netdev - Re: consistency for statistics with XDP mode

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Message-ID: <20181130204148.22f16080@cakuba.netronome.com>
Date:   Fri, 30 Nov 2018 20:41:48 -0800
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     David Ahern <dsahern@...il.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Saeed Mahameed <saeedm@...lanox.com>,
        "toke@...e.dk" <toke@...e.dk>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "pstaszewski@...are.pl" <pstaszewski@...are.pl>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "jasowang@...hat.com" <jasowang@...hat.com>,
        "brouer@...hat.com" <brouer@...hat.com>
Subject: Re: consistency for statistics with XDP mode

On Fri, 30 Nov 2018 13:35:53 -0700, David Ahern wrote:
> On 11/30/18 1:30 PM, Michael S. Tsirkin wrote:
> >>>> I would like to see basic packets, bytes, and dropped counters
> >>>> tracked
> >>>> for Rx and Tx via the standard netdev counters for all devices.   
> >>
> >> The problem of reporting XDP_DROP in the netedev drop counter is that
> >> they don't fit this counter description : "no space in linux buffers"
> >> and it will be hard for the user to determine whether these drops are
> >> coming from XDP or because no buffer is available, which will make it
> >> impossible to estimate packet rate performance without looking at
> >> ethtool stats.
> >> And reporting XDP_DROP in the netdev rx packets counter is somehow
> >> misleading.. since those packets never made it out of this driver.. 
> >>
> >>
> >> And reporting XDP_DROP in the netdev rx packets counter is somehow
> >> misleading.. since those packets never made it out of this driver..  
> > 
> > I think I agree. XDP needs minimal overhead - if user wants to do
> > counters then user can via maps. And in a sense XDP dropping packet
> > is much like e.g. TCP dropping packet - it is not counted
> > against the driver since it's not driver's fault.
> >   
> 
> XDP dropping a packet is completely different.
> 
> stats are important. packets disappearing with no counters -- standard
> counters visible by standard tools -- is a user nightmare. If the
> agreement is for XDP drops to be in driver level (e.g., xdp_drop) that
> is fine since it is still retrievable by ethtool -S (existing APIs and
> existing tools).

I don't think that's completely fair.  Disappearing packets are a
nightmare, but if the user installed a program which silently drops
packets without incrementing any counter it's their own fault.  If
cls_bpf returns STOLEN or TRAP, I don't think that's gonna get counted
anywhere.

I don't think DPDK drivers maintain "just in case" statistics, either..