On Mon, Jan 20, 2025 at 1:29 PM Arnd Bergmann <arnd@xxxxxxxxxx> wrote:
>
> From: Arnd Bergmann <arnd@xxxxxxxx>
>
> 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@xxxxxxxxxxxxxxx
> Cc: Han Shen <shenhan@xxxxxxxxxx>
> Cc: Masahiro Yamada <masahiroy@xxxxxxxxxx>
> Cc: Nathan Chancellor <nathan@xxxxxxxxxx>
> Cc: Kees Cook <kees@xxxxxxxxxx>
> 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@xxxxxxxx>
> ---
> 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
>
