netdev - Re: [PATCH net v3] tun: add missing rx stats accounting in tun_xdp

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Date: Wed, 17 Jan 2024 10:42:24 -0500
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Yunjian Wang <wangyunjian@...wei.com>, 
 willemdebruijn.kernel@...il.com, 
 jasowang@...hat.com, 
 kuba@...nel.org, 
 davem@...emloft.net
Cc: netdev@...r.kernel.org, 
 linux-kernel@...r.kernel.org, 
 xudingke@...wei.com, 
 Yunjian Wang <wangyunjian@...wei.com>
Subject: Re: [PATCH net v3] tun: add missing rx stats accounting in
 tun_xdp_act

Yunjian Wang wrote:
> The TUN can be used as vhost-net backend, and it is necessary to count
> the packets transmitted from TUN to vhost-net/virtio-net. However, there
> are some places in the receive path that were not taken into account
> when using XDP. The commit 8ae1aff0b331 ("tuntap: split out XDP logic")
> only includes dropped counter for XDP_DROP, XDP_ABORTED, and invalid
> XDP actions. It would be beneficial to also include new accounting for
> successfully received bytes using dev_sw_netstats_rx_add and introduce
> new dropped counter for XDP errors on XDP_TX and XDP_REDIRECT.

>From the description it is clear that these are two separate changes
wrapped into one patch. I should have flagged this previously.

Ack on returning the error counter that was previously present and
matches the Fixes tag.

For the second change, I had to check a few other XDP capable drivers
to verify that it is indeed common to count such packets.

> Fixes: 8ae1aff0b331 ("tuntap: split out XDP logic")
> Signed-off-by: Yunjian Wang <wangyunjian@...wei.com>
> ---
> v3: update commit log and code
> v2: add Fixes tag
> ---
>  drivers/net/tun.c | 14 +++++++++-----
>  1 file changed, 9 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index afa5497f7c35..0704a17e74e1 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -1625,18 +1625,15 @@ static struct sk_buff *__tun_build_skb(struct tun_file *tfile,
>  static int tun_xdp_act(struct tun_struct *tun, struct bpf_prog *xdp_prog,
>  		       struct xdp_buff *xdp, u32 act)
>  {
> -	int err;
> +	unsigned int datasize = xdp->data_end - xdp->data;
> +	int err = 0;
>  
>  	switch (act) {
>  	case XDP_REDIRECT:
>  		err = xdp_do_redirect(tun->dev, xdp, xdp_prog);
> -		if (err)
> -			return err;
>  		break;
>  	case XDP_TX:
>  		err = tun_xdp_tx(tun->dev, xdp);
> -		if (err < 0)
> -			return err;
>  		break;
>  	case XDP_PASS:
>  		break;
> @@ -1651,6 +1648,13 @@ static int tun_xdp_act(struct tun_struct *tun, struct bpf_prog *xdp_prog,
>  		break;
>  	}
>  
> +	if (err < 0) {
> +		act = err;
> +		dev_core_stats_rx_dropped_inc(tun->dev);
> +	} else if (act == XDP_REDIRECT || act == XDP_TX) {
> +		dev_sw_netstats_rx_add(tun->dev, datasize);
> +	}
> +

Let's avoid adding yet another branch and just do these operations in
the case statements, like XDP_DROP.

>  	return act;
>  }
>  
> -- 
> 2.41.0
>