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Date:   Thu, 21 Jan 2021 17:52:34 -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>,
        Fangrui Song <maskray@...gle.com>
Subject: Re: [PATCH v5] pgo: add clang's Profile Guided Optimization infrastructure

On Thu, Jan 21, 2021 at 5:49 PM Sedat Dilek <sedat.dilek@...il.com> wrote:
>
> 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.

Does the rebuild produce the hash warnings previously reported?

Can you send your .config for this?
-- 
Thanks,
~Nick Desaulniers

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