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

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Message-ID: <Z-REG5kkE1RahBW2@google.com>
Date: Wed, 26 Mar 2025 11:14:51 -0700
From: Namhyung Kim <namhyung@...nel.org>
To: Howard Chu <howardchu95@...il.com>
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,

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?

Thanks,
Namhyung

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