linux-kernel - Re: [PATCH v3 RESEND] function_graph: Enable funcgraph-args and funcgraph-retaddr to work simultaneously

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Message-ID: <20251112103804.4a43591a@gandalf.local.home>
Date: Wed, 12 Nov 2025 10:38:04 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: Donglin Peng <dolinux.peng@...il.com>
Cc: Masami Hiramatsu <mhiramat@...nel.org>,
 linux-trace-kernel@...r.kernel.org, linux-kernel@...r.kernel.org, Donglin
 Peng <pengdonglin@...omi.com>, Sven Schnelle <svens@...ux.ibm.com>
Subject: Re: [PATCH v3 RESEND] function_graph: Enable funcgraph-args and
 funcgraph-retaddr to work simultaneously

On Wed, 12 Nov 2025 17:00:10 +0800
Donglin Peng <dolinux.peng@...il.com> wrote:

> I have a question regarding the behavior of the funcgraph-args option.
> 
> Currently, when the funcgraph-args flag is set, the function arguments
> are saved.
> However, when the flag is cleared, the trace output still displays the function
> arguments unconditionally.

Yes. The flag affects the behavior of the recording and not the displaying
of the arguments. There's no reason *not* to show them if they have already
been recorded. Recording the arguments causes a noticeable overhead which
is why it is optional.

> 
> I expected that the flag would control both saving and displaying the arguments.
> But it seems that the current design only controls the saving. This
> behavior seems
> inconsistent.

Not at all. The option is only for the recording. Otherwise, one might
expect than they could do the recording and then after the fact enable
"func-args" and expect to see them. That would be more inconsistent.

> 
> I think we should add a check for the flag in the display part,
> specifically before
> calling print_function_args, to ensure that the arguments are only
> displayed when
> the flag is set.
> 
> What are your thoughts?

Big nope!

The retaddr is different, as the information is somewhat redundant to the
call chain and may cause more noise then needed.

-- Steve