Subject: Re: [PATCH 2/3] vhost-net: Unify the code of mergeable and big buffer handling

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 17 Jan 2011 10:36:45 +0200

On Mon, Jan 17, 2011 at 04:11:08PM +0800, Jason Wang wrote:
> Codes duplication were found between the handling of mergeable and big
> buffers, so this patch tries to unify them. This could be easily done
> by adding a quota to the get_rx_bufs() which is used to limit the
> number of buffers it returns (for mergeable buffer, the quota is
> simply UIO_MAXIOV, for big buffers, the quota is just 1), and then the
> previous handle_rx_mergeable() could be resued also for big buffers.
> 
> Signed-off-by: Jason Wang <[email-masked]>

We actually started this way. However the code turned out
to have measureable overhead when handle_rx_mergeable
is called with quota 1.
So before applying this I'd like to see some data
to show this is not the case anymore.

> ---
>  drivers/vhost/net.c |  128 +++------------------------------------------------
>  1 files changed, 7 insertions(+), 121 deletions(-)
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 95e49de..c32a2e4 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -227,6 +227,7 @@ static int peek_head_len(struct sock *sk)
>   * @iovcount	- returned count of io vectors we fill
>   * @log		- vhost log
>   * @log_num	- log offset
> + * @quota       - headcount quota, 1 for big buffer
>   *	returns number of buffer heads allocated, negative on error
>   */
>  static int get_rx_bufs(struct vhost_virtqueue *vq,
> @@ -234,7 +235,8 @@ static int get_rx_bufs(struct vhost_virtqueue *vq,
>  		       int datalen,
>  		       unsigned *iovcount,
>  		       struct vhost_log *log,
> -		       unsigned *log_num)
> +		       unsigned *log_num,
> +		       unsigned int quota)
>  {
>  	unsigned int out, in;
>  	int seg = 0;
> @@ -242,7 +244,7 @@ static int get_rx_bufs(struct vhost_virtqueue *vq,
>  	unsigned d;
>  	int r, nlogs = 0;
>  
> -	while (datalen > 0) {
> +	while (datalen > 0 && headcount < quota) {
>  		if (unlikely(seg >= UIO_MAXIOV)) {
>  			r = -ENOBUFS;
>  			goto err;
> @@ -282,116 +284,7 @@ err:
>  
>  /* Expects to be always run from workqueue - which acts as
>   * read-size critical section for our kind of RCU. */
> -static void handle_rx_big(struct vhost_net *net)
> -{
> -	struct vhost_virtqueue *vq = &net->dev.vqs[VHOST_NET_VQ_RX];
> -	unsigned out, in, log, s;
> -	int head;
> -	struct vhost_log *vq_log;
> -	struct msghdr msg = {
> -		.msg_name = NULL,
> -		.msg_namelen = 0,
> -		.msg_control = NULL, /* FIXME: get and handle RX aux data. */
> -		.msg_controllen = 0,
> -		.msg_iov = vq->iov,
> -		.msg_flags = MSG_DONTWAIT,
> -	};
> -
> -	struct virtio_net_hdr hdr = {
> -		.flags = 0,
> -		.gso_type = VIRTIO_NET_HDR_GSO_NONE
> -	};
> -
> -	size_t len, total_len = 0;
> -	int err;
> -	size_t hdr_size;
> -	struct socket *sock = rcu_dereference(vq->private_data);
> -	if (!sock || skb_queue_empty(&sock->sk->sk_receive_queue))
> -		return;
> -
> -	mutex_lock(&vq->mutex);
> -	vhost_disable_notify(vq);
> -	hdr_size = vq->vhost_hlen;
> -
> -	vq_log = unlikely(vhost_has_feature(&net->dev, VHOST_F_LOG_ALL)) ?
> -		vq->log : NULL;
> -
> -	for (;;) {
> -		head = vhost_get_vq_desc(&net->dev, vq, vq->iov,
> -					 ARRAY_SIZE(vq->iov),
