linux-kernel - Re: [PATCH] pgo: add clang's Profile Guided Optimization infrastructure

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Message-ID: <CAKwvOd==OfKHKzhWVGAQs5bFJ9qSbm8WRkX7qDF+FQd-p1gdNQ@mail.gmail.com>
Date:   Thu, 21 Jan 2021 17:34:29 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Sedat Dilek <sedat.dilek@...il.com>
Cc:     Bill Wendling <morbo@...gle.com>, Jonathan Corbet <corbet@....net>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Clang-Built-Linux ML <clang-built-linux@...glegroups.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Nathan Chancellor <natechancellor@...il.com>,
        Sami Tolvanen <samitolvanen@...gle.com>
Subject: Re: [PATCH] pgo: add clang's Profile Guided Optimization infrastructure

On Wed, Jan 20, 2021 at 6:21 PM Sedat Dilek <sedat.dilek@...il.com> wrote:
>
> Hi,
>
> When I looked through the code I wondered why we do not add a
> "CONFIG_PGO_CLANG_PROFDATA" which can be helpful when doing the PGO
> rebuild with a vmlinux.profdata.
>
> This introduces a "PGO_PROFDATA" to turn on/off to pass
> "-fprofile-use=vmlinux.profdata" (see CFLAGS_PGO_CLANG_PROFDATA in
> top-level Makefile).
>
> If we turn off via "PGO_PROFILE := n" in several Makefiles - we should
> do the same and add "PGO_PROFDATA := n" to the same Makefiles?
>
> Please see the attached diff.

This is a good idea; something that I brought up in initial code
review (on github).  Would it be ok with you to land the core first,
then follow up with this suggestion?

Also, AutoFDO production builds are so incredibly similar to PGO
builds that I could see a possible path forward:
1. land PGO upstream
2. adds docs for AutoFDO
3. consider a config for hardcoding the location of the profiling data
so that we don't need to specify it at the command line invocation of
make.
-- 
Thanks,
~Nick Desaulniers