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Message-ID: <CAF1bQ=Tp8Dc=jpNNq+B+qj90bowod8dQ1JYRsM4q5ARdf=Jd_Q@mail.gmail.com>
Date: Mon, 21 Oct 2024 16:32:00 -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>,
Breno Leitao <leitao@...ian.org>, Brian Gerst <brgerst@...il.com>,
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>,
Josh Poimboeuf <jpoimboe@...nel.org>, Juergen Gross <jgross@...e.com>,
Justin Stitt <justinstitt@...gle.com>, Kees Cook <kees@...nel.org>,
"Mike Rapoport (IBM)" <rppt@...nel.org>, Nathan Chancellor <nathan@...nel.org>,
Nick Desaulniers <ndesaulniers@...gle.com>, Nicolas Schier <nicolas@...sle.eu>,
"Paul E. McKenney" <paulmck@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
Sami Tolvanen <samitolvanen@...gle.com>, Thomas Gleixner <tglx@...utronix.de>,
Wei Yang <richard.weiyang@...il.com>, workflows@...r.kernel.org,
Miguel Ojeda <miguel.ojeda.sandonis@...il.com>, Maksim Panchenko <max4bolt@...il.com>, x86@...nel.org,
linux-arch@...r.kernel.org, linux-doc@...r.kernel.org,
linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
llvm@...ts.linux.dev, Sriraman Tallam <tmsriram@...gle.com>
Subject: Re: [PATCH v4 4/6] AutoFDO: Enable -ffunction-sections for the
AutoFDO build
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@...nel.org> wrote:
>
> On Tue, Oct 15, 2024 at 6:33 AM Rong Xu <xur@...gle.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@...gle.com>
> > Signed-off-by: Han Shen <shenhan@...gle.com>
> > Signed-off-by: Rong Xu <xur@...gle.com>
> > Suggested-by: Sriraman Tallam <tmsriram@...gle.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
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