Re: [GIT PULL] Clang feature updates for v5.14-rc1

From: Nick Desaulniers <ndesaulniers@google.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Bill Wendling <morbo@google.com>,
	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>,
	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
Date: Fri, 2 Jul 2021 10:26:40 -0700	[thread overview]
Message-ID: <CAKwvOd=-qdp6xcuVb-fSni6X-0UuJ6GM5+TJdWSkAfvNmqfZ+w@mail.gmail.com> (raw)
In-Reply-To: <YN8NGLPQ4Cqanc48@hirez.programming.kicks-ass.net>

On Fri, Jul 2, 2021 at 5:57 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> 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?

Are we able to collect LBR samples from __init code?  I can imagine
trying to launch perf from init/pid 1, but I suspect at that point
it's way too late.

Increasingly, boot times of hosts (and virtualized guests) are
becoming important to us, both in the datacenters and on mobile.
-- 
Thanks,
~Nick Desaulniers