netdev - Re: consistency for statistics with XDP mode

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Date:   Fri, 30 Nov 2018 13:35:53 -0700
From:   David Ahern <dsahern@...il.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>,
        Saeed Mahameed <saeedm@...lanox.com>
Cc:     "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 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).