From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 1D85675811 for ; Wed, 19 Jun 2024 10:51:58 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1718794319; cv=none; b=MOMlLgmXKqptceQVbr+tAVUKqv3sWTF+VPzWYl9Er8s6HnmK9Dfy6tgID+ReB4IrSJYiHseQcypVQxUiMsM1We4mKeSiX8+cvztARRLWMaGMxB+5YxAxudcX+nsPA8ghVRm2vJU4mVuSMhoCzbY7W0gQ+fAGUEpEyXx9tLENZ3E= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1718794319; c=relaxed/simple; bh=CuaRLi1tmzTJ7D6ZP1HcAdkBlKhP69emUoOTwxf5CUQ=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=UiHEiTeTqWIkUrG8ZVrPV1yNE3n1g7FTe808QwnAAVLHtofGX0jsytf1W1R/Sa9Yj/qei2J0XkMAXbVnKeboZ0clJfbEtJVg9NUJahFwz8pj2G/txerRIcdbwO+eK1pL2/NYms59NG6eyV5uDpBCbO0CxByxTPQAdX4LOngGJk0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=nZjuewND; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="nZjuewND" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 24EEBC4AF1C; Wed, 19 Jun 2024 10:51:57 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1718794318; bh=CuaRLi1tmzTJ7D6ZP1HcAdkBlKhP69emUoOTwxf5CUQ=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=nZjuewNDEY8VQy9xTsHZOYX8OnKxmaTrrFsMLPXM5izlaSvlWsOtCOurc9yH6VBSK EDij7bo5v09YhXDMoOTIgltHowyh1OvWDva717GPEVCHgsdS0Sf0ZhUELX3hJ92TYt M38eWAIRTTwAgr/ZZ2zUyyeIssqD4B0dZh7XeZho= Date: Wed, 19 Jun 2024 12:51:55 +0200 From: Greg KH To: Nathan Chancellor Cc: stable@vger.kernel.org Subject: Re: [PATCH 6.1.y] kbuild: Remove support for Clang's ThinLTO caching Message-ID: <2024061937-footpad-altitude-1462@gregkh> References: <2024061340-troubling-automated-9989@gregkh> <20240613183322.1088226-1-nathan@kernel.org> Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20240613183322.1088226-1-nathan@kernel.org> On Thu, Jun 13, 2024 at 11:33:22AM -0700, Nathan Chancellor wrote: > commit aba091547ef6159d52471f42a3ef531b7b660ed8 upstream. > > There is an issue in clang's ThinLTO caching (enabled for the kernel via > '--thinlto-cache-dir') with .incbin, which the kernel occasionally uses > to include data within the kernel, such as the .config file for > /proc/config.gz. For example, when changing the .config and rebuilding > vmlinux, the copy of .config in vmlinux does not match the copy of > .config in the build folder: > > $ echo 'CONFIG_LTO_NONE=n > CONFIG_LTO_CLANG_THIN=y > CONFIG_IKCONFIG=y > CONFIG_HEADERS_INSTALL=y' >kernel/configs/repro.config > > $ make -skj"$(nproc)" ARCH=x86_64 LLVM=1 clean defconfig repro.config vmlinux > ... > > $ grep CONFIG_HEADERS_INSTALL .config > CONFIG_HEADERS_INSTALL=y > > $ scripts/extract-ikconfig vmlinux | grep CONFIG_HEADERS_INSTALL > CONFIG_HEADERS_INSTALL=y > > $ scripts/config -d HEADERS_INSTALL > > $ make -kj"$(nproc)" ARCH=x86_64 LLVM=1 vmlinux > ... > UPD kernel/config_data > GZIP kernel/config_data.gz > CC kernel/configs.o > ... > LD vmlinux > ... > > $ grep CONFIG_HEADERS_INSTALL .config > # CONFIG_HEADERS_INSTALL is not set > > $ scripts/extract-ikconfig vmlinux | grep CONFIG_HEADERS_INSTALL > CONFIG_HEADERS_INSTALL=y > > Without '--thinlto-cache-dir' or when using full LTO, this issue does > not occur. > > Benchmarking incremental builds on a few different machines with and > without the cache shows a 20% increase in incremental build time without > the cache when measured by touching init/main.c and running 'make all'. > > ARCH=arm64 defconfig + CONFIG_LTO_CLANG_THIN=y on an arm64 host: > > Benchmark 1: With ThinLTO cache > Time (mean ± σ): 56.347 s ± 0.163 s [User: 83.768 s, System: 24.661 s] > Range (min … max): 56.109 s … 56.594 s 10 runs > > Benchmark 2: Without ThinLTO cache > Time (mean ± σ): 67.740 s ± 0.479 s [User: 718.458 s, System: 31.797 s] > Range (min … max): 67.059 s … 68.556 s 10 runs > > Summary > With ThinLTO cache ran > 1.20 ± 0.01 times faster than Without ThinLTO cache > > ARCH=x86_64 defconfig + CONFIG_LTO_CLANG_THIN=y on an x86_64 host: > > Benchmark 1: With ThinLTO cache > Time (mean ± σ): 85.772 s ± 0.252 s [User: 91.505 s, System: 8.408 s] > Range (min … max): 85.447 s … 86.244 s 10 runs > > Benchmark 2: Without ThinLTO cache > Time (mean ± σ): 103.833 s ± 0.288 s [User: 232.058 s, System: 8.569 s] > Range (min … max): 103.286 s … 104.124 s 10 runs > > Summary > With ThinLTO cache ran > 1.21 ± 0.00 times faster than Without ThinLTO cache > > While it is unfortunate to take this performance improvement off the > table, correctness is more important. If/when this is fixed in LLVM, it > can potentially be brought back in a conditional manner. Alternatively, > a developer can just disable LTO if doing incremental compiles quickly > is important, as a full compile cycle can still take over a minute even > with the cache and it is unlikely that LTO will result in functional > differences for a kernel change. > > Cc: stable@vger.kernel.org > Fixes: dc5723b02e52 ("kbuild: add support for Clang LTO") > Reported-by: Yifan Hong > Closes: https://github.com/ClangBuiltLinux/linux/issues/2021 > Reported-by: Masami Hiramatsu > Closes: https://lore.kernel.org/r/20220327115526.cc4b0ff55fc53c97683c3e4d@kernel.org/ > Signed-off-by: Nathan Chancellor > Signed-off-by: Masahiro Yamada > [nathan: Address conflict in Makefile] > Signed-off-by: Nathan Chancellor > --- > Makefile | 5 ++--- > 1 file changed, 2 insertions(+), 3 deletions(-) This applied to 5.15.y, not 6.1.y :( Can you rebase and resend a fix for 6.1.y? thanks, greg k-h