linux-kernel - Re: [PATCH 2/3] perf: collect BPF metadata from existing BPF programs

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Message-ID: <CAP_z_Cg0ZCfvEFpJpvhuRcUkjV_paCODw2J61D3YQMm7dg0aGg@mail.gmail.com>
Date: Tue, 3 Jun 2025 15:29:35 -0700
From: Blake Jones <blakejones@...gle.com>
To: Namhyung Kim <namhyung@...nel.org>
Cc: Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	Andrii Nakryiko <andrii@...nel.org>, Martin KaFai Lau <martin.lau@...ux.dev>, 
	Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>, John Fastabend <john.fastabend@...il.com>, 
	KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>, 
	Jiri Olsa <jolsa@...nel.org>, Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, 
	Arnaldo Carvalho de Melo <acme@...nel.org>, Mark Rutland <mark.rutland@....com>, 
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Ian Rogers <irogers@...gle.com>, 
	Adrian Hunter <adrian.hunter@...el.com>, Kan Liang <kan.liang@...ux.intel.com>, 
	Chun-Tse Shao <ctshao@...gle.com>, Zhongqiu Han <quic_zhonhan@...cinc.com>, 
	James Clark <james.clark@...aro.org>, Charlie Jenkins <charlie@...osinc.com>, 
	Andi Kleen <ak@...ux.intel.com>, Dmitry Vyukov <dvyukov@...gle.com>, Leo Yan <leo.yan@....com>, 
	Yujie Liu <yujie.liu@...el.com>, Graham Woodward <graham.woodward@....com>, 
	Yicong Yang <yangyicong@...ilicon.com>, Ben Gainey <ben.gainey@....com>, 
	linux-kernel@...r.kernel.org, bpf@...r.kernel.org, 
	linux-perf-users@...r.kernel.org
Subject: Re: [PATCH 2/3] perf: collect BPF metadata from existing BPF programs

On Tue, Jun 3, 2025 at 3:10 PM Namhyung Kim <namhyung@...nel.org> wrote:
> > Hmmm. Is that documented and tested anywhere? Offhand it sounds like an
> > implementation detail that I wouldn't feel great about depending on -
> > certainly not without a strong guarantee that it wouldn't change.
>
> Good point.  Maybe BPF folks have some idea?
>
> Anyway the current code generates them together in a function.
>
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/events/core.c?h=v6.15#n9825

It certainly does, yeah. But I don't want to have that become another
instance of https://www.hyrumslaw.com/.

> > Can you say more about why the duplicated records concern you?
>
> More data means more chance to lost something.  I don't expect this is
> gonna be a practical concern but in general we should pursue less data.

That makes sense. In this case, it will only show up for BPF programs that
define "bpf_metadata_" variables (which is already an opt-in action), and
the number of variables a given program defines is likely to be quite small.
So I think the cost of the marginal increase in data generated is outweighed
by the usability and reliability benefits of being able to match these events
1:1 with the KSYMBOL events. If this proves to be a problem in practice,
it can be revisited.

Blake