RE: [PATCH net v3] tun: add missing rx stats accounting in tun_xdp_act

[Willem de Bruijn]

wangyunjian wrote:
> > -----Original Message-----
> > From: Willem de Bruijn [mailto:willemdebruijn.kernel@xxxxxxxxx]
> > Sent: Wednesday, January 17, 2024 11:42 PM
> > To: wangyunjian <wangyunjian@xxxxxxxxxx>;
> > willemdebruijn.kernel@xxxxxxxxx; jasowang@xxxxxxxxxx; kuba@xxxxxxxxxx;
> > davem@xxxxxxxxxxxxx
> > Cc: netdev@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; xudingke
> > <xudingke@xxxxxxxxxx>; wangyunjian <wangyunjian@xxxxxxxxxx>
> > 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.
> 
> Do I need to split these two modifications into 2 patches?
> 1. only fix dropped counter
> 2. add new accounting for successfully received bytes
> Or:
> Only fix dropped counter?

It's definitely good to fix both.

It might be a bit pedantic, but two separate patches is more correct.

The second fix, add missing byte counter, goes back to the original
introduction of XDP for tun, so has a different tag:

Fixes: 761876c857cb ("tap: XDP support")

> 
> > 
> > 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@xxxxxxxxxx>
> > > ---
> > > 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.
> 
> Fix it like this?

Perhaps avoid computing datasize is all paths, when it is not used
in common XDP_PASS, high performance XDP_DROP and a few others. Not
sure whether (all) compilers would optimze that.

        dev_core_stats_rx_dropped_inc(tun->dev, xdp,
                                      xdp->data_end - xdp->data);


> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -1625,18 +1625,25 @@ 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)
>  {
> +       unsigned int datasize = xdp->data_end - xdp->data;
>         int err;
>  
>         switch (act) {
>         case XDP_REDIRECT:
>                 err = xdp_do_redirect(tun->dev, xdp, xdp_prog);
> -               if (err)
> +               if (err) {
> +                       dev_core_stats_rx_dropped_inc(tun->dev);
>                         return err;
> +               }
> +               dev_sw_netstats_rx_add(tun->dev, datasize);
>                 break;
>         case XDP_TX:
>                 err = tun_xdp_tx(tun->dev, xdp);
> -               if (err < 0)
> +               if (err < 0) {
> +                       dev_core_stats_rx_dropped_inc(tun->dev);
>                         return err;
> +               }
> +               dev_sw_netstats_rx_add(tun->dev, datasize);
>                 break;
>         case XDP_PASS:
> 
> > 
> > >  	return act;
> > >  }
> > >
> > > --
> > > 2.41.0
> > >
> > 
> > 
>