From: Sedat Dilek <sedat.dilek@gmail.com>
To: Arnd Bergmann <arnd@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>,
Ionela Voinescu <ionela.voinescu@arm.com>,
Kees Cook <keescook@chromium.org>, Arnd Bergmann <arnd@arndb.de>,
Catalin Marinas <catalin.marinas@arm.com>,
Nick Desaulniers <ndesaulniers@google.com>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
Kristina Martsenko <kristina.martsenko@arm.com>,
Ard Biesheuvel <ardb@kernel.org>,
Nathan Chancellor <nathan@kernel.org>,
Clang-Built-Linux ML <clang-built-linux@googlegroups.com>,
Mark Brown <broonie@kernel.org>, Andrew Scull <ascull@google.com>,
Marc Zyngier <maz@kernel.org>,
Geert Uytterhoeven <geert+renesas@glider.be>,
Vincenzo Frascino <vincenzo.frascino@arm.com>,
Will Deacon <will@kernel.org>,
David Brazdil <dbrazdil@google.com>,
Linux ARM <linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH] [RFC] arm64: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION
Date: Fri, 26 Feb 2021 10:05:25 +0100 [thread overview]
Message-ID: <CA+icZUW9Hzm_ZZtLiJu8pVeOmYJMOaXkV4Om_u4yZKYRVWNddA@mail.gmail.com> (raw)
In-Reply-To: <CAK8P3a1QmS6g3cHTR93wWPkKs8vaP6RJEb8nZbTMLX+xphLJRw@mail.gmail.com>
On Fri, Feb 26, 2021 at 9:14 AM Arnd Bergmann <arnd@kernel.org> wrote:
>
> On Fri, Feb 26, 2021 at 1:36 AM Sedat Dilek <sedat.dilek@gmail.com> wrote:
> >
> > On Thu, Feb 25, 2021 at 12:21 PM Arnd Bergmann <arnd@kernel.org> wrote:
> > >
> > > From: Arnd Bergmann <arnd@arndb.de>
> > >
> > > When looking at kernel size optimizations, I found that arm64
> > > does not currently support HAVE_LD_DEAD_CODE_DATA_ELIMINATION,
> > > which enables the --gc-sections flag to the linker.
> > >
> > > I see that for a defconfig build with llvm, there are some
> > > notable improvements from enabling this, in particular when
> > > combined with the recently added CONFIG_LTO_CLANG_THIN
> > > and CONFIG_TRIM_UNUSED_KSYMS:
> > >
> > > text data bss dec hex filename
> > > 16570322 10998617 506468 28075407 1ac658f defconfig/vmlinux
> > > 16318793 10569913 506468 27395174 1a20466 trim_defconfig/vmlinux
> > > 16281234 10984848 504291 27770373 1a7be05 gc_defconfig/vmlinux
> > > 16029705 10556880 504355 27090940 19d5ffc gc+trim_defconfig/vmlinux
> > > 17040142 11102945 504196 28647283 1b51f73 thinlto_defconfig/vmlinux
> > > 16788613 10663201 504196 27956010 1aa932a thinlto+trim_defconfig/vmlinux
> > > 16347062 11043384 502499 27892945 1a99cd1 gc+thinlto_defconfig/vmlinux
> > > 15759453 10532792 502395 26794640 198da90 gc+thinlto+trim_defconfig/vmlinux
> > >
> >
> > Thanks for the numbers.
> > Does CONFIG_TRIM_UNUSED_KSYMS=y have an impact to the build-time (and
> > disc-usage - negative way means longer/bigger)?
> > Do you have any build-time for the above numbers?
>
> They are in the mailing list archive I linked to:
>
> ==== defconfig ====
> 332.001786355 seconds time elapsed
> 8599.464163000 seconds user
> 676.919635000 seconds sys
> ==== trim_defconfig ====
> 448.378576012 seconds time elapsed
> 10735.489271000 seconds user
> 964.006504000 seconds sys
> ==== gc_defconfig ====
> 324.347492236 seconds time elapsed
> 8465.785800000 seconds user
> 614.899797000 seconds sys
> ==== gc+trim_defconfig ====
> 429.188875620 seconds time elapsed
> 10203.759658000 seconds user
> 871.307973000 seconds sys
> ==== thinlto_defconfig ====
> 389.793540200 seconds time elapsed
> 9491.665320000 seconds user
> 664.858109000 seconds sys
> ==== thinlto+trim_defconfig ====
> 580.431820561 seconds time elapsed
> 11429.515538000 seconds user
> 1056.985745000 seconds sys
> ==== gc+thinlto_defconfig ====
> 389.484364525 seconds time elapsed
> 9473.831980000 seconds user
> 675.057675000 seconds sys
> ==== gc+thinlto+trim_defconfig ====
> 580.824912807 seconds time elapsed
> 11433.650337000 seconds user
> 1049.845569000 seconds sys
>
Thanks for the numbers Arnd.
