Re: [PATCH 1/3] vhost-net: check the support of mergeable buffer outside the receive loop

Jason Wang
2011-01-18T04:26:29Z

Michael S. Tsirkin writes:
> On Mon, Jan 17, 2011 at 04:10:59PM +0800, Jason Wang wrote:
> > No need to check the support of mergeable buffer inside the recevie
> > loop as the whole handle_rx()_xx is in the read critical region. So
> > this patch move it ahead of the receiving loop.
> >
> > Signed-off-by: Jason Wang <jasowang@redhat.com>
>
> Well feature check is mostly just features & bit
> so why would it be slower? Because of the rcu
> adding memory barriers? Do you observe a
> measureable speedup with this patch?
>

I do not measure the performance for just this patch, maybe not obvious. And it
can also help the code unification.

> Apropos, I noticed that the check in vhost_has_feature
> is wrong: it uses the same kind of RCU as the
> private pointer. So we'll have to fix that properly
> by adding more lockdep classes, but for now
> we'll need to make
> the check 1 || lockdep_is_held(&dev->mutex);
> and add a TODO.
>

Not sure, lockdep_is_head(&dev->mutex) maybe not accurate but sufficient, as it
was always held in the read critical region.

> > ---
> > drivers/vhost/net.c | 5 +++--
> > 1 files changed, 3 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> > index 14fc189..95e49de 100644
> > --- a/drivers/vhost/net.c
> > +++ b/drivers/vhost/net.c
> > @@ -411,7 +411,7 @@ static void handle_rx_mergeable(struct vhost_net *net)
> > };
> >
> > size_t total_len = 0;
> > - int err, headcount;
> > + int err, headcount, mergeable;
> > size_t vhost_hlen, sock_hlen;
> > size_t vhost_len, sock_len;
> > struct socket *sock = rcu_dereference(vq->private_data);
> > @@ -425,6 +425,7 @@ static void handle_rx_mergeable(struct vhost_net *net)
> >
> > vq_log = unlikely(vhost_has_feature(&net->dev, VHOST_F_LOG_ALL)) ?
> > vq->log : NULL;
> > + mergeable = vhost_has_feature(&net->dev, VIRTIO_NET_F_MRG_RXBUF);
> >
> > while ((sock_len = peek_head_len(sock->sk))) {
> > sock_len += sock_hlen;
> > @@ -474,7 +475,7 @@ static void handle_rx_mergeable(struct vhost_net *net)
> > break;
> > }
> > /* TODO: Should check and handle checksum. */
> > - if (vhost_has_feature(&net->dev, VIRTIO_NET_F_MRG_RXBUF) &&
> > + if (likely(mergeable) &&
> > memcpy_toiovecend(vq->hdr, (unsigned char *)&headcount,
> > offsetof(typeof(hdr), num_buffers),
> > sizeof hdr.num_buffers)) {