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

[Sedat Dilek]

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

- 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