From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: [PATCH 1/3] vhost-net: check the support of mergeable buffer outside the receive loop Date: Tue, 18 Jan 2011 06:36:50 +0200 Message-ID: <20110118043650.GA2233@redhat.com> References: <20110117081058.18900.67456.stgit@dhcp-91-7.nay.redhat.com.englab.nay.redhat.com> <20110117084644.GB23479@redhat.com> <19765.5737.352179.50100@gargle.gargle.HOWL> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <19765.5737.352179.50100@gargle.gargle.HOWL> Sender: linux-kernel-owner@vger.kernel.org To: Jason Wang Cc: virtualization@lists.osdl.org, netdev@vger.kernel.org, kvm@vger.kernel.org, linux-kernel@vger.kernel.org List-Id: virtualization@lists.linuxfoundation.org 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 > > > > 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. > > > --- > > > 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)) {