netdev - Re: [PATCH v2 net-next 00/26] net: introduce and use generic XDP stats

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Message-ID: <20211130114624.5b1f5f61@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Tue, 30 Nov 2021 11:46:24 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     David Ahern <dsahern@...il.com>
Cc:     Alexander Lobakin <alexandr.lobakin@...el.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jesse Brandeburg <jesse.brandeburg@...el.com>,
        Michal Swiatkowski <michal.swiatkowski@...ux.intel.com>,
        Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        Jonathan Corbet <corbet@....net>,
        Shay Agroskin <shayagr@...zon.com>,
        Arthur Kiyanovski <akiyano@...zon.com>,
        David Arinzon <darinzon@...zon.com>,
        Noam Dagan <ndagan@...zon.com>,
        Saeed Bishara <saeedb@...zon.com>,
        Ioana Ciornei <ioana.ciornei@....com>,
        Claudiu Manoil <claudiu.manoil@....com>,
        Tony Nguyen <anthony.l.nguyen@...el.com>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
        Marcin Wojtas <mw@...ihalf.com>,
        Russell King <linux@...linux.org.uk>,
        Saeed Mahameed <saeedm@...dia.com>,
        Leon Romanovsky <leon@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        John Fastabend <john.fastabend@...il.com>,
        Edward Cree <ecree.xilinx@...il.com>,
        Martin Habets <habetsm.xilinx@...il.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        KP Singh <kpsingh@...nel.org>,
        Lorenzo Bianconi <lorenzo@...nel.org>,
        Yajun Deng <yajun.deng@...ux.dev>,
        Sergey Ryazanov <ryazanov.s.a@...il.com>,
        David Ahern <dsahern@...nel.org>,
        Andrei Vagin <avagin@...il.com>,
        Johannes Berg <johannes.berg@...el.com>,
        Vladimir Oltean <vladimir.oltean@....com>,
        Cong Wang <cong.wang@...edance.com>, netdev@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-rdma@...r.kernel.org, bpf@...r.kernel.org,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v2 net-next 00/26] net: introduce and use generic XDP
 stats

On Tue, 30 Nov 2021 10:38:14 -0700 David Ahern wrote:
> On 11/30/21 10:04 AM, Jakub Kicinski wrote:
> > On Tue, 30 Nov 2021 17:34:54 +0100 Alexander Lobakin wrote:  
> >> I know about ETHTOOL_STAT_NOT_SET, but RTNL xstats doesn't use this,
> >> does it?  
> > 
> > Not sure if you're asking me or Dave but no, to my knowledge RTNL does
> > not use such semantics today. But the reason is mostly because there
> > weren't many driver stats added there. Knowing if an error counter is
> > not supported or supporter and 0 is important for monitoring. Even if
> > XDP stats don't have a counter which may not be supported today it's
> > not a good precedent to make IMO.
> 
> Today, stats are sent as a struct so skipping stats whose value is 0 is
> not an option. When using individual attributes for the counters this
> becomes an option. Given there is no value in sending '0' why do it?

To establish semantics of what it means that the statistic is not
reported. If we need to save space we'd need an extra attr with 
a bitmap of "these stats were skipped because they were zero".
Or conversely some way of querying supported stats.

> Is your pushback that there should be a uapi to opt-in to this behavior?

Not where I was going with it, but it is an option. If skipping 0s was
controlled by a flag a dump without such flag set would basically serve
as a way to query supported stats.