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

[Sedat Dilek <sedat.dilek@gmail.com>]

On Mon, Jan 11, 2021 at 10:17 AM Bill Wendling <morbo@google.com> wrote:
>
> On Mon, Jan 11, 2021 at 12:39 AM Sedat Dilek <sedat.dilek@gmail.com> wrote:
> >
> > On Mon, Jan 11, 2021 at 9:18 AM 'Bill Wendling' via Clang Built Linux
> > <clang-built-linux@googlegroups.com> 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 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.)
>
> > 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.
>
> > 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.
>
> > 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 -