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

Kees Cook
2021-04-07T21:22:50Z

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.

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?

Thanks!

-Kees

--
Kees Cook