linux-kernel - Re: [PATCH net-next 8/8] vhost_net: use lockless peeking for skb array during busy polling

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Date:   Thu, 30 Mar 2017 05:33:33 +0300
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next 8/8] vhost_net: use lockless peeking for skb
 array during busy polling

On Thu, Mar 30, 2017 at 10:16:15AM +0800, Jason Wang wrote:
> 
> 
> On 2017年03月29日 20:07, Michael S. Tsirkin wrote:
> > On Tue, Mar 21, 2017 at 12:04:47PM +0800, Jason Wang wrote:
> > > For the socket that exports its skb array, we can use lockless polling
> > > to avoid touching spinlock during busy polling.
> > > 
> > > Signed-off-by: Jason Wang <jasowang@...hat.com>
> > > ---
> > >   drivers/vhost/net.c | 7 +++++--
> > >   1 file changed, 5 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> > > index 53f09f2..41153a3 100644
> > > --- a/drivers/vhost/net.c
> > > +++ b/drivers/vhost/net.c
> > > @@ -551,10 +551,13 @@ static int peek_head_len(struct vhost_net_virtqueue *rvq, struct sock *sk)
> > >   	return len;
> > >   }
> > > -static int sk_has_rx_data(struct sock *sk)
> > > +static int sk_has_rx_data(struct vhost_net_virtqueue *rvq, struct sock *sk)
> > >   {
> > >   	struct socket *sock = sk->sk_socket;
> > > +	if (rvq->rx_array)
> > > +		return !__skb_array_empty(rvq->rx_array);
> > > +
> > >   	if (sock->ops->peek_len)
> > >   		return sock->ops->peek_len(sock);
> > I don't see which patch adds __skb_array_empty.
> 
> This is not something new, it was introduced by ad69f35d1dc0a ("skb_array:
> array based FIFO for skbs").
> 
> Thanks

Same comment about a compiler barrier applies then.

> > 
> > > @@ -579,7 +582,7 @@ static int vhost_net_rx_peek_head_len(struct vhost_net *net,
> > >   		endtime = busy_clock() + vq->busyloop_timeout;
> > >   		while (vhost_can_busy_poll(&net->dev, endtime) &&
> > > -		       !sk_has_rx_data(sk) &&
> > > +		       !sk_has_rx_data(rvq, sk) &&
> > >   		       vhost_vq_avail_empty(&net->dev, vq))
> > >   			cpu_relax();
> > > -- 
> > > 2.7.4