> -					 &out, &in,
> -					 vq_log, &log);
> -		/* On error, stop handling until the next kick. */
> -		if (unlikely(head < 0))
> -			break;
> -		/* OK, now we need to know about added descriptors. */
> -		if (head == vq->num) {
> -			if (unlikely(vhost_enable_notify(vq))) {
> -				/* They have slipped one in as we were
> -				 * doing that: check again. */
> -				vhost_disable_notify(vq);
> -				continue;
> -			}
> -			/* Nothing new?  Wait for eventfd to tell us
> -			 * they refilled. */
> -			break;
> -		}
> -		/* We don't need to be notified again. */
> -		if (out) {
> -			vq_err(vq, "Unexpected descriptor format for RX: "
> -			       "out %d, int %d\n",
> -			       out, in);
> -			break;
> -		}
> -		/* Skip header. TODO: support TSO/mergeable rx buffers. */
> -		s = move_iovec_hdr(vq->iov, vq->hdr, hdr_size, in);
> -		msg.msg_iovlen = in;
> -		len = iov_length(vq->iov, in);
> -		/* Sanity check */
> -		if (!len) {
> -			vq_err(vq, "Unexpected header len for RX: "
> -			       "%zd expected %zd\n",
> -			       iov_length(vq->hdr, s), hdr_size);
> -			break;
> -		}
> -		err = sock->ops->recvmsg(NULL, sock, &msg,
> -					 len, MSG_DONTWAIT | MSG_TRUNC);
> -		/* TODO: Check specific error and bomb out unless EAGAIN? */
> -		if (err < 0) {
> -			vhost_discard_vq_desc(vq, 1);
> -			break;
> -		}
> -		/* TODO: Should check and handle checksum. */
> -		if (err > len) {
> -			pr_debug("Discarded truncated rx packet: "
> -				 " len %d > %zd\n", err, len);
> -			vhost_discard_vq_desc(vq, 1);
> -			continue;
> -		}
> -		len = err;
> -		err = memcpy_toiovec(vq->hdr, (unsigned char *)&hdr, hdr_size);
> -		if (err) {
> -			vq_err(vq, "Unable to write vnet_hdr at addr %p: %d\n",
> -			       vq->iov->iov_base, err);
> -			break;
> -		}
> -		len += hdr_size;
> -		vhost_add_used_and_signal(&net->dev, vq, head, len);
> -		if (unlikely(vq_log))
> -			vhost_log_write(vq, vq_log, log, len);
> -		total_len += len;
> -		if (unlikely(total_len >= VHOST_NET_WEIGHT)) {
> -			vhost_poll_queue(&vq->poll);
> -			break;
> -		}
> -	}
> -
> -	mutex_unlock(&vq->mutex);
> -}
> -
> -/* Expects to be always run from workqueue - which acts as
> - * read-size critical section for our kind of RCU. */
> -static void handle_rx_mergeable(struct vhost_net *net)
> +static void handle_rx(struct vhost_net *net)
>  {
>  	struct vhost_virtqueue *vq = &net->dev.vqs[VHOST_NET_VQ_RX];
>  	unsigned uninitialized_var(in), log;
> @@ -431,7 +324,8 @@ static void handle_rx_mergeable(struct vhost_net *net)
>  		sock_len += sock_hlen;
>  		vhost_len = sock_len + vhost_hlen;
>  		headcount = get_rx_bufs(vq, vq->heads, vhost_len,
> -					&in, vq_log, &log);
> +					&in, vq_log, &log,
> +					likely(mergeable) ? UIO_MAXIOV : 1);
>  		/* On error, stop handling until the next kick. */
>  		if (unlikely(headcount < 0))
>  			break;
> @@ -497,14 +391,6 @@ static void handle_rx_mergeable(struct vhost_net *net)
>  	mutex_unlock(&vq->mutex);
>  }
>  
> -static void handle_rx(struct vhost_net *net)
> -{
> -	if (vhost_has_feature(&net->dev, VIRTIO_NET_F_MRG_RXBUF))
> -		handle_rx_mergeable(net);
> -	else
> -		handle_rx_big(net);
> -}
> -
>  static void handle_tx_kick(struct vhost_work *work)
>  {
>  	struct vhost_virtqueue *vq = container_of(work, struct vhost_virtqueue,