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 0BE8D14B06E for ; Thu, 13 Jun 2024 18:33:41 +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=1718303622; cv=none; b=aP667He+2p5hvMXGc4IPigwE/HRhEf3T5Hty7CCa3KJ6VKiioTAFJ95ISCDH3NW0nc8K117FzL9VtD/FtJkQcvr2u3TkyB844jVHB2rzcPbWZhL+jBxrk07nxutUXFIbPW4VbLnmgS8oswcNHE3pI7ogjQlCvkUsPJTXH5kkyYA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1718303622; c=relaxed/simple; bh=8u0Y6svM88axULhOTqGS1VfCafqaKn6kBA1ZAFcW4hg=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version:Content-Type; b=Z4+okKnra2OxyKvfrMeexh85lQeBqg7dhtfD67YCVUzZRlNGDu9hK2qb00A6DFXhrRBvp0XY+YaQwgvvnBpMRpnhRHWzRB5qAcZdzHjxz/LS6oeGryr3Aluc8HdC6aSrnRgZooNCQyVlgMC/k6oFcJSSRMDw+j4kF1cH19B5+u8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=NkBCvSoN; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="NkBCvSoN" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 495FCC2BBFC; Thu, 13 Jun 2024 18:33:41 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1718303621; bh=8u0Y6svM88axULhOTqGS1VfCafqaKn6kBA1ZAFcW4hg=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=NkBCvSoN8Ev36e0RvfYdBENNgQ0HmgS4YA1UnYPr5YQAJz8B8B9ngT3gjD8g4wv/1 /XzKxgp0rjBjwPTWRQMg8VhLnHRGw9Q6TognXjvWH4NJHNQmHPdFgX+V1bSuN8t0Fe hSaG0iPDhlojU0ZEtV0Mk4m27ULndV4mJNLueq6t+NGVySstJOSH9yvJvV6aWLWhKI 6d6f6pF6UL80xHZicTH1GyRR9TgZdigDbQAObxuI1jHZW+ftPOoJkohIX8hY4pQbGo RLWtjv61k7We+ZmF60jxgV83QRkycV+CNqGagV4DuKJHJUAQV/gQow34Kj9rR+HOqt TCT/fH3ChvpGA== From: Nathan Chancellor To: gregkh@linuxfoundation.org Cc: nathan@kernel.org, stable@vger.kernel.org Subject: [PATCH 6.1.y] kbuild: Remove support for Clang's ThinLTO caching Date: Thu, 13 Jun 2024 11:33:22 -0700 Message-ID: <20240613183322.1088226-1-nathan@kernel.org> X-Mailer: git-send-email 2.45.2 In-Reply-To: <2024061340-troubling-automated-9989@gregkh> References: <2024061340-troubling-automated-9989@gregkh> 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-Transfer-Encoding: 8bit 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(-) diff --git a/Makefile b/Makefile index c5147f1c46f8..abe7ba05155b 100644 --- a/Makefile +++ b/Makefile @@ -980,7 +980,6 @@ endif ifdef CONFIG_LTO_CLANG ifdef CONFIG_LTO_CLANG_THIN CC_FLAGS_LTO := -flto=thin -fsplit-lto-unit -KBUILD_LDFLAGS += --thinlto-cache-dir=$(extmod_prefix).thinlto-cache else CC_FLAGS_LTO := -flto endif @@ -1588,7 +1587,7 @@ endif # CONFIG_MODULES # Directories & files removed with 'make clean' CLEAN_FILES += include/ksym vmlinux.symvers modules-only.symvers \ modules.builtin modules.builtin.modinfo modules.nsdeps \ - compile_commands.json .thinlto-cache rust/test rust/doc \ + compile_commands.json rust/test rust/doc \ .vmlinux.objs .vmlinux.export.c # Directories & files removed with 'make mrproper' @@ -1884,7 +1883,7 @@ PHONY += compile_commands.json clean-dirs := $(KBUILD_EXTMOD) clean: rm-files := $(KBUILD_EXTMOD)/Module.symvers $(KBUILD_EXTMOD)/modules.nsdeps \ - $(KBUILD_EXTMOD)/compile_commands.json $(KBUILD_EXTMOD)/.thinlto-cache + $(KBUILD_EXTMOD)/compile_commands.json PHONY += prepare # now expand this into a simple variable to reduce the cost of shell evaluations base-commit: ae9f2a70d69e9c840ee1eda201f09662ca7e2038 -- 2.45.2