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

Michael S. Tsirkin
2011-01-17T08:37:09Z

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 <jasowang@redhat.com>

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,