diff mbox series

kbuild: Remove support for Clang's ThinLTO caching

Message ID 20240501-kbuild-llvm-drop-thinlto-cache-v1-1-c117cc50a24b@kernel.org (mailing list archive)
State New
Headers show
Series kbuild: Remove support for Clang's ThinLTO caching | expand

Commit Message

Nathan Chancellor May 1, 2024, 10:55 p.m. UTC 
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>
---
This is an alternative (and arguably more robust) fix to Yifan's patch
at https://lore.kernel.org/20240429220756.979347-2-elsk@google.com/.
---
 Makefile | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)


---
base-commit: e67572cd2204894179d89bd7b984072f19313b03
change-id: 20240501-kbuild-llvm-drop-thinlto-cache-f497770692f8

Best regards,

Comments

Masahiro Yamada May 3, 2024, 10:19 a.m. UTC | #1 
On Thu, May 2, 2024 at 7:55 AM Nathan Chancellor <nathan@kernel.org> 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@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>

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@google.com/.
> ---
>  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@kernel.org>
>


--
Best Regards
Masahiro Yamada
diff mbox series

Patch

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