linux-kernel - Re: [PATCH v3] kbuild: distributed build support for Clang ThinLTO

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Message-ID: <CAF1bQ=QSBOdvqqBPPv70z1hLX4echmfzBDCu5o4LtKrZEaAJUw@mail.gmail.com>
Date: Tue, 27 May 2025 14:05:12 -0700
From: Rong Xu <xur@...gle.com>
To: Masahiro Yamada <masahiroy@...nel.org>
Cc: Nathan Chancellor <nathan@...nel.org>, Eric Naim <dnaim@...hyos.org>, 
	Nicolas Schier <nicolas.schier@...ux.dev>, 
	Nick Desaulniers <nick.desaulniers+lkml@...il.com>, Bill Wendling <morbo@...gle.com>, 
	Justin Stitt <justinstitt@...gle.com>, Miguel Ojeda <ojeda@...nel.org>, 
	Thomas Gleixner <tglx@...utronix.de>, Alice Ryhl <aliceryhl@...gle.com>, 
	Sami Tolvanen <samitolvanen@...gle.com>, "Mike Rapoport (Microsoft)" <rppt@...nel.org>, 
	Rafael Aquini <aquini@...hat.com>, Michael Ellerman <mpe@...erman.id.au>, 
	Stafford Horne <shorne@...il.com>, Christophe Leroy <christophe.leroy@...roup.eu>, 
	Teresa Johnson <tejohnson@...gle.com>, linux-kernel@...r.kernel.org, 
	linux-kbuild@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH v3] kbuild: distributed build support for Clang ThinLTO

On Mon, May 26, 2025 at 6:12 AM Masahiro Yamada <masahiroy@...nel.org> wrote:
>
> On Thu, May 22, 2025 at 6:35 AM <xur@...gle.com> wrote:
> >
> > From: Rong Xu <xur@...gle.com>
> >
> > Add distributed ThinLTO build support for the Linux kernel.
> > This new mode offers several advantages: (1) Increased
> > flexibility in handling user-specified build options.
> > (2) Improved user-friendliness for developers. (3) Greater
> > convenience for integrating with objtool and livepatch.
> >
> > Note that "distributed" in this context refers to a term
> > that differentiates in-process ThinLTO builds by invoking
> > backend compilation through the linker, not necessarily
> > building in distributed environments.
> >
> > Distributed ThinLTO is enabled via the
> > `CONFIG_LTO_CLANG_THIN_DIST` Kconfig option. For example:
> >  > make LLVM=1 defconfig
> >  > scripts/config -e LTO_CLANG_THIN_DIST
> >  > make LLVM=1 oldconfig
> >  > make LLVM=1 vmlinux -j <..>
> >
> > The implementation changes the top-level Makefile with a
> > macro for generating `vmlinux.o` for distributed ThinLTO
> > builds. It uses the existing Kbuild infrastructure to
> > perform two recursive passes through the subdirectories.
> > The first pass generates LLVM IR object files, similar to
> > in-process ThinLTO. Following the thin-link stage, a second
> > pass compiles these IR files into the final native object
> > files. The build rules and actions for this two-pass process
> > are primarily implemented in `scripts/Makefile.build`.
> >
> > Currently, this patch focuses on building the main kernel
> > image (`vmlinux`) only. Support for building kernel modules
> > using this method is planned for a subsequent patch.
> >
> > Tested on the following arch: x86, arm64, loongarch, and
> > riscv.
> >
> > Some implementation details can be found here:
> > https://discourse.llvm.org/t/rfc-distributed-thinlto-build-for-kernel/85934
> >
> > Signed-off-by: Rong Xu <xur@...gle.com>
> > ---
> > Changelog since v1:
> > - Updated the description in arch/Kconfig based on feedback
> >   from Nathan Chancellor
> > - Revised file suffixes: .final_o -> .o.thinlto.native, and
> >   .final_a -> .a.thinlto.native
> > - Updated list of ignored files in .gitignore
> >
> > Changelog since v2:
> > - Changed file suffixes: .o.thinlto.native -> .o_thinlto_native,
> >   and .a.thinlto.native -> .a_thinlto_native so that basename
> >   works as intended.
> > - Tested the patch with AutoFDO and Propeller.
> > ---
> >  .gitignore                        |  3 ++
> >  MAINTAINERS                       |  5 +++
> >  Makefile                          | 40 ++++++++++++++++++++---
> >  arch/Kconfig                      | 19 +++++++++++
> >  scripts/Makefile.build            | 52 +++++++++++++++++++++++++++---
> >  scripts/Makefile.lib              |  7 +++-
> >  scripts/Makefile.vmlinux_o        | 16 +++++++---
> >  scripts/Makefile.vmlinux_thinlink | 53 +++++++++++++++++++++++++++++++
> >  scripts/head-object-list.txt      |  1 +
> >  9 files changed, 181 insertions(+), 15 deletions(-)
> >  create mode 100644 scripts/Makefile.vmlinux_thinlink
>
> I re-implemented the Makefiles to avoid
> the second recursion and hacky ifdefs.
> Attached.
>
> The topic branch is available in
>
> git://git.kernel.org/pub/scm/linux/kernel/git/masahiroy/linux-kbuild.git
> thinlto-dist-refactor
>
> I only compile and boot tested on x86 QEMU.
This implementation does look cleaner. But with one issue: it has a
unified cflags for
all the BE compilations. This means per-file flags, such as
CFLAGS_fork.o = -fabc,
are lost during the BE compilation for fork.thinlto_native.o.

This exact issue was what I aimed to prevent with the two-recursion approach.
If we must avoid two recursions, perhaps we could leverage the saved per-file
pre-link commands (i.e., .*.o.cmd)?

Regards,

-Rong

>
>
> --
> Best Regards
> Masahiro Yamada