Re: [PATCH RFC 0/3] tun zerocopy stats

[David Miller <davem@davemloft.net>]

From: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Date: Fri,  6 Oct 2017 18:25:13 -0400

> From: Willem de Bruijn <willemb@google.com>
> 
> Add zerocopy transfer statistics to the vhost_net/tun zerocopy path.
> 
> I've been using this to verify recent changes to zerocopy tuning [1].
> Sharing more widely, as it may be useful in similar future work.
> 
> Use ethtool stats as interface, as these are defined per device
> driver and can easily be extended.
> 
> Make the zerocopy release callback take an extra hop through the tun
> driver to allow the driver to increment its counters.
> 
> Care must be taken to avoid adding an alloc/free to this hot path.
> Since the caller already must allocate a ubuf_info, make it allocate
> two at a time and grant one to the tun device.
> 
>  1/3: introduce ethtool stats (`ethtool -S $DEV`) for tun devices
>  2/3: add zerocopy tx and tx_err counters
>  3/3: convert vhost_net to pass a pair of ubuf_info to tun
> 
> [1] http://patchwork.ozlabs.org/patch/822613/

This looks mostly fine to me, but I don't know enough about how vhost
and tap interact to tell whether this makes sense to upstream.

What are the runtime costs for these new statistics?