Re: [PATCH v9] pgo: add clang's Profile Guided Optimization infrastructure

Fāng-ruì Sòng
2021-04-07T21:44:57Z

On Wed, Apr 7, 2021 at 2:22 PM Kees Cook <keescook@chromium.org> wrote:
>
> On Wed, Apr 07, 2021 at 02:17:04PM -0700, 'Bill Wendling' via Clang Built Linux wrote:
> > From: Sami Tolvanen <samitolvanen@google.com>
> >
> > Enable the use of clang's Profile-Guided Optimization[1]. To generate a
> > profile, the kernel is instrumented with PGO counters, a representative
> > workload is run, and the raw profile data is collected from
> > /sys/kernel/debug/pgo/profraw.
> >
> > The raw profile data must be processed by clang's "llvm-profdata" tool
> > before it can be used during recompilation:
> >
> > $ cp /sys/kernel/debug/pgo/profraw vmlinux.profraw
> > $ llvm-profdata merge --output=vmlinux.profdata vmlinux.profraw
> >
> > Multiple raw profiles may be merged during this step.
> >
> > The data can now be used by the compiler:
> >
> > $ make LLVM=1 KCFLAGS=-fprofile-use=vmlinux.profdata ...
> >
> > This initial submission is restricted to x86, as that's the platform we
> > know works. This restriction can be lifted once other platforms have
> > been verified to work with PGO.
> >
> > Note that this method of profiling the kernel is clang-native, unlike
> > the clang support in kernel/gcov.
> >
> > [1] https://clang.llvm.org/docs/UsersManual.html#profile-guided-optimization
> >
> > Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
> > Co-developed-by: Bill Wendling <morbo@google.com>
> > Signed-off-by: Bill Wendling <morbo@google.com>
> > Tested-by: Nick Desaulniers <ndesaulniers@google.com>
> > Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
> > Reviewed-by: Fangrui Song <maskray@google.com>
>
> Thanks for sending this again! I'm looking forward to using it.

Yay. Quite excited about that:)

> Masahiro and Andrew, unless one of you would prefer to take this in your
> tree, I figure I can snag it to send to Linus.
>
> Anyone else have feedback?

I have carefully compared the implementation and the original
implementation in llvm-project/compiler-rt.
This looks great.
Also very happy about the cleaner include/asm-generic/vmlinux.lds.h now.

Just adding a note here for folks who may want to help test the
not-yet-common option LD_DEAD_CODE_DATA_ELIMINATION.
--gc-sections may not work perfectly with some advanced PGO features
before Clang 13 (not broken but probably just in an inferior state).
There were some upstream changes in this area recently and I think as
of my https://reviews.llvm.org/D97649 things should be perfect with GC
now.
This does not deserve any comment without more testing, though.

Thanks for already carrying my Reviewed-by tag.

> Thanks!
>
> -Kees
>
> --
> Kees Cook

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