[PATCH 5.15.y] kbuild: Remove support for Clang's ThinLTO caching

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 elsk@google.com Closes: https://github.com/ClangBuiltLinux/linux/issues/2021 Reported-by: Masami Hiramatsu mhiramat@kernel.org Closes: https://lore.kernel.org/r/20220327115526.cc4b0ff55fc53c97683c3e4d@kernel.org... Signed-off-by: Nathan Chancellor nathan@kernel.org Signed-off-by: Masahiro Yamada masahiroy@kernel.org [nathan: Address conflict in Makefile] Signed-off-by: Nathan Chancellor nathan@kernel.org --- Makefile | 5 ++--- 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/Makefile b/Makefile index bfc863d71978..a9118f982484 100644 --- a/Makefile +++ b/Makefile @@ -959,7 +959,6 @@ 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 @@ -1560,7 +1559,7 @@ 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 + compile_commands.json

# Directories & files removed with 'make mrproper' MRPROPER_FILES += include/config include/generated \ @@ -1788,7 +1787,7 @@ 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: c61bd26ae81a896c8660150b4e356153da30880a