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

On Sun, Jan 17, 2021 at 4:27 PM Sedat Dilek [off-list ref] wrote:

[ big snip ]

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[ 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.

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