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

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Message-ID: <CAF1bQ=TjpUrEgiqepyaGAiDoFM8jzozzxW=0YvTXpFY64YoTzw@mail.gmail.com>
Date: Fri, 1 Nov 2024 11:37:13 -0700
From: Rong Xu <xur@...gle.com>
To: Masahiro Yamada <masahiroy@...nel.org>
Cc: Alice Ryhl <aliceryhl@...gle.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	Arnd Bergmann <arnd@...db.de>, Bill Wendling <morbo@...gle.com>, Borislav Petkov <bp@...en8.de>, 
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	Dave Hansen <dave.hansen@...ux.intel.com>, David Li <davidxl@...gle.com>, 
	Han Shen <shenhan@...gle.com>, Heiko Carstens <hca@...ux.ibm.com>, "H. Peter Anvin" <hpa@...or.com>, 
	Ingo Molnar <mingo@...hat.com>, Jann Horn <jannh@...gle.com>, Jonathan Corbet <corbet@....net>, 
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	Justin Stitt <justinstitt@...gle.com>, Kees Cook <kees@...nel.org>, 
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	llvm@...ts.linux.dev
Subject: Re: [PATCH v6 3/7] Adjust symbol ordering in text output section

Current order is:
.text.hot, .text, .text_unlikely, .text.unknown, .text.asan

The patch reorders them to:
.text.asan, .text.unknown, .text_unlikely, .text.hot, .text

The majority of the code resides in three sections: .text.hot, .text, and
 .text.unlikely, with .text.unknown containing a negligible amount.
.text.asan is only generated in ASAN builds.

Our primary goal is to group code segments based on their execution
frequency (hotness).

First, we want to place .text.hot adjacent to .text. Since we cannot put
.text.hot after .text (Due to constraints with -ffunction-sections,
placing .text.hot after .text is problematic), we need to put
.text.hot before .text.

Then it comes to .text.unlikely, we cannot put it after .text
(same -ffunction-sections issue) . Therefore, we'll position .text.unlikely
before .text.hot.

.text.unknown and .tex.asan follow the same logic.

This revised ordering effectively reverses the original arrangement (for
.text.unlikely, .text.unknown, and .tex.asan), maintaining a similar level of
affinity between sections.

I hope this explains the reason for the new ordering.

-Rong

On Fri, Nov 1, 2024 at 11:06 AM Masahiro Yamada <masahiroy@...nel.org> wrote:
>
> On Sat, Oct 26, 2024 at 7:14 AM Rong Xu <xur@...gle.com> 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.
>
>
> The fixed patterns are currently listed in this order:
>
>   .text.hot, .text_unlikely, .text.unknown, .text.asan.
>
> You reorder them to:
>
>   .text.asan, .text.unknown, .text.unlikely, .text.hot
>
>
> I believe it is better to describe your thoughts
> about the reshuffling among the fixed pattern sections.
>
> Otherwise, It is unclear to me.
>
>
>
>
> --
> Best Regards
> Masahiro Yamada