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

[Masahiro Yamada <masahiroy@kernel.org>]

On Tue, Oct 22, 2024 at 8:28 AM Rong Xu <xur@google.com> wrote:
>
> On Sun, Oct 20, 2024 at 8:18 PM Masahiro Yamada <masahiroy@kernel.org> wrote:
> >
> > On Tue, Oct 15, 2024 at 6:33 AM Rong Xu <xur@google.com> 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@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>
> > > ---
> > >  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.


Same.

Adding two __split_text_start and __split_text_end markers
do not affect anything. It just increases the kallsyms table slightly.

You can do it unconditionally.



>
> >
> >
> > 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.


Should be explained in the commit description.



--
Best Regards
Masahiro Yamada