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Date:   Fri, 22 Jan 2021 02:49:00 +0100
From:   Sedat Dilek <sedat.dilek@...il.com>
To:     Nick Desaulniers <ndesaulniers@...gle.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>,
        Fangrui Song <maskray@...gle.com>
Subject: Re: [PATCH v5] pgo: add clang's Profile Guided Optimization infrastructure

On Fri, Jan 22, 2021 at 2:43 AM Nick Desaulniers
<ndesaulniers@...gle.com> wrote:
>
> On Wed, Jan 20, 2021 at 6:03 PM 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?
>
> I was able to build+boot with LLVM_IAS=1 on my personal laptop (no
> dwarf 5, just mainline+v5).
>

To clarify:

I can build a PGO-enabled Linux-kernel and boot it.
Afterwards generate a vmlinux.profdata.
In a next step: A rebuild without PGO-Kconfig disabled + LLVM_IAS=1
does not boot.

- Sedat -

> >
> > 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 agree, providing test results with patches that haven't landed yet
> can cloud the interpretation of results.  It would be helpful to drop
> local patch sets before trying this.
>
> If the resulting image still isn't working for you, can you please
> provide your config? Surely we'd be able to reproduce boot failures in
> QEMU?  Nothing comes to mind about a change of assemblers causing an
> issue; I would assume assembly cannot be instrumented by the compiler
> (even though the compiler is the "driver" of the assembler).
>
> The hash warnings are certainly curious.
> IndexedInstrProfReader::getInstrProfRecord() is the only place in LLVM
> sources that can emit that.
> --
> Thanks,
> ~Nick Desaulniers

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