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

Bill Wendling
2021-06-12T19:29:02Z

On Sat, Jun 12, 2021 at 12:10 PM Bill Wendling <morbo@google.com> wrote:
> ")On Sat, Jun 12, 2021 at 11:15 AM Peter Zijlstra <peterz@infradead.org> wrote:
> > On Sat, Jun 12, 2021 at 10:25:57AM -0700, Bill Wendling wrote:
> > > On Sat, Jun 12, 2021 at 9:59 AM Peter Zijlstra <peterz@infradead.org> wrote:
> > > >
> > > > On Wed, Apr 07, 2021 at 02:17:04PM -0700, Bill Wendling 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.
> > > >
> > > > *sigh*, and not a single x86 person on Cc, how nice :-/
> > > >
> > > This tool is generic and, despite the fact that it's first enabled for
> > > x86, it contains no x86-specific code. The reason we're restricting it
> > > to x86 is because it's the platform we tested on.
> >
> > You're modifying a lot of x86 files, you don't think it's good to let us
> > know? Worse, afaict this -fprofile-generate changes code generation,
> > and we definitely want to know about that.
> >
> I got the list of people to add from the scripts/get_maintainer.pl.
> The files you list below are mostly changes in Makefile, so it added
> the kbuild maintainers and list. There's a small change to the linker
> script to add the clang PGO data section, which is defined in
> "include/asm-generic/vmlinux.lds.h". Using the "kernel/gcov" initial
> implementation as a guildlline
> (2521f2c228ad750701ba4702484e31d876dbc386), there's one intel people
> CC'ed, but he didn't sign off on it. These patches were available for
> review for months now, and posted to all of the lists and CC'ed to the
> people from scripts/get_maintainers.pl. Perhaps that program should be
> improved?
>
Correction: I see now that it lists X86 maintainers. That was somehow
missed in my initial submission. Sorry about that. Please add any
reviewers you think are necessary.

-bw