Re: [PATCH V3 2/2] vhost_net: conditionally enable tx polling

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Jun 01, 2016 at 01:56:34AM -0400, Jason Wang wrote:
> We always poll tx for socket, this is sub optimal since:
> 
> - it will be only used when we exceed the sndbuf of the socket.
> - since we use two independent polls for tx and vq, this will slightly
>   increase the waitqueue traversing time and more important, vhost
>   could not benefit from commit
>   9e641bdcfa4ef4d6e2fbaa59c1be0ad5d1551fd5 ("net-tun: restructure
>   tun_do_read for better sleep/wakeup efficiency") even if we've
>   stopped rx polling during handle_rx since tx poll were still left in
>   the waitqueue.
> 
> Fix this by conditionally enable tx polling only when -EAGAIN were
> met.
> 
> Test shows about 8% improvement on guest rx pps.
> 
> Before: ~1350000
> After:  ~1460000
> 
> Signed-off-by: Jason Wang <jasowang@redhat.com>
> ---
>  drivers/vhost/net.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 1d3e45f..e75ffcc 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -378,6 +378,7 @@ static void handle_tx(struct vhost_net *net)
>  		goto out;
>  
>  	vhost_disable_notify(&net->dev, vq);
> +	vhost_net_disable_vq(net, vq);
>  
>  	hdr_size = nvq->vhost_hlen;
>  	zcopy = nvq->ubufs;
> @@ -459,6 +460,8 @@ static void handle_tx(struct vhost_net *net)
>  					% UIO_MAXIOV;
>  			}
>  			vhost_discard_vq_desc(vq, 1);
> +			if (err == -EAGAIN)
> +				vhost_net_enable_vq(net, vq);
>  			break;
>  		}
>  		if (err != len)

This seems rather risky. What if TX failed for some other reason?
Polling won't ever be re-enabled ...


> -- 
> 1.8.3.1