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Date:   Fri, 2 Jul 2021 14:56:56 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Bill Wendling <morbo@google.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Kees Cook <keescook@chromium.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Bill Wendling <wcw@google.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Fangrui Song <maskray@google.com>,
        Heiko Carstens <hca@linux.ibm.com>,
        Jarmo Tiitto <jarmo.tiitto@gmail.com>,
        Lukas Bulwahn <lukas.bulwahn@gmail.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Miguel Ojeda <ojeda@kernel.org>,
        Nathan Chancellor <nathan@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Peter Oberparleiter <oberpar@linux.ibm.com>,
        Sami Tolvanen <samitolvanen@google.com>,
        Will Deacon <will@kernel.org>
Subject: Re: [GIT PULL] Clang feature updates for v5.14-rc1
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On Fri, Jul 02, 2021 at 05:46:46AM -0700, Bill Wendling wrote:
> On Tue, Jun 29, 2021 at 2:04 PM Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> >
> > On Tue, Jun 29, 2021 at 1:44 PM Kees Cook <keescook@chromium.org> wrote:
> > > >
> > > > And it causes the kernel to be bigger and run slower.
> > >
> > > Right -- that's expected. It's not designed to be the final kernel
> > > someone uses. :)
> >
> > Well, from what I've seen, you actually want to run real loads in
> > production environments for PGO to actually be anything but a bogus
> > "performance benchmarks only" kind of thing.
> >
> The reason we use PGO in this way is because we _cannot_ release a
> kernel into production that hasn't had PGO applied to it. The
> performance of a non-PGO'ed kernel is a non-starter for rollout. We
> try our best to replicate this environment for the benchmarks, which
> is the only sane way to do this. I can't imagine that we're the only
> ones who run up against this chicken-and-egg problem.
> 
> For why we don't use sampling, PGO gives a better performance boost
> from an instrumented kernel rather than a sampled profile. I'll work
> on getting statistics to show this.

I've asked this before; *what* is missing from LBR samples that's
reponsible for the performance gap?