Michael S. Tsirkin writes:
> On Tue, Jan 18, 2011 at 12:26:17PM +0800, Jason Wang wrote:
> > 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@xxxxxxxxxx>
> > >
> > > 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.
>
> Not really, when we call vhost_has_feature from the vq handling thread
> it's not.
>
Seems not, see vhost_net_ioctl().
> > > > ---
> > > > 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)) {
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