Subject: Re: [PATCH net] vhost_net: correctly check tx avail during rx busy polling

From: "Michael S. Tsirkin" <[email-masked]>
Date: Fri, 1 Sep 2017 18:51:26 +0300

On Fri, Sep 01, 2017 at 05:02:50PM +0800, Jason Wang wrote:
> We check tx avail through vhost_enable_notify() in the past which is
> wrong since it only checks whether or not guest has filled more
> available buffer since last avail idx synchronization which was just
> done by vhost_vq_avail_empty() before. What we really want is checking
> pending buffers in the avail ring.

These are rx buffers, right? I'm not even sure why do we need to poll
for them. Running out of rx buffers is a slow path.

> Fix this by calling
> vhost_vq_avail_empty() instead.
> 
> This issue could be noticed by doing netperf TCP_RR benchmark as
> client from guest (but not host). With this fix, TCP_RR from guest to
> localhost restores from 1375.91 trans per sec to 55235.28 trans per
> sec on my laptop (Intel(R) Core(TM) i7-5600U CPU @ 2.60GHz).
> 
> Fixes: 030881372460 ("vhost_net: basic polling support")
> Signed-off-by: Jason Wang <[email-masked]>
> ---
> - The patch is needed for -stable
> ---
>  drivers/vhost/net.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 06d0448..1b68253 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -634,7 +634,7 @@ static int vhost_net_rx_peek_head_len(struct vhost_net *net, struct sock *sk)

In fact why does it poll the ring at all? I thought this function's
job is to poll the socket, isn't it?

>  
>  		preempt_enable();
>  
> -		if (vhost_enable_notify(&net->dev, vq))
> +		if (!vhost_vq_avail_empty(&net->dev, vq))
>  			vhost_poll_queue(&vq->poll);
>  		mutex_unlock(&vq->mutex);

Adding more contex:

                mutex_lock(&vq->mutex);
                vhost_disable_notify(&net->dev, vq);

                preempt_disable();
                endtime = busy_clock() + vq->busyloop_timeout;

                while (vhost_can_busy_poll(&net->dev, endtime) &&
                       !sk_has_rx_data(sk) &&
                       vhost_vq_avail_empty(&net->dev, vq))
                        cpu_relax();
                
                preempt_enable();
        
                if (vhost_enable_notify(&net->dev, vq))
                        vhost_poll_queue(&vq->poll);
                mutex_unlock(&vq->mutex);

                len = peek_head_len(rvq, sk);

If you drop this we'll exit the function with notifications
disabled. Seems wrong to me.

> -- 
> 2.7.4