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

Sedat Dilek
2021-01-18T17:31:56Z

On Mon, Jan 18, 2021 at 1:39 PM Sedat Dilek <sedat.dilek@gmail.com> wrote:
>
> On Mon, Jan 18, 2021 at 3:32 AM Bill Wendling <morbo@google.com> wrote:
> >
> > On Sun, Jan 17, 2021 at 4:27 PM Sedat Dilek <sedat.dilek@gmail.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.

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

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.

Thanks.

Regards,
- Sedat -

>
> > > I would like to know what the impact of the Clang's Integrated
> > > Assembler and DWARF v5 are.
> > >
> > > I dropped both means...
> > >
> > > 1. Do not pass "LLVM_IAS=1" to make.
> > > 2. Use default DWARF v2 (with Nick's patchset: CONFIG_DEBUG_INFO_DWARF2=y).
> > >
> > > ...for a successfull build and boot on bare metal.
> > >
> >
> > [Next message]
> >
> > > On each rebuild I need to pass to make ...?
> > >
> > > LLVM=1 -fprofile-use=vmlinux.profdata
> > >
> > Yes.
> >
> > > Did you try together with passing LLVM_IAS=1 to make?
> >
> > One of my tests was with the integrated assembler enabled. Are you
> > finding issues with it?
> >
> > The problem with using top-of-tree clang is that it's not necessarily
> > stable. You could try using the clang 11.x release (changing the
> > "CLANG_VERSION >= 120000" in kernel/pgo/Kconfig/ to "CLANG_VERSION >=
> > 110000").
> >
> > -bw