Re: [PATCH 6.1.y] kbuild: Remove support for Clang's ThinLTO caching

[Greg KH <gregkh@linuxfoundation.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 <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(-)

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