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

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Message-ID: <CAErzpmt8Dsq3f6ZUvtmc25hvy0NH6uRxBHPSxeZhkFfzkdJ04g@mail.gmail.com>
Date: Thu, 13 Nov 2025 09:48:52 +0800
From: Donglin Peng <dolinux.peng@...il.com>
To: Steven Rostedt <rostedt@...dmis.org>
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, Nov 12, 2025 at 11:37 PM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> 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.

Thank you for the clarification.

I noticed that when funcgraph-args is enabled, it registers
trace_graph_entry_args
to replace trace_graph_entry. The only difference is whether a valid
fregs pointer
is passed.

To reduce overhead, I propose consolidating the two entry functions. We could
maintain only trace_graph_entry and pass the fregs parameter to graph_entry.
Within __graph_entry, we can then conditionally record arguments based on
'TRACE_GRAPH_ARGS && !!fregs'.

Additionally, I've confirmed that trace_graph_entry is exclusively used by the
function_graph tracer. Other tracers like irqsoff and wakeup directly call
__trace_graph_entry with a NULL fregs pointer. Therefore, this consolidation
would not impact those tracers.

What are your thoughts on this approach?

Thanks,
Donglin


>
> >
> > 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