From: Nicolas Pitre <nico@fluxnic.net>
To: Masahiro Yamada <masahiroy@kernel.org>
Cc: linux-kbuild@vger.kernel.org, Christoph Hellwig <hch@lst.de>,
Linus Torvalds <torvalds@linux-foundation.org>,
Jessica Yu <jeyu@kernel.org>,
linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org
Subject: Re: [PATCH v2 3/4] kbuild: re-implement CONFIG_TRIM_UNUSED_KSYMS to make it work in one-pass
Date: Tue, 9 Mar 2021 12:36:15 -0500 (EST) [thread overview]
Message-ID: <354sr3np-67o8-oss9-813s-p2qoro06p4o@syhkavp.arg> (raw)
In-Reply-To: <20210309151737.345722-4-masahiroy@kernel.org>
On Wed, 10 Mar 2021, Masahiro Yamada wrote:
> Commit a555bdd0c58c ("Kbuild: enable TRIM_UNUSED_KSYMS again, with some
> guarding") re-enabled this feature, but Linus is still unhappy about
> the build time.
>
> The reason of the slowness is the recursion - this basically works in
> two loops.
>
> In the first loop, Kbuild builds the entire tree based on the temporary
> autoksyms.h, which contains macro defines to control whether their
> corresponding EXPORT_SYMBOL() is enabled or not, and also gathers all
> symbols required by modules. After the tree traverse, Kbuild updates
> autoksyms.h and triggers the second loop to rebuild source files whose
> EXPORT_SYMBOL() needs flipping.
>
> This commit re-implements CONFIG_TRIM_UNUSED_KSYMS to make it work in
> one pass. In the new design, unneeded EXPORT_SYMBOL() instances are
> trimmed by the linker instead of the preprocessor.
>
> After the tree traverse, a linker script snippet <generated/keep-ksyms.h>
> is generated. It feeds the list of necessary sections to vmlinus.lds.S
> and modules.lds.S. The other sections fall into /DISCARD/.
>
> Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
I'm not sure I do understand every detail here, especially since it is
so far away from the version that I originally contributed. But the
concept looks good.
I still think that there is no way around a recursive approach to get
the maximum effect with LTO, but given that true LTO still isn't applied
to mainline after all those years, the recursive approach brings
nothing. Maybe that could be revisited if true LTO ever makes it into
mainline, and the desire to reduce the binary size is still relevant
enough to justify it.
Acked-by: Nicolas Pitre <nico@fluxnic.net>
Nicolas
next prev parent reply other threads:[~2021-03-09 17:36 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-09 15:17 [PATCH v2 0/4] kbuild: build speed improvement of CONFIG_TRIM_UNUSED_KSYMS Masahiro Yamada
2021-03-09 15:17 ` [PATCH v2 1/4] export.h: make __ksymtab_strings per-symbol section Masahiro Yamada
2021-03-09 15:17 ` [PATCH v2 2/4] kbuild: separate out vmlinux.lds generation Masahiro Yamada
2021-03-09 15:17 ` [PATCH v2 3/4] kbuild: re-implement CONFIG_TRIM_UNUSED_KSYMS to make it work in one-pass Masahiro Yamada
2021-03-09 17:36 ` Nicolas Pitre [this message]
2021-03-09 18:11 ` Masahiro Yamada
2021-03-09 19:54 ` Nicolas Pitre
2021-03-09 20:11 ` Rasmus Villemoes
2021-03-09 20:45 ` Nicolas Pitre
2021-03-17 15:48 ` kernel test robot
2021-03-09 15:17 ` [PATCH v2 4/4] kbuild: remove guarding from TRIM_UNUSED_KSYMS Masahiro Yamada
2021-03-09 19:54 ` Linus Torvalds
2021-03-10 12:55 ` kernel test robot
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