linux-kernel - Re: [PATCH v4 1/6] Add AutoFDO support for Clang build

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Message-ID: <CAK7LNAQ5yNKvZDtJuvo9Lt4rZwLSv0UN4=Ff=WcCDy1CCEpQ7Q@mail.gmail.com>
Date: Mon, 21 Oct 2024 01:32:54 +0900
From: Masahiro Yamada <masahiroy@...nel.org>
To: Rong Xu <xur@...gle.com>
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>, 
	Krzysztof Pszeniczny <kpszeniczny@...gle.com>, Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH v4 1/6] Add AutoFDO support for Clang build

On Tue, Oct 15, 2024 at 6:33 AM Rong Xu <xur@...gle.com> wrote:


> +Customization
> +=============
> +
> +You can enable or disable AutoFDO build for individual file and directories by
> +adding a line similar to the following to the respective kernel Makefile:


Perhaps, it might be worth mentioning that kernel space objects are
covered by default.

Then, people would understand ':= y' will be less common than ':= n'.




> +
> +- For enabling a single file (e.g. foo.o) ::
> +
> +   AUTOFDO_PROFILE_foo.o := y
> +
> +- For enabling all files in one directory ::
> +
> +   AUTOFDO_PROFILE := y
> +
> +- For disabling one file ::
> +
> +   AUTOFDO_PROFILE_foo.o := n
> +
> +- For disabling all files in one directory ::
> +
> +   AUTOFDO_PROFILE := n
> +




> +3) Run the load tests. The '-c' option in perf specifies the sample
> +   event period. We suggest using a suitable prime number, like 500009,
> +   for this purpose.
> +
> +   - For Intel platforms::
> +
> +      $ perf record -e BR_INST_RETIRED.NEAR_TAKEN:k -a -N -b -c <count> -o <perf_file> -- <loadtest>
> +
> +   - For AMD platforms: For Intel platforms:


I guess this is a copy-paste mistake.


For AMD platforms: For Intel platforms:

   ->

For AMD platforms:






> +   (https://github.com/google/autofdo),  version v0.30.1 or later.


Please one space instead of two after the comma.







> diff --git a/scripts/Makefile.autofdo b/scripts/Makefile.autofdo
> new file mode 100644
> index 000000000000..1c9f224bc221
> --- /dev/null
> +++ b/scripts/Makefile.autofdo
> @@ -0,0 +1,23 @@
> +# SPDX-License-Identifier: GPL-2.0
> +
> +# Enable available and selected Clang AutoFDO features.
> +
> +CFLAGS_AUTOFDO_CLANG := -fdebug-info-for-profiling -mllvm -enable-fs-discriminator=true -mllvm -improved-fs-discriminator=true
> +
> +# If CONFIG_DEBUG_INFO is not enabled, set -gmlt option.


Meaningless comment. It explains too obvious code.


> +ifndef CONFIG_DEBUG_INFO
> +  CFLAGS_AUTOFDO_CLANG += -gmlt
> +endif




> diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
> index 01a9f567d5af..e85d6ac31bd9 100644
> --- a/scripts/Makefile.lib
> +++ b/scripts/Makefile.lib
> @@ -191,6 +191,16 @@ _c_flags += $(if $(patsubst n%,, \
>         -D__KCSAN_INSTRUMENT_BARRIERS__)
>  endif
>
> +#
> +# Enable Clang's AutoFDO build flags for a file or directory depending on
> +# variables AUTOFDO_PROFILE_obj.o and AUTOFDO_PROFILE.
> +#


This comment would give the wrong understanding that this flag is opt-in.


The comment for KASAN correctly describes that it is enabled by default,
and can be opted out using KASAN_SANITIZE_*.





--
Best Regards


Masahiro Yamada