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

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Message-ID: <aD9yte49C_BM5oA9@google.com>
Date: Tue, 3 Jun 2025 15:09:57 -0700
From: Namhyung Kim <namhyung@...nel.org>
To: Blake Jones <blakejones@...gle.com>
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 03, 2025 at 02:54:50PM -0700, Blake Jones wrote:
> Hi Namhyung,
> 
> On Tue, Jun 3, 2025 at 2:44 PM Namhyung Kim <namhyung@...nel.org> wrote:
> > > > IIUC the metadata is collected for each BPF program which may have
> > > > multiple subprograms.  Then this patch creates multiple PERF_RECORD_
> > > > BPF_METADATA for each subprogram, right?
> > > >
> > > > Can it be shared using the BPF program ID?
> > >
> > > In theory, yes, it could be shared. But I want to be able to correlate them
> > > with the corresponding PERF_RECORD_KSYMBOL events, and KSYMBOL events for
> > > subprograms don't have the full-program ID, so I wouldn't be able to do that.
> >
> > It's unfortunate that KSYMBOL doesn't have the program ID, but IIRC the
> > following BPF_EVENT should have it.  I think it's safe to think KSYMBOLs
> > belong to the BPF_EVENT when they are from the same thread.
> 
> 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

> 
> 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.

Thanks,
Namhyung