linux-kernel - Re: [PATCH] [RFC, DO NOT APPLY] vmlinux.lds: revert link speed regression

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Message-ID: <CAF1bQ=QFxE8AvnpOeSjSeL1buxDDACKVNufLjw99cQir0pyS_Q@mail.gmail.com>
Date: Tue, 21 Jan 2025 09:45:54 -0800
From: Rong Xu <xur@...gle.com>
To: Arnd Bergmann <arnd@...nel.org>
Cc: linux-kbuild@...r.kernel.org, Masahiro Yamada <masahiroy@...nel.org>, 
	linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>, regressions@...ts.linux.dev, 
	Han Shen <shenhan@...gle.com>, Nathan Chancellor <nathan@...nel.org>, Kees Cook <kees@...nel.org>, 
	Jann Horn <jannh@...gle.com>, Ard Biesheuvel <ardb@...nel.org>, linux-arch@...r.kernel.org
Subject: Re: [PATCH] [RFC, DO NOT APPLY] vmlinux.lds: revert link speed regression

On Mon, Jan 20, 2025 at 1:29 PM Arnd Bergmann <arnd@...nel.org> wrote:
>
> From: Arnd Bergmann <arnd@...db.de>
>
> I noticed a regression in the time it takes to fully link some randconfig
> kernels and bisected this to commit 0043ecea2399 ("vmlinux.lds.h: Adjust
> symbol ordering in text output section"), which (among other changes) moves
> .text.unlikely ahead of .text.
>
> Partially reverting this makes the final link over six times faster again,
> back to what it was in linux-6.12:
>
>                 linux-6.12      linux-6.13
> ld.lld v20      1.2s            1.2s
> ld.bfd v2.36    3.2s            5.2s
> ld.bfd v2.39    59s             388s
>
> According to the commit description, that revert is not allowed here
> because with CONFIG_LD_DEAD_CODE_DATA_ELIMINATION, the .text.unlikely
> section name conflicts with the function-section names. On the other
> hand, the excessive link time happens both with and without that
> option, so the order could be conditional.

Yes. The order could be conditional. As a matter of fact, the first
version was conditional.
I changed it based on the reviewer comments to reduce conditions for
more maintainable code.
I would like to work from the ld.bfd side to see if we can fix the problem.

-Rong

>
> I did not try to bisect the linker beyond trying multiple versions
> I had installed already, and it does feel like the behavior of recent
> versions (tested 2.39 and 2.42 with identical results) is broken in
> some form that earlier versions were not. According to 'perf', most
> of the time is spent in elf_link_adjust_relocs() and ext64l_r_offset().
>
> I also did not try to narrow the problem down to specific kernel
> configuration options, but from my first impression it does appear
> to be rare, and unrelated to the Propeller options added in 6.13.
>
> Cc: regressions@...ts.linux.dev
> Cc: Han Shen <shenhan@...gle.com>
> Cc: Masahiro Yamada <masahiroy@...nel.org>
> Cc: Nathan Chancellor <nathan@...nel.org>
> Cc: Kees Cook <kees@...nel.org>
> Fixes: 0043ecea2399 ("vmlinux.lds.h: Adjust symbol ordering in text output section")
> Link: https://pastebin.com/raw/sWpbkapL (config)
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
>  include/asm-generic/vmlinux.lds.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> index 54504013c749..61fa047023b5 100644
> --- a/include/asm-generic/vmlinux.lds.h
> +++ b/include/asm-generic/vmlinux.lds.h
> @@ -588,10 +588,10 @@ defined(CONFIG_AUTOFDO_CLANG) || defined(CONFIG_PROPELLER_CLANG)
>                 *(.text.asan.* .text.tsan.*)                            \
>                 *(.text.unknown .text.unknown.*)                        \
>                 TEXT_SPLIT                                              \
> -               TEXT_UNLIKELY                                           \
>                 . = ALIGN(PAGE_SIZE);                                   \
>                 TEXT_HOT                                                \
>                 *(TEXT_MAIN .text.fixup)                                \
> +               TEXT_UNLIKELY                                           \
>                 NOINSTR_TEXT                                            \
>                 *(.ref.text)
>
> --
> 2.39.5
>