer
> sockets through msg_control of sendmsg().
> 
> When XDP is enabled on the TUN/TAP, TUN/TAP can process XDP inside a
> loop without caring GUP thus it can do batch map flushing. When XDP is
> not enabled or not supported, the underlayer socket need to build skb
> and pass it to network core. The batched packet submission allows us
> to do batching like netif_receive_skb_list() in the future.
> 
> This saves lots of indirect calls for better cache utilization. For
> the case that we can't so batching e.g when sndbuf is limited or
> packet size is too large, we will go for usual one packet per
> sendmsg() way.
> 
> Doing testpmd on various setups gives us:
> 
> Test                /+pps%
> XDP_DROP on TAP     /+44.8%
> XDP_REDIRECT on TAP /+29%
> macvtap (skb)       /+26%
> 
> Netperf tests shows obvious improvements for small packet transmission:
> 
> size/session/+thu%/+normalize%
>    64/     1/   +2%/    0%
>    64/     2/   +3%/   +1%
>    64/     4/   +7%/   +5%
>    64/     8/   +8%/   +6%
>   256/     1/   +3%/    0%
>   256/     2/  +10%/   +7%
>   256/     4/  +26%/  +22%
>   256/     8/  +27%/  +23%
>   512/     1/   +3%/   +2%
>   512/     2/  +19%/  +14%
>   512/     4/  +43%/  +40%
>   512/     8/  +45%/  +41%
>  1024/     1/   +4%/    0%
>  1024/     2/  +27%/  +21%
>  1024/     4/  +38%/  +73%
>  1024/     8/  +15%/  +24%
>  2048/     1/  +10%/   +7%
>  2048/     2/  +16%/  +12%
>  2048/     4/    0%/   +2%
>  2048/     8/    0%/   +2%
>  4096/     1/  +36%/  +60%
>  4096/     2/  -11%/  -26%
>  4096/     4/    0%/  +14%
>  4096/     8/    0%/   +4%
> 16384/     1/   -1%/   +5%
> 16384/     2/    0%/   +2%
> 16384/     4/    0%/   -3%
> 16384/     8/    0%/   +4%
> 65535/     1/    0%/  +10%
> 65535/     2/    0%/   +8%
> 65535/     4/    0%/   +1%
> 65535/     8/    0%/   +3%
> 
> Signed-off-by: Jason Wang <[email-masked]>
> ---
>  drivers/vhost/net.c | 164 ++++++++++++++++++++++++++++++++++++++++----
>  1 file changed, 151 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index fb01ce6d981c..1dd4239cbff8 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -116,6 +116,7 @@ struct vhost_net_virtqueue {
>  	 * For RX, number of batched heads
>  	 */
>  	int done_idx;
> +	int batched_xdp;

Pls add a comment documenting what does this new field do.

>  	/* an array of userspace buffers info */
>  	struct ubuf_info *ubuf_info;
>  	/* Reference counting for outstanding ubufs.
> @@ -123,6 +124,7 @@ struct vhost_net_virtqueue {
>  	struct vhost_net_ubuf_ref *ubufs;
>  	struct ptr_ring *rx_ring;
>  	struct vhost_net_buf rxq;
> +	struct xdp_buff xdp[VHOST_NET_BATCH];

This is a bit too much to have inline. 64 entries 48 bytes each, and we
have 2 of these structures, that's already >4K.

>  };
>  
>  struct vhost_net {
> @@ -338,6 +340,11 @@ static bool vhost_sock_zcopy(struct socket *sock)
>  		sock_flag(sock->sk, SOCK_ZEROCOPY);
>  }
>  
> +static bool vhost_sock_xdp(struct socket *sock)
> +{
> +	return sock_flag(sock->sk, SOCK_XDP);

what if an xdp program is attached while this processing
is going on? Flag value will be wrong - is this safe
and why? Pls add a comment.

