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

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Message-ID: <20251112210236.07deb6b2@gandalf.local.home>
Date: Wed, 12 Nov 2025 21:02:36 -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 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?

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.

-- Steve