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

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Date:   Mon, 14 Jun 2021 11:01:13 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Bill Wendling <morbo@...gle.com>
Cc:     Kees Cook <keescook@...gle.com>, Jonathan Corbet <corbet@....net>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Nathan Chancellor <natechancellor@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Fangrui Song <maskray@...gle.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        andreyknvl@...il.com, dvyukov@...gle.com, elver@...gle.com,
        johannes.berg@...el.com, oberpar@...ux.vnet.ibm.com
Subject: Re: [PATCH v9] pgo: add clang's Profile Guided Optimization
 infrastructure

On Sat, Jun 12, 2021 at 01:56:41PM -0700, Bill Wendling wrote:
> On Sat, Jun 12, 2021 at 1:25 PM Peter Zijlstra <peterz@...radead.org> wrote:
> > On Sat, Jun 12, 2021 at 12:10:03PM -0700, Bill Wendling wrote:
> > Yes it is, but is that sufficient in this case? It very much isn't for
> > KASAN, UBSAN, and a whole host of other instrumentation crud. They all
> > needed their own 'bugger-off' attributes.
> >
> > > > We've got KCOV and GCOV support already. Coverage is also not an
> > > > argument mentioned anywhere else. Coverage can go pound sand, we really
> > > > don't need a third means of getting that.
> > > >
> > > Those aren't useful for clang-based implementations. And I like to
> > > look forward to potential improvements.
> >
> > I look forward to less things doing the same over and over. The obvious
> > solution if of course to make clang use what we have, not the other way
> > around.
> >
> That is not the obvious "solution".

Because having GCOV, KCOV and PGO all do essentially the same thing
differently, makes heaps of sense?

I understand that the compilers actually generates radically different
instrumentation for the various cases, but essentially they're all
collecting (function/branch) arcs.

I'm thinking it might be about time to build _one_ infrastructure for
that and define a kernel arc format and call it a day.

Note that if your compiler does arcs with functions (like gcc, unlike
clang) we can also trivially augment the arcs with PMU counter data. I
once did that for userspace.