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

[Bill Wendling]

On Mon, Jan 11, 2021 at 12:39 AM Sedat Dilek <sedat.dilek@xxxxxxxxx> wrote:
>
> On Mon, Jan 11, 2021 at 9:18 AM 'Bill Wendling' via Clang Built Linux
> <clang-built-linux@xxxxxxxxxxxxxxxx> wrote:
> >
> > From: Sami Tolvanen <samitolvanen@xxxxxxxxxx>
> >
> > 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?

-bw