Linux Virtualization - Mar 2017 - [PATCH net-next 8/8] vhost_net: use lockless peeking for skb array during busy polling

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 at redhat.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.
> @@ -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

On 2017?03?29? 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 at redhat.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
>
>> @@ -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

On Thu, Mar 30, 2017 at 10:16:15AM +0800, Jason Wang
wrote:
> 
> 
> On 2017?03?29? 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 at redhat.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