[PATCH net-next v2 03/10] tun: switch to net core provided statistics counters

[Jakub Kicinski]

On Fri, 6 Nov 2020 09:27:45 +0100 Heiner Kallweit wrote:
> On 06.11.2020 08:48, Heiner Kallweit wrote:
> > On 06.11.2020 02:14, Jakub Kicinski wrote:  
> >> On Wed, 4 Nov 2020 15:25:24 +0100 Heiner Kallweit wrote:  
> >>> @@ -1066,7 +1054,7 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev)
> >>>  	return NETDEV_TX_OK;
> >>>  
> >>>  drop:
> >>> -	this_cpu_inc(tun->pcpu_stats->tx_dropped);
> >>> +	dev->stats.tx_dropped++;
> >>>  	skb_tx_error(skb);
> >>>  	kfree_skb(skb);
> >>>  	rcu_read_unlock();  
> >>
> >> This is no longer atomic. Multiple CPUs may try to update it at the
> >> same time.
> >>
> >> Do you know what the story on dev->rx_dropped is? The kdoc says drivers
> >> are not supposed to use it but:
> >>
> >> drivers/net/ipvlan/ipvlan_core.c:               atomic_long_inc(&skb->dev->rx_dropped);
> >> drivers/net/macvlan.c:  atomic_long_inc(&skb->dev->rx_dropped);
> >> drivers/net/vxlan.c:            atomic_long_inc(&vxlan->dev->rx_dropped);
> >>
> >> Maybe tun can use it, too?
> >>  
> > Thanks, yes that should be possible. Here we speak about tx_dropped,
> > but AFAICS the same applies as for rx_dropped. Will change it accordingly
> > in a v3.
> >   
> For rx_dropped and tx_dropped it's easy, however tun also has a per-cpu
> counter for rx_frame_errors that is incremented if virtio_net_hdr_to_skb()
> fails. Not sure how to deal best with this one.

Umpf, yeah, so I'd probably add an atomic long to struct tun_struct,
but then you'll need to keep the ndo implementation instead of using
dev_get_tstats64 directly. I can't think of a better way, tho.