Messages in this thread | | | From | Nick Desaulniers <> | Date | Fri, 2 Jul 2021 10:26:40 -0700 | Subject | Re: [GIT PULL] Clang feature updates for v5.14-rc1 |
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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
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