On Tue, Nov 28, 2017 at 07:53:33PM +0200, Michael S. Tsirkin wrote:
> 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.
>
Absolutely we want that, sorry for missed you here, Matthew. It is more
like you have figured out this issue independently all by yourself with
a wide assortment of extremely quick tests(tools, throughput, slub leak,
etc), and I am happy that I have the opportunity to do the paperwork on
behalf of you.:) Thanks a lot Matthew.
Wei
> 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
