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

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Date:   Thu, 21 Jan 2021 03:03:15 +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 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.
>
> -bw

View attachment "0001-pgo-Allow-to-use-clang-v11.0.1.patch" of type "text/x-patch" (841 bytes)