linux-kernel - Re: [PATCH v5] pgo: add clang's Profile Guided Optimization infrastructure

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Date:   Thu, 21 Jan 2021 23:44:31 +0100
From:   Sedat Dilek <sedat.dilek@...il.com>
To:     Bill Wendling <morbo@...gle.com>
Cc:     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>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Fangrui Song <maskray@...gle.com>
Subject: Re: [PATCH v5] pgo: add clang's Profile Guided Optimization infrastructure

On Thu, Jan 21, 2021 at 3:03 AM Sedat Dilek <sedat.dilek@...il.com> wrote:
>
> On Mon, Jan 18, 2021 at 10:56 PM Bill Wendling <morbo@...gle.com> wrote:
> >
> > On Mon, Jan 18, 2021 at 9:26 AM Sedat Dilek <sedat.dilek@...il.com> wrote:
> > >
> > > On Mon, Jan 18, 2021 at 1:39 PM Sedat Dilek <sedat.dilek@...il.com> wrote:
> > > >
> > > > On Mon, Jan 18, 2021 at 3:32 AM Bill Wendling <morbo@...gle.com> wrote:
> > > > >
> > > > > On Sun, Jan 17, 2021 at 4:27 PM Sedat Dilek <sedat.dilek@...il.com> wrote:
> > > > > >
> > > > > > [ big snip ]
> > > > >
> > > > > [More snippage.]
> > > > >
> > > > > > [ CC Fangrui ]
> > > > > >
> > > > > > With the attached...
> > > > > >
> > > > > >    [PATCH v3] module: Ignore _GLOBAL_OFFSET_TABLE_ when warning for
> > > > > > undefined symbols
> > > > > >
> > > > > > ...I was finally able to boot into a rebuild PGO-optimized Linux-kernel.
> > > > > > For details see ClangBuiltLinux issue #1250 "Unknown symbol
> > > > > > _GLOBAL_OFFSET_TABLE_ loading kernel modules".
> > > > > >
> > > > > Thanks for confirming that this works with the above patch.
> > > > >
> > > > > > @ Bill Nick Sami Nathan
> > > > > >
> > > > > > 1, Can you say something of the impact passing "LLVM_IAS=1" to make?
> > > > >
> > > > > The integrated assembler and this option are more-or-less orthogonal
> > > > > to each other. One can still use the GNU assembler with PGO. If you're
> > > > > having an issue, it may be related to ClangBuiltLinux issue #1250.
> > > > >
> > > > > > 2. Can you please try Nick's DWARF v5 support patchset v5 and
> > > > > > CONFIG_DEBUG_INFO_DWARF5=y (see attachments)?
> > > > > >
> > > > > I know Nick did several tests with PGO. He may have looked into it
> > > > > already, but we can check.
> > > > >
> > > >
> > > > Reproducible.
> > > >
> > > > LLVM_IAS=1 + DWARF5 = Not bootable
> > > >
> > > > I will try:
> > > >
> > > > LLVM_IAS=1 + DWARF4
> > > >
> > >
> > > I was not able to boot into such a built Linux-kernel.
> > >
> > PGO will have no effect on debugging data. If this is an issue with
> > DWARF, then it's likely orthogonal to the PGO patch.
> >
> > > For me worked: DWARF2 and LLVM_IAS=1 *not* set.
> > >
> > > Of course, this could be an issue with my system's LLVM/Clang.
> > >
> > > Debian clang version
> > > 12.0.0-++20210115111113+45ef053bd709-1~exp1~20210115101809.3724
> > >
> > Please use the official clang 11.0.1 release
> > (https://releases.llvm.org/download.html), modifying the
> > kernel/pgo/Kconfig as I suggested above. The reason we specify clang
> > 12 for the minimal version is because of an issue that was recently
> > fixed.
> >
>
> I downgraded to clang-11.1.0-rc1.
> ( See attachment. )
>
> Doing the first build with PGO enabled plus DWARF5 and LLVM_IAS=1 works.
>
> But again after generating vmlinux.profdata and doing the PGO-rebuild
> - the resulting Linux-kernel does NOT boot in QEMU or on bare metal.
> With GNU/as I can boot.
>
> So this is independent of DWARF v4 or DWARF v5 (LLVM_IAS=1 and DWARF
> v2 is not allowed).
> There is something wrong (here) with passing LLVM_IAS=1 to make when
> doing the PGO-rebuild.
>
> Can someone please verify and confirm that the PGO-rebuild with
> LLVM_IAS=1 boots or boots not?
>
> Thanks.
>
> - Sedat -
>
> > > Can you give me a LLVM commit-id where you had success with LLVM_IAS=1
> > > and especially CONFIG_DEBUG_INFO_DWARF5=y?
> > > Success means I was able to boot in QEMU and/or bare metal.
> > >
> > The DWARF5 patch isn't in yet, so I don't want to rely upon it too much.
> >

I passed LLVM_IAS=1 with KAFLAGS=-fprofile-use=vmlinux.profdata:

/usr/bin/perf_5.10 stat make V=1 -j4 HOSTCC=clang HOSTCXX=clang++
HOSTLD=ld.lld CC=clang LD=ld.lld PAHOLE=/opt/paho
le/bin/pahole LOCALVERSION=-2-amd64-clang11-pgo KBUILD_VERBOSE=1
KBUILD_BUILD_HOST=iniza KBUILD_BUILD_USER=sedat.dilek@...il.com
KBUILD_BUILD_TIMESTAMP=2021-01-21 bind
eb-pkg KDEB_PKGVERSION=5.11.0~rc4-2~bullseye+dileks1 LLVM=1
KCFLAGS=-fprofile-use=vmlinux.profdata LLVM_IAS=1
KAFLAGS=-fprofile-use=vmlinux.profdata

The resulting Linux-kernel does not boot.

But I see in the build-log these warnings:

warning: arch/x86/platform/efi/quirks.c: Function control flow change
detected (hash mismatch) efi_arch_mem_reserve Hash = 73770966985
[-Wbackend-plugin]
warning: arch/x86/platform/efi/efi.c: Function control flow change
detected (hash mismatch) efi_attr_is_visible Hash = 57959232386
[-Wbackend-plugin]
warning: arch/x86/boot/compressed/string.c: Function control flow
change detected (hash mismatch) memcmp Hash = 12884901887
[-Wbackend-plugin]
warning: arch/x86/boot/compressed/string.c: Function control flow
change detected (hash mismatch) bcmp Hash = 12884901887
[-Wbackend-plugin]
warning: arch/x86/boot/compressed/string.c: Function control flow
change detected (hash mismatch) strcmp Hash = 44149752232
[-Wbackend-plugin]
warning: arch/x86/boot/compressed/string.c: Function control flow
change detected (hash mismatch) strnlen Hash = 29212902728
[-Wbackend-plugin]
warning: arch/x86/boot/compressed/string.c: Function control flow
change detected (hash mismatch) simple_strtoull Hash =
288230479369728480 [-Wbackend-plugin]
warning: arch/x86/boot/compressed/string.c: Function control flow
change detected (hash mismatch) strstr Hash = 76464046323
[-Wbackend-plugin]
warning: arch/x86/boot/compressed/string.c: Function control flow
change detected (hash mismatch) strchr Hash = 30948479515
[-Wbackend-plugin]
warning: arch/x86/boot/compressed/string.c: Function control flow
change detected (hash mismatch) kstrtoull Hash = 288230543187488006
[-Wbackend-plugin]

What does "Function control flow change detected (hash mismatch)" mean?
Related with my boot problems?

- Sedat -