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

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Message-ID: <aEDEw7bCDAtEXfGC@x1>
Date: Wed, 4 Jun 2025 19:12:19 -0300
From: Arnaldo Carvalho de Melo <acme@...nel.org>
To: Namhyung Kim <namhyung@...nel.org>
Cc: Blake Jones <blakejones@...gle.com>, Song Liu <song@...nel.org>,
	Jiri Olsa <jolsa@...nel.org>, 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>,
	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>, Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, 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 Wed, Jun 04, 2025 at 02:40:34PM -0700, Namhyung Kim wrote:
> On Tue, Jun 03, 2025 at 03:29:35PM -0700, Blake Jones wrote:
> > 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/.

> Thanks for sharing this.

> I'm curious about the semantics of the KSYMBOL and BPF_EVENT.  And I
> feel like there should be a connection between them.

So, the comment in:

tools/perf/util/bpf-event.c

Is:

 * Synthesize PERF_RECORD_KSYMBOL and PERF_RECORD_BPF_EVENT for one bpf
 * program. One PERF_RECORD_BPF_EVENT is generated for the program. And
 * one PERF_RECORD_KSYMBOL is generated for each sub program.

which is not so nicely worded tho :-\

"One KSYMBOL per program", followed by "one KSYMBOL per sub program".

But that matches the referenced:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/events/core.c?h=v6.15#n9825

So, for these bpf_metadata_ variables, would that be strictly per
program or would it be perf 'sub program'?

Couldn't get an answer from looking at tools/bpf/bpftool/prog.c, but
seems to be with progs, not subprogs, i.e. just the PERF_RECORD_KSYMBOL
associated with progs (not subprogs) will have those variables.

But then it seems those variables _are_ associated with at least one
PERF_RECORD_KSYMBOL, right?

- Arnaldo

> Song and Jiri, what do you think?

> Thanks,
> Namhyung

> > > > 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.