Re: [PATCH v4 4/6] AutoFDO: Enable -ffunction-sections for the AutoFDO build

[Rong Xu <xur@google.com>]

The answers are the same as the reply in [PATCH v4 5/6]

On Sun, Oct 20, 2024 at 7:26 PM Masahiro Yamada <masahiroy@kernel.org> wrote:
>
> On Tue, Oct 15, 2024 at 6:33 AM Rong Xu <xur@google.com> wrote:
> >
> > Enable -ffunction-sections by default for the AutoFDO build.
> >
> > With -ffunction-sections, the compiler places each function in its own
> > section named .text.function_name instead of placing all functions in
> > the .text section. In the AutoFDO build, this allows the linker to
> > utilize profile information to reorganize functions for improved
> > utilization of iCache and iTLB.
> >
> > 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>
> > ---
> >  include/asm-generic/vmlinux.lds.h | 37 ++++++++++++++++++++++++-------
> >  scripts/Makefile.autofdo          |  2 +-
> >  2 files changed, 30 insertions(+), 9 deletions(-)
> >
> > diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> > index 5df589c60401..ace617d1af9b 100644
> > --- a/include/asm-generic/vmlinux.lds.h
> > +++ b/include/asm-generic/vmlinux.lds.h
> > @@ -95,18 +95,25 @@
> >   * With LTO_CLANG, the linker also splits sections by default, so we need
> >   * these macros to combine the sections during the final link.
> >   *
> > + * With LTO_CLANG, the linker also splits sections by default, so we need
> > + * these macros to combine the sections during the final link.
> > + *
> >   * RODATA_MAIN is not used because existing code already defines .rodata.x
> >   * sections to be brought in with rodata.
> >   */
> > -#if defined(CONFIG_LD_DEAD_CODE_DATA_ELIMINATION) || defined(CONFIG_LTO_CLANG)
> > +#if defined(CONFIG_LD_DEAD_CODE_DATA_ELIMINATION) || defined(CONFIG_LTO_CLANG) || \
> > +defined(CONFIG_AUTOFDO_CLANG)
> >  #define TEXT_MAIN .text .text.[0-9a-zA-Z_]*
> > +#else
> > +#define TEXT_MAIN .text
> > +#endif
> > +#if defined(CONFIG_LD_DEAD_CODE_DATA_ELIMINATION) || defined(CONFIG_LTO_CLANG)
> >  #define DATA_MAIN .data .data.[0-9a-zA-Z_]* .data..L* .data..compoundliteral* .data.$__unnamed_* .data.$L*
> >  #define SDATA_MAIN .sdata .sdata.[0-9a-zA-Z_]*
> >  #define RODATA_MAIN .rodata .rodata.[0-9a-zA-Z_]* .rodata..L*
> >  #define BSS_MAIN .bss .bss.[0-9a-zA-Z_]* .bss..L* .bss..compoundliteral*
> >  #define SBSS_MAIN .sbss .sbss.[0-9a-zA-Z_]*
> >  #else
> > -#define TEXT_MAIN .text
> >  #define DATA_MAIN .data
> >  #define SDATA_MAIN .sdata
> >  #define RODATA_MAIN .rodata
> > @@ -549,6 +556,20 @@
> >                 __cpuidle_text_end = .;                                 \
> >                 __noinstr_text_end = .;
> >
> > +#ifdef CONFIG_AUTOFDO_CLANG
> > +#define TEXT_HOT                                                       \
> > +               __hot_text_start = .;                                   \
> > +               *(.text.hot .text.hot.*)                                \
> > +               __hot_text_end = .;
> > +#define TEXT_UNLIKELY                                                  \
> > +               __unlikely_text_start = .;                              \
> > +               *(.text.unlikely .text.unlikely.*)                      \
> > +               __unlikely_text_end = .;
> > +#else
> > +#define TEXT_HOT *(.text.hot .text.hot.*)
> > +#define TEXT_UNLIKELY *(.text.unlikely .text.unlikely.*)
> > +#endif
>
>
>
> Again, why is this 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.

>
>
> The only difference is *_start and *_end symbols are defined
> when CONFIG_AUTOFDO_CLANG=y.
>
> And, where are these symbols 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.

>
>
>
>
>
>
>
>
>
>
>
> > +
> >  /*
> >   * .text section. Map to function alignment to avoid address changes
> >   * during second ld run in second ld pass when generating System.map
> > @@ -557,30 +578,30 @@
> >   * code elimination or function-section is enabled. Match these symbols
> >   * first when in these builds.
> >   */
> > -#if defined(CONFIG_LD_DEAD_CODE_DATA_ELIMINATION) || defined(CONFIG_LTO_CLANG)
> > +#if defined(CONFIG_LD_DEAD_CODE_DATA_ELIMINATION) || defined(CONFIG_LTO_CLANG) || \
> > +defined(CONFIG_AUTOFDO_CLANG)
> >  #define TEXT_TEXT                                                      \
> >                 ALIGN_FUNCTION();                                       \
> >                 *(.text.asan.* .text.tsan.*)                            \
> >                 *(.text.unknown .text.unknown.*)                        \
> > -               *(.text.unlikely .text.unlikely.*)                      \
> > +               TEXT_UNLIKELY                                           \
> >                 . = ALIGN(PAGE_SIZE);                                   \
> > -               *(.text.hot .text.hot.*)                                \
> > +               TEXT_HOT                                                \
> >                 *(TEXT_MAIN .text.fixup)                                \
> >                 NOINSTR_TEXT                                            \
> >                 *(.ref.text)
> >  #else
> >  #define TEXT_TEXT                                                      \
> >                 ALIGN_FUNCTION();                                       \
> > -               *(.text.hot .text.hot.*)                                \
> > +               TEXT_HOT                                                \
> >                 *(TEXT_MAIN .text.fixup)                                \
> > -               *(.text.unlikely .text.unlikely.*)                      \
> > +               TEXT_UNLIKELY                                           \
> >                 *(.text.unknown .text.unknown.*)                        \
> >                 NOINSTR_TEXT                                            \
> >                 *(.ref.text)                                            \
> >                 *(.text.asan.* .text.tsan.*)
> >  #endif
> >
> > -
> >  /* sched.text is aling to function alignment to secure we have same
> >   * address even at second ld pass when generating System.map */
> >  #define SCHED_TEXT                                                     \
> > diff --git a/scripts/Makefile.autofdo b/scripts/Makefile.autofdo
> > index 1c9f224bc221..9c9a530ef090 100644
> > --- a/scripts/Makefile.autofdo
> > +++ b/scripts/Makefile.autofdo
> > @@ -10,7 +10,7 @@ ifndef CONFIG_DEBUG_INFO
> >  endif
> >
> >  ifdef CLANG_AUTOFDO_PROFILE
> > -  CFLAGS_AUTOFDO_CLANG += -fprofile-sample-use=$(CLANG_AUTOFDO_PROFILE)
> > +  CFLAGS_AUTOFDO_CLANG += -fprofile-sample-use=$(CLANG_AUTOFDO_PROFILE) -ffunction-sections
> >  endif
> >
> >  ifdef CONFIG_LTO_CLANG_THIN
> > --
> > 2.47.0.rc1.288.g06298d1525-goog
> >
> >
>
>
> --
> Best Regards
> Masahiro Yamada