linux-kernel - Re: [PATCH v2] kbuild: distributed build support for Clang ThinLTO

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Message-ID: <CAF1bQ=TQSQ75qCyQYegW7kq8WdyB-SNViraOuBvgGgAyUPS54Q@mail.gmail.com>
Date: Wed, 21 May 2025 09:53:30 -0700
From: Rong Xu <xur@...gle.com>
To: Eric Naim <dnaim@...hyos.org>
Cc: Masahiro Yamada <masahiroy@...nel.org>, Nathan Chancellor <nathan@...nel.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 v2] kbuild: distributed build support for Clang ThinLTO

On Wed, May 21, 2025 at 9:40 AM Rong Xu <xur@...gle.com> wrote:
>
> On Mon, May 19, 2025 at 10:22 PM Eric Naim <dnaim@...hyos.org> wrote:
> >
> > Hi Rong Xu,
> >
> > On 5/8/25 04:55, 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
> > > ---
> > >  .gitignore                        |  2 ++
> > >  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, 180 insertions(+), 15 deletions(-)
> > >  create mode 100644 scripts/Makefile.vmlinux_thinlink
> > >
> >
> > I noticed that both Makefile.autofdo and Makefile.propeller add extra linker flags when building with ThinLTO. Did you miss updating that or is the omission there intentional?
>
> Thanks for catching this! One good aspect of distributed build mode is
> that we no longer need the extra linker flags -- most of them are just
> to pass the options to the BE compilation.
> So this patch does not need these linker options.  But for the
> Propeller build, we still need to pass one of the two profiles to the
> final link, and I'll be sure to incorporate that into the patch.

One clarification: We don't need to change scripts/Makefile.propeller
as the link
ordering profile is already passed to the final link under
CLANG_PROPELLER_PROFILE_PREFIX.

All the --lto-* options are only for in-process thinlto builds.

>
> However, I do need to make a change regarding file suffixes. The
> is_kernel_obj macro in the Makefile.build uses the basename command.
> The issue is that basename extracts everything before the last period
> in a filename. So, for a file named "foo.o.thinlto.native", basename
> returns "foo.o.thinlto", but kbuild expects it to return "foo".
>
> To fix this, I'll adjust the suffixes to ".a_thinlto_native" and
> ".o_thinlto_native". I'll send the patch v3 shortly.
>
> Thanks,
>
> -Rong
> >
> > --
> > Regards,
> >   Eric