On Tue, Nov 28, 2017 at 12:17:16PM -0500, w...@redhat.com wrote:
> From: Wei Xu <w...@redhat.com>
>
> Matthew found a roughly 40% tcp throughput regression with commit
> c67df11f(vhost_net: try batch dequing from skb array) as discussed
> in the following thread:
> https://www.mail-archive.com/netdev@vger.kernel.org/msg187936.html
>
> Eventually we figured out that it was a skb leak in handle_rx()
> when sending packets to the VM. This usually happens when a guest
> can not drain out vq as fast as vhost fills in, afterwards it sets
> off the traffic jam and leaks skb(s) which occurs as no headcount
> to send on the vq from vhost side.
>
> This can be avoided by making sure we have got enough headcount
> before actually consuming a skb from the batched rx array while
> transmitting, which is simply done by deferring it a moment later
> in this patch.
>
> Signed-off-by: Wei Xu <w...@redhat.com>
Given the amount of effort Matthew has put into this,
you definitely want
Reported-by: Matthew Rosato <mjros...@linux.vnet.ibm.com>
here.
Let's give credit where credit is due.
Thanks a lot Matthew!
> ---
> drivers/vhost/net.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 8d626d7..e76535e 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -778,8 +778,6 @@ 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);
> @@ -809,6 +807,8 @@ static void handle_rx(struct vhost_net *net)
> */
> iov_iter_advance(&msg.msg_iter, vhost_hlen);
> }
> + if (nvq->rx_array)
> + msg.msg_control = vhost_net_buf_consume(&nvq->rxq);
> err = sock->ops->recvmsg(sock, &msg,
> sock_len, MSG_DONTWAIT | MSG_TRUNC);
> /* Userspace might have consumed the packet meanwhile:
> --
> 1.8.3.1
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