linux-kernel - Re: [PATCH v1] perf doc: Improve perf trace's doc on --force-btf option

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Message-ID: <CAH0uvojANF=3y252epqFaxB5FXQ36-UrBnJ63bY6Wwr6BAxCZA@mail.gmail.com>
Date: Wed, 26 Mar 2025 11:22:22 -0700
From: Howard Chu <howardchu95@...il.com>
To: Namhyung Kim <namhyung@...nel.org>
Cc: acme@...nel.org, mingo@...hat.com, mark.rutland@....com, 
	alexander.shishkin@...ux.intel.com, jolsa@...nel.org, irogers@...gle.com, 
	adrian.hunter@...el.com, peterz@...radead.org, kan.liang@...ux.intel.com, 
	linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] perf doc: Improve perf trace's doc on --force-btf option

Hello Namhyung,

On Wed, Mar 26, 2025 at 11:14 AM Namhyung Kim <namhyung@...nel.org> wrote:
>
> Hello,
>
> On Wed, Mar 26, 2025 at 09:04:16AM -0700, Howard Chu wrote:
> > As mentioned by Arnaldo in a past discussion (Link:
> > https://lore.kernel.org/linux-perf-users/Z2m0pWiQgZNXhvxK@x1/), the
> > current documentation on --force-btf is vague and inaccurate, this patch
> > aims to provide a clearer explanation of the option.
> >
> > Signed-off-by: Howard Chu <howardchu95@...il.com>
> > Suggested-by: Arnaldo Carvalho de Melo <acme@...nel.org>
> > ---
> >  tools/perf/Documentation/perf-trace.txt | 8 ++++----
> >  1 file changed, 4 insertions(+), 4 deletions(-)
> >
> > diff --git a/tools/perf/Documentation/perf-trace.txt b/tools/perf/Documentation/perf-trace.txt
> > index 887dc37773d0..cf2445d2ba3d 100644
> > --- a/tools/perf/Documentation/perf-trace.txt
> > +++ b/tools/perf/Documentation/perf-trace.txt
> > @@ -246,10 +246,10 @@ the thread executes on the designated CPUs. Default is to monitor all CPUs.
> >       arguments to strings (pid to comm, syscall id to syscall name, etc).
> >
> >  --force-btf::
> > -     Use btf_dump to pretty print syscall argument data, instead of using hand-crafted pretty
> > -     printers. This option is intended for testing BTF integration in perf trace. btf_dump-based
> > -     pretty-printing serves as a fallback to hand-crafted pretty printers, as the latter can
> > -     better pretty-print integer flags and struct pointers.
> > +     Use libbpf's btf_dump to pretty print syscall arguments, utilizing only the debug
> > +     information from BTF. This option is intended for testing BTF integration in perf
> > +     trace. Using libbpf is a fallback to perf trace's own prettifiers generated from the
> > +     kernel source, as the latter can better pretty-print integer flags and struct members.
>
> "the latter" means our own prettifier, right?  Then why do we need the
> fallback as it's better and always works?

Honestly, for debugging—if BTF exists, it always works, yes. I don't
know if libbpf is 'better' because our prettifiers are prettier :),
yet there are so few of them.

Thanks,
Howard

>
> Thanks,
> Namhyung
>
> >
> >  PAGEFAULTS
> >  ----------
> > --
> > 2.45.2
> >