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

[Peter Zijlstra]

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?