From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 0423947F2F7; Tue, 15 Sep 2026 11:16:40 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789471001; cv=none; b=ZlAr46vHxHwelmHyfzlbwyC6K52aXAJwZ9UxiOLqAZRWD7x5P2vXs0Uwv0p4iyfPVBIkZhHw1v+hPPWBmtyyam2cCP5yRTPrLi4rJ/Jjhz6OrFLA3SQ5ZuBuKGBTEIJK62W41SCamOpb6ueJTc+WWeawIx42nu7HhTfbLr6SV4c= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789471001; c=relaxed/simple; bh=8jGKX9lxad0nC5LwC+m3/xZlEuIw43z0LUda/hlK958=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=XkemP/FZOM9cMn9WGK8XwtLVQvVenCTlE9y8z5tTFp9hSxxwIBoho8sS0BAdDYx0Qzj8vq/37gfHlR1ONmUA1hmqrdDnfE4ADCHFU7YcQVWNktxAAJfifnyIBMXAsPkHEP1gUKCnTfXR6q81ySKol9b6um8lsiaySy+fEfvM9LY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Q++L+p3W; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="Q++L+p3W" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 5C04A1F000FF; Tue, 15 Sep 2026 11:16:28 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1789470999; bh=8jGKX9lxad0nC5LwC+m3/xZlEuIw43z0LUda/hlK958=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=Q++L+p3WYA6AP7ADzOymmxif7GVxlUsd1TMY/VWqmWYAk0XvwziMicEQbz+6DNao7 XcPR++La0+G90EC9iwDjmpnkVeOifjMFofsHBu77Xi5mnhjyguFYPGvKBIUrquwQCI iiCNgS0iUL+FOLtDfRnYLFB+s6SvqNFqz3K69R94xTah6cUkSNdn52CWUZue0M+5dj +CrqVU1SNkWozON17mn8scd2A3/hV0UD6WFULz9ATw+O6zePrJc7pOK13WWXtgy/yq DD0zN4DWDoecYccw8W4FesguEdT1rAdhHxZy1ZHBQenSINumCyPKtJXWVqnfdmsQoZ Uqd2TxsmCMfLg== Date: Tue, 15 Sep 2026 12:16:25 +0100 From: "Lorenzo Stoakes (ARM)" To: Kees Cook Cc: Linus Torvalds , Nathan Chancellor , Nicolas Schier , Nick Desaulniers , Bill Wendling , Justin Stitt , Masahiro Yamada , Alexey Gladkov , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H. Peter Anvin" , Paul Walmsley , Palmer Dabbelt , Albert Ou , Alexandre Ghiti , Arnd Bergmann , Catalin Marinas , Will Deacon , Mark Rutland , Ard Biesheuvel , Ilias Apalodimas , Josh Poimboeuf , Peter Zijlstra , Miguel Ojeda , Boqun Feng , Gary Guo , =?utf-8?B?QmrDtnJu?= Roy Baron , Benno Lossin , Andreas Hindborg , Alice Ryhl , Trevor Gross , Danilo Krummrich , Daniel Almeida , Tamir Duberstein , Alexandre Courbot , Onur =?utf-8?B?w5Z6a2Fu?= , Jonathan Corbet , Randy Dunlap , "Gustavo A. R. Silva" , linux-kbuild@vger.kernel.org, linux-kernel@vger.kernel.org, llvm@lists.linux.dev, linux-riscv@lists.infradead.org, linux-arch@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-efi@vger.kernel.org, rust-for-linux@vger.kernel.org, linux-doc@vger.kernel.org, Jens Axboe , linux-hardening@vger.kernel.org Subject: Re: [PATCH v2 18/21] kbuild: rust: optionally parallelise rustc front end Message-ID: References: <20260914-build-speedup-v2-0-39817ec5db23@kernel.org> <20260914-build-speedup-v2-18-39817ec5db23@kernel.org> <202609141124.F24B6F77@keescook> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Tue, Sep 15, 2026 at 12:09:04PM +0100, Lorenzo Stoakes (ARM) wrote: > On Mon, Sep 14, 2026 at 11:32:14AM -0700, Kees Cook wrote: > > On Mon, Sep 14, 2026 at 10:22:17AM +0100, Lorenzo Stoakes (ARM) wrote: > > > The threads are taken from make's jobserver, so a parallel build is not > > > oversubscribed. It was found that benefits level off at 8 threads (16 was > > > found to be around the same, and 32 slower). > > > > I think it's worth noting that this comes from rustc itself (it has > > built-in support for the make jobserver). See commit ecab4115c44c > > ("kbuild: mark `rustc` (and others) invocations as recursive"). > > > > What happens when there is no jobserver, e.g. "make" without -j, or > > "make -j1"? > > > > > [...] > > > rustc has a parallel front end, enabled with -Zthreads, available since > > > rust 1.84 [0] and therefore on every rustc the kernel supports. > > > > > > It is not yet stable and its output is not reproducible, so it cannot be > > > [...] > > > +The number of threads rustc's parallel front end may use, passed to it as > > > +``-Zthreads``. Unset by default, as the parallel front end is not yet > > > +stable and its output is not reproducible. The gain levels off at 8 > > > +threads. > > > > Okay, unstable, not reproducible. > > > > > Generated output was confirmed byte-for-byte identical. > > > > But this says otherwise? I am confused. Reproducible or not? On this though - the lack of reproducibility was the reports in the rust world of the -Zthreads flag sometimes resulting in that. I never observed that locally at all, and so locally it was all byte-for-byte identical. It could have been worded better :) > > Well, we can set all of this aside as, based on Miguel's feedback, I think > dropping this commit is best :) > > > > > -- > > Kees Cook > > -- > Cheers, Lorenzo -- Cheers, Lorenzo