linux-kernel - Re: [PATCH v9 2/4] tracing: Add a tracepoint verification check at build time

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Message-ID: <aPf5TVismRLkQQOE@levanger>
Date: Tue, 21 Oct 2025 23:21:17 +0200
From: Nicolas Schier <nsc@...nel.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Steven Rostedt <rostedt@...nel.org>, linux-kernel@...r.kernel.org,
	linux-trace-kernel@...r.kernel.org, linux-kbuild@...r.kernel.org,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arnd Bergmann <arnd@...db.de>,
	Masahiro Yamada <masahiroy@...nel.org>,
	Nathan Chancellor <nathan@...nel.org>,
	Nick Desaulniers <nick.desaulniers+lkml@...il.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Randy Dunlap <rdunlap@...radead.org>,
	Stephen Rothwell <sfr@...b.auug.org.au>
Subject: Re: [PATCH v9 2/4] tracing: Add a tracepoint verification check at
 build time

On Tue, Oct 21, 2025 at 03:47:37PM -0400, Steven Rostedt wrote:
> On Fri, 17 Oct 2025 22:15:20 +0200
> Nicolas Schier <nsc@...nel.org> wrote:
> 
> > > +# To check for unused tracepoints (tracepoints that are defined but never
> > > +# called), run with:
> > > +#
> > > +# make UT=1
> > > +#
> > > +# Each unused tracepoints can take up to 5KB of memory in the running kernel.
> > > +# It is best to remove any that are not used.
> > > +
> > > +ifeq ("$(origin UT)", "command line")
> > > +  WARN_ON_UNUSED_TRACEPOINTS := $(UT)
> > > +endif
> > > +
> > > +export WARN_ON_UNUSED_TRACEPOINTS  
> > 
> > Is there a special reason why you chose to introduce a new command-line
> > variable instead of extending KBUILD_EXTRA_WARN / W ?
> 
> Honestly, I didn't think about using KBUILD_EXTRA_WARN. I also want this
> option to go away after we remove the current unused tracepoints so that
> any new ones will always cause a warning.
> 
> The only reason not to make it always warn is because I don't want to add
> warnings for the existing code. I'm working on having outreachy projects to
> remove the currently unused tracepoints. Once that is done, then this
> option is going to go away and the build will always warn on unused
> tracepoints.
> 
> I thought it might be easier to remove it without any issues if it's a new
> command line that goes away in the future.
> 
> Looking at EXTRA_WARN, it appears to be for basic issues with the code and
> adds new C compiler warning flags. This isn't exactly the same.
> 
> If you think it makes sense to extend EXTRA_WARN, I can still go ahead and
> do that.

thanks for clarification!  For completeness: KBUILD_EXTRA_WARN is also
used for non-C related checks (cp. scripts/misc-check).
I somehow missed that UT= shall exist temporarily only - if this is
still the plan, I don't see a strong reason to put to much work into
integration in KBUILD_EXTRA_WARN.

Kind regards
Nicolas