Re: [PATCH v4 5/6] AutoFDO: Enable machine function split optimization for AutoFDO

[Rong Xu]

On Sun, Oct 20, 2024 at 8:18 PM Masahiro Yamada <masahiroy@xxxxxxxxxx> wrote:
>
> On Tue, Oct 15, 2024 at 6:33 AM Rong Xu <xur@xxxxxxxxxx> wrote:
> >
> > Enable the machine function split optimization for AutoFDO in Clang.
> >
> > Machine function split (MFS) is a pass in the Clang compiler that
> > splits a function into hot and cold parts. The linker groups all
> > cold blocks across functions together. This decreases hot code
> > fragmentation and improves iCache and iTLB utilization.
> >
> > MFS requires a profile so this is enabled only for the AutoFDO builds.
> >
> > Co-developed-by: Han Shen <shenhan@xxxxxxxxxx>
> > Signed-off-by: Han Shen <shenhan@xxxxxxxxxx>
> > Signed-off-by: Rong Xu <xur@xxxxxxxxxx>
> > Suggested-by: Sriraman Tallam <tmsriram@xxxxxxxxxx>
> > Suggested-by: Krzysztof Pszeniczny <kpszeniczny@xxxxxxxxxx>
> > ---
> >  include/asm-generic/vmlinux.lds.h | 6 ++++++
> >  scripts/Makefile.autofdo          | 2 ++
> >  2 files changed, 8 insertions(+)
> >
> > diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> > index ace617d1af9b..20e46c0917db 100644
> > --- a/include/asm-generic/vmlinux.lds.h
> > +++ b/include/asm-generic/vmlinux.lds.h
> > @@ -565,9 +565,14 @@ defined(CONFIG_AUTOFDO_CLANG)
> >                 __unlikely_text_start = .;                              \
> >                 *(.text.unlikely .text.unlikely.*)                      \
> >                 __unlikely_text_end = .;
> > +#define TEXT_SPLIT                                                     \
> > +               __split_text_start = .;                                 \
> > +               *(.text.split .text.split.[0-9a-zA-Z_]*)                \
> > +               __split_text_end = .;
> >  #else
> >  #define TEXT_HOT *(.text.hot .text.hot.*)
> >  #define TEXT_UNLIKELY *(.text.unlikely .text.unlikely.*)
> > +#define TEXT_SPLIT
> >  #endif
>
>
> Why conditional?

The condition is to ensure that we don't change the default kernel
build by any means.
The new code will introduce a few new symbols.

>
>
> Where are __unlikely_text_start and __unlikely_text_end used?

These new symbols are currently unreferenced within the kernel source tree.
However, they provide a valuable means of identifying hot and cold
sections of text,
and how large they are. I think they are useful information.

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> --
> Best Regards
> Masahiro Yamada