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

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Message-ID: <CAErzpmsn-uys3VdchF_CKBPUQT+328wtaKnbufoDi-o9x7eHaA@mail.gmail.com>
Date: Thu, 13 Nov 2025 10:38:27 +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 Thu, Nov 13, 2025 at 10:02 AM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> On Thu, 13 Nov 2025 09:48:52 +0800
> Donglin Peng <dolinux.peng@...il.com> wrote:
>
> > 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'.
>
> What overhead are you reducing?  Why add a branch statement in a critical path?

Thanks. During testing, I found that enabling funcgraph-args incurs significant
overhead (344ms) compared to other trace options (near-zero).

# time echo 1 > options/funcgraph-retaddr

real    0m0.000s
user    0m0.000s
sys     0m0.000s

# time  echo 1 > options/funcgraph-args

real    0m0.344s
user    0m0.000s
sys     0m0.344s

So I thought it may need to be optimized.

>
> The graph_entry() should not be looking at the flags argument. It's called
> by *every function*. Also note, I recently fixed the flags to be per
> instance and not global. Which means testing the flags would require
> indirection lookups.

Thanks, I agree and it indeed introduces extra branch instruction and
may introduce more overhead when using indirection lookups.

>
> -- Steve