> So HAVE_LD_DEAD_CODE_DATA_ELIMINATION is a small improvement
> on build time (since it can spend less time linking), while
> CONFIG_TRIM_UNUSED_KSYMS slows it down quite a bit. Combining
> CONFIG_TRIM_UNUSED_KSYMS with CONFIG_THINLTO is really
> slow because here most of the time is spent in the final link (especially
> when you have many CPU cores to do the earlier bits quickly), but then
> it does the link twice.
>
My first pre-v5.12-rc1 kernel-build was with Clang-ThinLTO enabled.
But with the next ones I jumped to Sami's Clang-CFI.
> > BTW, is CONFIG_LD_DEAD_CODE_DATA_ELIMINATION=y setable for x86 (64bit)?
> > ( Did not look or check for it. )
>
> No, in mainline, HAVE_LD_DEAD_CODE_DATA_ELIMINATION is currently
> only selected on MIPS and PowerPC. I only sent experimental patches to
> enable it on arm64 and m68k, but have not tried booting them. If you
> select the symbol on x86, you should see similar results.
>
OK, i see:
$ git grep HAVE_LD_DEAD_CODE_DATA_ELIMINATION arch/mips/
arch/mips/Kconfig: select HAVE_LD_DEAD_CODE_DATA_ELIMINATION
$ git grep HAVE_LD_DEAD_CODE_DATA_ELIMINATION arch/powerpc/
arch/powerpc/Kconfig: select HAVE_LD_DEAD_CODE_DATA_ELIMINATION
So, I need to add this to arch/x86/Kconfig.
You happen to know if changes to arch/x86/kernel/vmlinux.lds.S
(sections) are needed?
Last question:
The last days I see a lot of fixes touching inlining with LLVM/Clang v13-git.
What git tag are you using?
What are your experiences?
Pending patches (kernel-side)?
I use:
$ /opt/llvm-toolchain/bin/clang --version
dileks clang version 13.0.0 (https://github.com/llvm/llvm-project.git
c465429f286f50e52a8d2b3b39f38344f3381cce)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /opt/llvm-toolchain/bin
My LLVM toolchain is ThinLTO+PGO optimized for Linux-kernel builds.
- Sedat -
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next prev parent reply other threads:[~2021-02-26 9:07 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-02-25 11:20 [PATCH] [RFC] arm64: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION Arnd Bergmann
2021-02-25 20:16 ` Kees Cook
2021-02-26 0:36 ` Sedat Dilek
2021-02-26 8:14 ` Arnd Bergmann
2021-02-26 9:05 ` Sedat Dilek [this message]
2021-02-26 9:51 ` Arnd Bergmann
2021-02-26 10:02 ` Sedat Dilek
2021-02-26 21:13 ` Fangrui Song
2021-02-27 9:49 ` Arnd Bergmann
2021-03-01 1:11 ` Nicholas Piggin
2021-03-10 20:49 ` Masahiro Yamada
2021-03-10 21:08 ` Arnd Bergmann
2021-03-10 21:24 ` Sedat Dilek
2021-03-10 21:47 ` Nicolas Pitre
2021-03-10 21:57 ` Sedat Dilek
2021-03-10 22:02 ` Nick Desaulniers
2021-03-10 22:08 ` Nicolas Pitre
2021-03-10 22:29 ` Fangrui Song
2021-03-10 21:45 ` Rasmus Villemoes
2021-03-10 21:19 ` Nicolas Pitre
2021-03-10 22:42 ` Fangrui Song
2021-03-17 14:37 ` Catalin Marinas
2021-03-17 16:18 ` Catalin Marinas
2021-03-18 8:41 ` Arnd Bergmann
2021-03-19 12:25 ` Catalin Marinas
2021-03-19 14:01 ` Arnd Bergmann
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