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Thu, 29 Oct 2026 01:49:45 -0400 (EDT) X-Mailer: MessagingEngine.com Webmail Interface X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 X-ThreadId: ADl5zPqKnVsk Date: Thu, 29 Oct 2026 05:49:24 +0000 From: "Chuck Wolber" To: "Sasha Levin" , "Peter Zijlstra" Cc: linux-kernel@vger.kernel.org, "Bill Wendling" , "Justin Stitt" , llvm@lists.linux.dev, "Marco Elver" , "Masami Hiramatsu" , "Miguel Ojeda" , "Nathan Chancellor" , "Nick Desaulniers" , "Thomas Gleixner" , wentaoz5@illinois.edu, chuck.wolber@boeing.com, "Josh Poimboeuf" Message-Id: <1f0ca403-551f-48c5-8e5b-1e07d3227f34@app.fastmail.com> In-Reply-To: <20261028164556.1610305-1-sashal@kernel.org> References: <20261028120034.871983-1-sashal@kernel.org> <20261028120034.871983-5-sashal@kernel.org> <20261028135140.GR2668564@noisy.programming.kicks-ass.net> <20261028140118.GB2669272@noisy.programming.kicks-ass.net> <20261028164556.1610305-1-sashal@kernel.org> Subject: Re: [RFC PATCH 04/20] compiler_types.h: Don't instrument __always_inline functions for coverage Content-Type: text/plain Content-Transfer-Encoding: 7bit On Wed, Oct 28, 2026, at 4:45 PM, Sasha Levin wrote: > On Wed, Oct 28, 2026 at 02:51:40PM +0100, Peter Zijlstra wrote: >>This is pretty vile. But also, since people were trying to use MC/DC >>for validation efforts and the like, how does this impact them? If I >>understand correctly their previous emails on the topic, this means >>they have to go manually audit every single __always_inline function. > > Yes, the __always_inline helpers join noinstr code as something that > has to be justified separately rather than covered by tests. Steve's > answer to your noinstr question in 2024 [1] was that this is workable > as long as the number of distinct categories stays small, and this > adds one (556 functions and 445 MC/DC conditions for the config in the > changelog). > > Chuck, Steve, does that match what you'd expect on your side? Yes, this is correct. And "small" is a relative term that should not be a factor that holds back this patchset. Perfect is the enemy of good and we can always improve on the level of coverage over time. >>Still, I'm left wondering if these tools should be fixed. For >>instance, while the front-end emits this instrumentation, the backend >>could easily remove it again when it finds itself in the context of a >>noinstr function after inlining. > > Yup, we should do that. Both GCC and Clang add the counters before > inlining and let always_inline win over the attribute mismatch, so > neither can do it today. We can look at improving them, but it's > probably outside of what this series can do. Agreed. This is a good example of how the level of coverage can be improved. ..Ch:W..