> +}
> +
>  /* In case of DMA done not in order in lower device driver for some reason.
>   * upend_idx is used to track end of used idx, done_idx is used to track head
>   * of used idx. Once lower device DMA done contiguously, we will signal KVM
> @@ -444,10 +451,36 @@ static void vhost_net_signal_used(struct vhost_net_virtqueue *nvq)
>  	nvq->done_idx = 0;
>  }
>  
> +static void vhost_tx_batch(struct vhost_net *net,
> +			   struct vhost_net_virtqueue *nvq,
> +			   struct socket *sock,
> +			   struct msghdr *msghdr)
> +{
> +	struct tun_msg_ctl ctl = {
> +		.type = nvq->batched_xdp << 16 | TUN_MSG_PTR,
> +		.ptr = nvq->xdp,
> +	};
> +	int err;
> +
> +	if (nvq->batched_xdp == 0)
> +		goto signal_used;
> +
> +	msghdr->msg_control = &ctl;
> +	err = sock->ops->sendmsg(sock, msghdr, 0);
> +	if (unlikely(err < 0)) {
> +		vq_err(&nvq->vq, "Fail to batch sending packets\n");
> +		return;
> +	}
> +
> +signal_used:
> +	vhost_net_signal_used(nvq);
> +	nvq->batched_xdp = 0;
> +}
> +
>  static int vhost_net_tx_get_vq_desc(struct vhost_net *net,
>  				    struct vhost_net_virtqueue *nvq,
>  				    unsigned int *out_num, unsigned int *in_num,
> -				    bool *busyloop_intr)
> +				    struct msghdr *msghdr, bool *busyloop_intr)
>  {
>  	struct vhost_virtqueue *vq = &nvq->vq;
>  	unsigned long uninitialized_var(endtime);
> @@ -455,8 +488,9 @@ static int vhost_net_tx_get_vq_desc(struct vhost_net *net,
>  				  out_num, in_num, NULL, NULL);
>  
>  	if (r == vq->num && vq->busyloop_timeout) {
> +		/* Flush batched packets first */
>  		if (!vhost_sock_zcopy(vq->private_data))
> -			vhost_net_signal_used(nvq);
> +			vhost_tx_batch(net, nvq, vq->private_data, msghdr);
>  		preempt_disable();
>  		endtime = busy_clock() + vq->busyloop_timeout;
>  		while (vhost_can_busy_poll(endtime)) {
> @@ -512,7 +546,7 @@ static int get_tx_bufs(struct vhost_net *net,
>  	struct vhost_virtqueue *vq = &nvq->vq;
>  	int ret;
>  
> -	ret = vhost_net_tx_get_vq_desc(net, nvq, out, in, busyloop_intr);
> +	ret = vhost_net_tx_get_vq_desc(net, nvq, out, in, msg, busyloop_intr);
>  
>  	if (ret < 0 || ret == vq->num)
>  		return ret;
> @@ -540,6 +574,83 @@ static bool tx_can_batch(struct vhost_virtqueue *vq, size_t total_len)
>  	       !vhost_vq_avail_empty(vq->dev, vq);
>  }
>  
> +#define VHOST_NET_RX_PAD (NET_IP_ALIGN + NET_SKB_PAD)

I wonder whether NET_IP_ALIGN make sense for XDP.

> +
> +static int vhost_net_build_xdp(struct vhost_net_virtqueue *nvq,
> +			       struct iov_iter *from)
> +{
> +	struct vhost_virtqueue *vq = &nvq->vq;
> +	struct socket *sock = vq->private_data;
> +	struct page_frag *alloc_frag = &current->task_frag;
> +	struct virtio_net_hdr *gso;
> +	struct xdp_buff *xdp = &nvq->xdp[nvq->batched_xdp];
> +	size_t len = iov_iter_count(from);
> +	int headroom = vhost_sock_xdp(sock) ? XDP_PACKET_HEADROOM : 0;
> +	int buflen = SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
> +	int pad = SKB_DATA_ALIGN(VHOST_NET_RX_PAD + headroom + nvq->sock_hlen);
> +	int sock_hlen = nvq->sock_hlen;
> +	void *buf;
> +	int copied;
> +
> +	if (unlikely(len < nvq->sock_hlen))
> +		return -EFAULT;
> +
> +	if (SKB_DATA_ALIGN(len + pad) +
> +	    SKB_DATA_ALIGN(sizeof(struct skb_shared_info)) > PAGE_SIZE)
> +		return -ENOSPC;
> +
> +	buflen += SKB_DATA_ALIGN(len + pad);
> +	alloc_frag->offset = ALIGN((u64)alloc_frag->offset, SMP_CACHE_BYTES);
> +	if (unlikely(!skb_page_frag_refill(buflen, alloc_frag, GFP_KERNEL)))
> +		return -ENOMEM;
> +
> +	buf = (char *)page_address(alloc_frag->page) + alloc_frag->offset;
> +
> +	/* We store two kinds of metadata in the header which will be
> +	 * used for XDP_PASS to do build_skb():
> +	 * offset 0: buflen
> +	 * offset sizeof(int): vnet header

Define a struct for the metadata then?

