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

[Nick Desaulniers]

On Fri, Jul 2, 2021 at 5:57 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxx> wrote:
> > >
> > > On Tue, Jun 29, 2021 at 1:44 PM Kees Cook <keescook@xxxxxxxxxxxx> 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