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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Thu, 28 Jan 2021 13:12:06 -0800
Message-ID: <CAKwvOdnUm2FqC0CEF3qFuMCaWoqiUMqr7ddMjA2UNsJugA9DNQ@mail.gmail.com>
Subject: Re: [PATCH v7] pgo: add clang's Profile Guided Optimization infrastructure
To:     Sedat Dilek <sedat.dilek@gmail.com>
Cc:     Bill Wendling <morbo@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Linux Doc Mailing List <linux-doc@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Nathan Chancellor <natechancellor@gmail.com>,
        Sami Tolvanen <samitolvanen@google.com>
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On Thu, Jan 28, 2021 at 12:46 PM Sedat Dilek <sedat.dilek@gmail.com> wrote:
>
> [ LLVM ]
>
> Today, I switched over to LLVM version 12.0.0-rc1.
>
>
> [ Step #1: 5.11.0-rc5-5-amd64-clang12-pgo ]
>
> My first kernel was built with CONFIG_PGO_CLANG=y and LLVM=1 plus LLVM_IAS=1.
>
> [ start-build_5.11.0-rc5-5-amd64-clang12-pgo.txt ]
> dileks    193090  193065  0 06:54 pts/2    00:00:00 /usr/bin/perf_5.10
> stat make V=1 -j4 HOSTCC=clang HOSTCXX=clang++ HOSTLD=ld.lld CC=clang
> LD=ld.lld LLVM=1 LLVM_IAS=1 PAHOLE=/opt/pahole/bin/pahole
> LOCALVERSION=-5-amd64-clang12-pgo KBUILD_VERBOSE=1
> KBUILD_BUILD_HOST=iniza KBUILD_BUILD_USER=sedat.dilek@gmail.com
> KBUILD_BUILD_TIMESTAMP=2021-01-28 bindeb-pkg
> KDEB_PKGVERSION=5.11.0~rc5-5~bullseye+dileks1
>
> Config: config-5.11.0-rc5-5-amd64-clang12-pgo
>
>
> [ Step #2: x86-64 defconfig & vmlinux.profdata ]
>
> Booted into 5.11.0-rc5-5-amd64-clang12-pgo and built an x86-64
> defconfig to generate/merge a vmlinux.profdata file.
>
> [ start-build_x86-64-defconfig.txt ]
> dileks     18430   15640  0 11:15 pts/2    00:00:00 make V=1 -j4
> HOSTCC=clang HOSTCXX=clang++ HOSTLD=ld.lld CC=clang LD=ld.lld LLVM=1
> LLVM_IAS=1
>
> Script: profile_clang-pgo.sh
> Config: dot-config.x86-64-defconfig
>
>
> [ Step #3.1: 5.11.0-rc5-6-amd64-clang12-pgo & GNU-AS ]
>
> The first rebuild with CONFIG_PGO_CLANG=n and "LLVM=1
> KCFLAGS=-fprofile-use=vmlinux.profdata".
> I was able to boot into this one.
> Used assembler: GNU-AS 2.35.1
>
> [ start-build_5.11.0-rc5-6-amd64-clang12-pgo.txt ]
> dileks     65734   65709  0 11:54 pts/2    00:00:00 /usr/bin/perf_5.10
> stat make V=1 -j4 HOSTCC=clang HOSTCXX=clang++ HOSTLD=ld.lld CC=clang
> LD=ld.lld PAHOLE=/opt/pahole/bin/pahole
> LOCALVERSION=-6-amd64-clang12-pgo KBUILD_VERBOSE=1
> KBUILD_BUILD_HOST=iniza KBUILD_BUILD_USER=sedat.dilek@gmail.com
> KBUILD_BUILD_TIMESTAMP=2021-01-28 bindeb-pkg
> KDEB_PKGVERSION=5.11.0~rc5-6~bullseye+dileks1 LLVM=1
> KCFLAGS=-fprofile-use=vmlinux.profdata
>
> Config: config-5.11.0-rc5-6-amd64-clang12-pgo
>
>
> [ Step #3.2: 5.11.0-rc5-7-amd64-clang12-pgo & Clang-IAS ]
>
> The second rebuild with CONFIG_PGO_CLANG=n and "LLVM=1
> KCFLAGS=-fprofile-use=vmlinux.profdata" plus LLVM_IAS=1.
> Compilable but NOT bootable in QEMU and on bare metal.
> Used assembler: Clang-IAS v12.0.0-rc1
>
> [ start-build_5.11.0-rc5-7-amd64-clang12-pgo.txt ]
> dileks      6545    6520  0 16:31 pts/2    00:00:00 /usr/bin/perf_5.10
> stat make V=1 -j4 HOSTCC=clang HOSTCXX=clang++ HOSTLD=ld.lld CC=clang
> LD=ld.lld PAHOLE=/opt/pahole/bin/pahole
> LOCALVERSION=-7-amd64-clang12-pgo KBUILD_VERBOSE=1
> KBUILD_BUILD_HOST=iniza KBUILD_BUILD_USER=sedat.dilek@gmail.com
> KBUILD_BUILD_TIMESTAMP=2021-01-28 bindeb-pkg
> KDEB_PKGVERSION=5.11.0~rc5-7~bullseye+dileks1 LLVM=1
> KCFLAGS=-fprofile-use=vmlinux.profdata LLVM_IAS=1
>
> Config: config-5.11.0-rc5-7-amd64-clang12-pgo
>
>
> [ Conclusion ]
>
> The only statement I can tell you is a "PGO optimized" rebuild with
> LLVM_IAS=1 is compilable but NOT bootable.

Thanks for the extensive testing and report. Can you compress, upload,
and post a link to your kernel image? I would like to take it for a
spin in QEMU and see if I can find what it's doing, then work
backwards from there.

-- 
Thanks,
~Nick Desaulniers