> +	 */
> +	copied = copy_page_from_iter(alloc_frag->page,
> +				     alloc_frag->offset + sizeof(int),
> +				     sock_hlen, from);
> +	if (copied != sock_hlen)
> +		return -EFAULT;
> +
> +	gso = (struct virtio_net_hdr *)(buf + sizeof(int));
> +
> +	if ((gso->flags & VIRTIO_NET_HDR_F_NEEDS_CSUM) &&
> +	    vhost16_to_cpu(vq, gso->csum_start) +
> +	    vhost16_to_cpu(vq, gso->csum_offset) + 2 >
> +	    vhost16_to_cpu(vq, gso->hdr_len)) {
> +		gso->hdr_len = cpu_to_vhost16(vq,
> +			       vhost16_to_cpu(vq, gso->csum_start) +
> +			       vhost16_to_cpu(vq, gso->csum_offset) + 2);
> +
> +		if (vhost16_to_cpu(vq, gso->hdr_len) > len)
> +			return -EINVAL;
> +	}
> +
> +	len -= sock_hlen;
> +	copied = copy_page_from_iter(alloc_frag->page,
> +				     alloc_frag->offset + pad,
> +				     len, from);
> +	if (copied != len)
> +		return -EFAULT;
> +
> +	xdp->data_hard_start = buf;
> +	xdp->data = buf + pad;
> +	xdp->data_end = xdp->data + len;
> +	*(int *)(xdp->data_hard_start) = buflen;
> +
> +	get_page(alloc_frag->page);
> +	alloc_frag->offset += buflen;
> +
> +	++nvq->batched_xdp;
> +
> +	return 0;
> +}
> +
>  static void handle_tx_copy(struct vhost_net *net, struct socket *sock)
>  {
>  	struct vhost_net_virtqueue *nvq = &net->vqs[VHOST_NET_VQ_TX];
> @@ -556,10 +667,14 @@ static void handle_tx_copy(struct vhost_net *net, struct socket *sock)
>  	size_t len, total_len = 0;
>  	int err;
>  	int sent_pkts = 0;
> +	bool bulking = (sock->sk->sk_sndbuf == INT_MAX);

What does bulking mean?

>  
>  	for (;;) {
>  		bool busyloop_intr = false;
>  
> +		if (nvq->done_idx == VHOST_NET_BATCH)
> +			vhost_tx_batch(net, nvq, sock, &msg);
> +
>  		head = get_tx_bufs(net, nvq, &msg, &out, &in, &len,
>  				   &busyloop_intr);
>  		/* On error, stop handling until the next kick. */
> @@ -577,14 +692,34 @@ static void handle_tx_copy(struct vhost_net *net, struct socket *sock)
>  			break;
>  		}
>  
> -		vq->heads[nvq->done_idx].id = cpu_to_vhost32(vq, head);
> -		vq->heads[nvq->done_idx].len = 0;
> -
>  		total_len += len;
> -		if (tx_can_batch(vq, total_len))
> -			msg.msg_flags |= MSG_MORE;
> -		else
> -			msg.msg_flags &= ~MSG_MORE;
> +
> +		/* For simplicity, TX batching is only enabled if
> +		 * sndbuf is unlimited.

What if sndbuf changes while this processing is going on?

> +		 */
> +		if (bulking) {
> +			err = vhost_net_build_xdp(nvq, &msg.msg_iter);
> +			if (!err) {
> +				goto done;
> +			} else if (unlikely(err != -ENOSPC)) {
> +				vhost_tx_batch(net, nvq, sock, &msg);
> +				vhost_discard_vq_desc(vq, 1);
> +				vhost_net_enable_vq(net, vq);
> +				break;
> +			}
> +
> +			/* We can't build XDP buff, go for single
> +			 * packet path but let's flush batched
> +			 * packets.
> +			 */
> +			vhost_tx_batch(net, nvq, sock, &msg);
> +			msg.msg_control = NULL;
> +		} else {
> +			if (tx_can_batch(vq, total_len))
> +				msg.msg_flags |= MSG_MORE;
> +			else
> +				msg.msg_flags &= ~MSG_MORE;
> +		}
>  
>  		/* TODO: Check specific error and bomb out unless ENOBUFS? */
>  		err = sock->ops->sendmsg(sock, &msg, len);
> @@ -596,15 +731,17 @@ static void handle_tx_copy(struct vhost_net *net, struct socket *sock)
>  		if (err != len)
>  			pr_debug("Truncated TX packet: len %d != %zd\n",
>  				 err, len);
> -		if (++nvq->done_idx >= VHOST_NET_BATCH)
> -			vhost_net_signal_used(nvq);
> +done:
> +		vq->heads[nvq->done_idx].id = cpu_to_vhost32(vq, head);
> +		vq->heads[nvq->done_idx].len = 0;
> +		++nvq->done_idx;
>  		if (vhost_exceeds_weight(++sent_pkts, total_len)) {
>  			vhost_poll_queue(&vq->poll);
>  			break;
>  		}
>  	}
>  
> -	vhost_net_signal_used(nvq);
> +	vhost_tx_batch(net, nvq, sock, &msg);
>  }
>  
>  static void handle_tx_zerocopy(struct vhost_net *net, struct socket *sock)
> @@ -1111,6 +1248,7 @@ static int vhost_net_open(struct inode *inode, struct file *f)
>  		n->vqs[i].ubuf_info = NULL;
>  		n->vqs[i].upend_idx = 0;
>  		n->vqs[i].done_idx = 0;
> +		n->vqs[i].batched_xdp = 0;
>  		n->vqs[i].vhost_hlen = 0;
>  		n->vqs[i].sock_hlen = 0;
>  		n->vqs[i].rx_ring = NULL;
> -- 
> 2.17.1