On Thu, May 2, 2024 at 7:55 AM Nathan Chancellor <nathan@xxxxxxxxxx> wrote:
>
> 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@xxxxxxxxxxxxxxx
> Fixes: dc5723b02e52 ("kbuild: add support for Clang LTO")
> Reported-by: Yifan Hong <elsk@xxxxxxxxxx>
> Closes: https://github.com/ClangBuiltLinux/linux/issues/2021
> Reported-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> Closes: https://lore.kernel.org/r/20220327115526.cc4b0ff55fc53c97683c3e4d@xxxxxxxxxx/
> Signed-off-by: Nathan Chancellor <nathan@xxxxxxxxxx>
Applied to linux-kbuild.
Thanks.
> ---
> This is an alternative (and arguably more robust) fix to Yifan's patch
> at https://lore.kernel.org/20240429220756.979347-2-elsk@xxxxxxxxxx/.
> ---
> Makefile | 5 ++---
> 1 file changed, 2 insertions(+), 3 deletions(-)
>
> diff --git a/Makefile b/Makefile
> index 40fb2ca6fe4c..6b6a4a757062 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -942,7 +942,6 @@ 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
> @@ -1477,7 +1476,7 @@ endif # CONFIG_MODULES
> # Directories & files removed with 'make clean'
> CLEAN_FILES += vmlinux.symvers modules-only.symvers \
> modules.builtin modules.builtin.modinfo modules.nsdeps \
> - compile_commands.json .thinlto-cache rust/test \
> + compile_commands.json rust/test \
> rust-project.json .vmlinux.objs .vmlinux.export.c
>
> # Directories & files removed with 'make mrproper'
> @@ -1783,7 +1782,7 @@ 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: e67572cd2204894179d89bd7b984072f19313b03
> change-id: 20240501-kbuild-llvm-drop-thinlto-cache-f497770692f8
>
> Best regards,
> --
> Nathan Chancellor <nathan@xxxxxxxxxx>
>
--
Best Regards
Masahiro Yamada
