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

[Greg KH <gregkh@linuxfoundation.org>]

On Wed, Jun 19, 2024 at 07:23:39AM -0700, Nathan Chancellor wrote:
> On Wed, Jun 19, 2024 at 12:51:55PM +0200, Greg KH wrote:
> > 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?
> 
> I don't understand how that is possible, this was generated directly on
> top of 6.1.93 (as evidenced by the base commit) and there were no
> changes to Makefile in 6.1.94. It still applies cleanly for me?
> 
>   $ curl -LSs https://lore.kernel.org/all/20240613183322.1088226-1-nathan@kernel.org/raw | patch -p1
>   patching file Makefile
> 

Very odd, I just tried it again and it worked, must have been a problem
on my side, sorry for the noise.

greg k-h