linux-kernel - Re: [PATCH v5 1/5] bug/kunit: Core support for suppressing warning backtraces

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Message-ID: <20250531074615.GA19817@noisy.programming.kicks-ass.net>
Date: Sat, 31 May 2025 09:46:15 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Alessandro Carminati <acarmina@...hat.com>
Cc: linux-kselftest@...r.kernel.org,
	Dan Carpenter <dan.carpenter@...aro.org>,
	Kees Cook <keescook@...omium.org>,
	Daniel Diaz <daniel.diaz@...aro.org>,
	David Gow <davidgow@...gle.com>,
	Arthur Grillo <arthurgrillo@...eup.net>,
	Brendan Higgins <brendan.higgins@...ux.dev>,
	Naresh Kamboju <naresh.kamboju@...aro.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Maxime Ripard <mripard@...nel.org>,
	Ville Syrjala <ville.syrjala@...ux.intel.com>,
	Daniel Vetter <daniel@...ll.ch>, Guenter Roeck <linux@...ck-us.net>,
	Alessandro Carminati <alessandro.carminati@...il.com>,
	Jani Nikula <jani.nikula@...el.com>,
	Jeff Johnson <jeff.johnson@....qualcomm.com>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Shuah Khan <skhan@...uxfoundation.org>,
	Linux Kernel Functional Testing <lkft@...aro.org>,
	dri-devel@...ts.freedesktop.org, kunit-dev@...glegroups.com,
	linux-kernel@...r.kernel.org, Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH v5 1/5] bug/kunit: Core support for suppressing warning
 backtraces

On Fri, May 30, 2025 at 04:01:40PM +0200, Peter Zijlstra wrote:

> I'm not really concerned with performance here, but more with the size
> of the code emitted by WARN_ONCE(). There are a *ton* of WARN sites,
> while only one report_bug() and printk().

We need a new and stronger unlikely(), resulting in the compiler being
forced to split a .cold sub-function/part which lives in .text.unlikely

At that point it becomes less of a concern I suppose. 

AFAIK the only means of achieving that with the current compilers is
doing a manual function split and marking the part __cold -- which is
unfortunate.

At some point GCC explored label attributes, and we were able to mark
labels with cold, but that never really worked / went anywhere.