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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 18 Jan 2011 06:36:50 +0200

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 <[email-masked]>
>  > 
>  > 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)) {