On Mon, Jan 11, 2021 at 10:17 AM Bill Wendling [off-list ref] wrote:

On Mon, Jan 11, 2021 at 12:39 AM Sedat Dilek [off-list ref] wrote:

quoted

On Mon, Jan 11, 2021 at 9:18 AM 'Bill Wendling' via Clang Built Linux
[off-list ref] wrote:

quoted

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 be used either by the compiler if LTO isn't enabled:

    ... -fprofile-use=vmlinux.profdata ...

or by LLD if LTO is enabled:

    ... -lto-cs-profile-file=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 and isn't
compatible with clang's gcov support in kernel/gcov.

[1] https://clang.llvm.org/docs/UsersManual.html#profile-guided-optimization

Hi Bill and Sami,

I have seen the pull-request in the CBL issue tracker and had some
questions in mind.

Good you send this.

First of all, I like to fetch any development stuff easily from a Git
repository.

The version in the pull-request in the CBL issue tracker is roughly
the same as this patch. (There are some changes, but they aren't
functionality changes.)

quoted

Can you offer this, please?
What is the base for your work?
I hope this is (fresh released) Linux v5.11-rc3.

This patch (and the PR on the CBL issue tracker) are from top-of-tree Linux.

quoted

I myself had some experiences with a PGO + ThinLTO optimized LLVM
toolchain built with the help of tc-build.
Here it takes very long to build it.

This means I have some profile-data archived.
Can I use it?

LLVM is more tolerant of "stale" profile data than gcov, so it's
possible that your archived profile data would still work, but I can't
guarantee that it will be better than using new profile data.

quoted

Is an own PGO + ThinLTO optimized LLVM toolchain pre-requirement for
this or not?
That is one of my important questions.

Do you mean that the LLVM tools (clang, llc, etc.) are compiled with
PGO + ThinLTO?

Yes.

- Sedat -