Re: [PATCH v6 3/7] Adjust symbol ordering in text output section

[Klara Modin <klarasmodin@gmail.com>]

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Hi,

On 2024-10-26 07:14, Rong Xu wrote:
> When the -ffunction-sections compiler option is enabled, each function
> is placed in a separate section named .text.function_name rather than
> putting all functions in a single .text section.
> 
> However, using -function-sections can cause problems with the
> linker script. The comments included in include/asm-generic/vmlinux.lds.h
> note these issues.:
>    “TEXT_MAIN here will match .text.fixup and .text.unlikely if dead
>     code elimination is enabled, so these sections should be converted
>     to use ".." first.”
> 
> It is unclear whether there is a straightforward method for converting
> a suffix to "..".
> 
> This patch modifies the order of subsections within the text output
> section. Specifically, it repositions sections with certain fixed patterns
> (for example .text.unlikely) before TEXT_MAIN, ensuring that they are
> grouped and matched together. It also places .text.hot section at the
> beginning of a page to help the TLB performance.
> 
> Note that the limitation arises because the linker script employs glob
> patterns instead of regular expressions for string matching. While there
> is a method to maintain the current order using complex patterns, this
> significantly complicates the pattern and increases the likelihood of
> errors.
> 
> This patch also changes vmlinux.lds.S for the sparc64 architecture to
> accommodate specific symbol placement requirements.

With this patch (622240ea8d71a75055399fd4b3cc2b190e44d2e2 in 
next-20241108) my Edgerouter 6P hangs on boot (Cavium Octeon III, 
mips64, running in big endian). It's using device tree passed from the 
vendored u-boot (attached in case it's relevant).

Disabling dead code elimination does not fix the issue.

Please let me know if there's anything else you need.

Regards,
Klara Modin

> 
> Co-developed-by: Han Shen <shenhan@google.com>
> Signed-off-by: Han Shen <shenhan@google.com>
> Signed-off-by: Rong Xu <xur@google.com>
> Suggested-by: Sriraman Tallam <tmsriram@google.com>
> Suggested-by: Krzysztof Pszeniczny <kpszeniczny@google.com>
> Tested-by: Yonghong Song <yonghong.song@linux.dev>
> Tested-by: Yabin Cui <yabinc@google.com>
> Change-Id: I5202d40bc7e24f93c2bfb2f0d987e9dc57dec1b1
> ---
>   arch/sparc/kernel/vmlinux.lds.S   |  5 +++++
>   include/asm-generic/vmlinux.lds.h | 19 ++++++++++++-------
>   2 files changed, 17 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/sparc/kernel/vmlinux.lds.S b/arch/sparc/kernel/vmlinux.lds.S
> index d317a843f7ea9..f1b86eb303404 100644
> --- a/arch/sparc/kernel/vmlinux.lds.S
> +++ b/arch/sparc/kernel/vmlinux.lds.S
> @@ -48,6 +48,11 @@ SECTIONS
>   	{
>   		_text = .;
>   		HEAD_TEXT
> +	        ALIGN_FUNCTION();
> +#ifdef CONFIG_SPARC64
> +	        /* Match text section symbols in head_64.S first */
> +	        *head_64.o(.text)
> +#endif
>   		TEXT_TEXT
>   		SCHED_TEXT
>   		LOCK_TEXT
> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> index eeadbaeccf88b..fd901951549c0 100644
> --- a/include/asm-generic/vmlinux.lds.h
> +++ b/include/asm-generic/vmlinux.lds.h
> @@ -553,19 +553,24 @@
>    * .text section. Map to function alignment to avoid address changes
>    * during second ld run in second ld pass when generating System.map
>    *
> - * TEXT_MAIN here will match .text.fixup and .text.unlikely if dead
> - * code elimination is enabled, so these sections should be converted
> - * to use ".." first.
> + * TEXT_MAIN here will match symbols with a fixed pattern (for example,
> + * .text.hot or .text.unlikely) if dead code elimination or
> + * function-section is enabled. Match these symbols first before
> + * TEXT_MAIN to ensure they are grouped together.
> + *
> + * Also placing .text.hot section at the beginning of a page, this
> + * would help the TLB performance.
>    */
>   #define TEXT_TEXT							\
>   		ALIGN_FUNCTION();					\
> +		*(.text.asan.* .text.tsan.*)				\
> +		*(.text.unknown .text.unknown.*)			\
> +		*(.text.unlikely .text.unlikely.*)			\
> +		. = ALIGN(PAGE_SIZE);					\
>   		*(.text.hot .text.hot.*)				\
>   		*(TEXT_MAIN .text.fixup)				\
> -		*(.text.unlikely .text.unlikely.*)			\
> -		*(.text.unknown .text.unknown.*)			\
>   		NOINSTR_TEXT						\
> -		*(.ref.text)						\
> -		*(.text.asan.* .text.tsan.*)
> +		*(.ref.text)
>   
>   
>   /* sched.text is aling to function alignment to secure we have same

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# bad: [291b13f025250e2fa3b5e2ff714aaaa227dff02c] of: WARN on deprecated #address-cells/#size-cells handling
git bisect start 'HEAD'
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git bisect good bfc64d9b7e8cac82be6b8629865e137d962578f8
# bad: [5090ed9c92a1ba84f8563486550c6bf0b39954f2] Merge branch 'master' of git://git.kernel.org/pub/scm/linux/kernel/git/herbert/cryptodev-2.6.git
git bisect bad 5090ed9c92a1ba84f8563486550c6bf0b39954f2
# bad: [dbaafa9b8d4a351fb925efb44f4177cabc95d26d] Merge branch 'fs-next' of linux-next
git bisect bad dbaafa9b8d4a351fb925efb44f4177cabc95d26d
# bad: [d81b3235857fe7abd24ab2928a36626a0b4734b1] Merge branch 'for-next' of git://git.kernel.org/pub/scm/linux/kernel/git/qcom/linux.git
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git bisect good 5d2ae3246a2d15e2d384637ee55fc0320546be63
# bad: [10540b9bb270cbffbc56ca2f828439675abbdccc] Merge branch 'for-next' of git://git.kernel.org/pub/scm/linux/kernel/git/masahiroy/linux-kbuild.git
git bisect bad 10540b9bb270cbffbc56ca2f828439675abbdccc
# good: [e038e395a05e3a1cf2f5450bb2d08adcffca80b4] Merge branch 'master' of git://git.kernel.org/pub/scm/linux/kernel/git/herbert/crypto-2.6.git
git bisect good e038e395a05e3a1cf2f5450bb2d08adcffca80b4
# good: [aded6a2e0817d76cdd8010ea7c0b217b55cbe78b] Merge branch 'for-linux-next-fixes' of https://gitlab.freedesktop.org/drm/misc/kernel.git
git bisect good aded6a2e0817d76cdd8010ea7c0b217b55cbe78b
# good: [375a4f4ea719ad68e305373cfe0f77ecd1378531] kconfig: qconf: remove unnecessary lastWindowClosed() signal connection
git bisect good 375a4f4ea719ad68e305373cfe0f77ecd1378531
# good: [397a479b511df4e6e7c665d7d8991943645b4cab] kbuild: simplify rustfmt target
git bisect good 397a479b511df4e6e7c665d7d8991943645b4cab
# bad: [84712ea5d9b003d298b0df6f957d06bbcac99ef5] AutoFDO: Enable -ffunction-sections for the AutoFDO build
git bisect bad 84712ea5d9b003d298b0df6f957d06bbcac99ef5
# good: [18e885099f1c52755f054202525cb60d3edcda44] objtool: Fix unreachable instruction warnings for weak functions
git bisect good 18e885099f1c52755f054202525cb60d3edcda44
# bad: [9a92584c8ef545cf92299e4cadbe2148b93dafa1] vmlinux.lds.h: Add markers for text_unlikely and text_hot sections
git bisect bad 9a92584c8ef545cf92299e4cadbe2148b93dafa1
# bad: [622240ea8d71a75055399fd4b3cc2b190e44d2e2] vmlinux.lds.h: Adjust symbol ordering in text output section
git bisect bad 622240ea8d71a75055399fd4b3cc2b190e44d2e2
# first bad commit: [622240ea8d71a75055399fd4b3cc2b190e44d2e2] vmlinux.lds.h: Adjust symbol ordering in text output section

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