[PATCH v2] KVM: arm64: Get rid of userspace_irqchip_in_use

[Raghavendra Rao Ananta]

On Wed, Oct 30, 2024 at 1:22 AM Marc Zyngier <maz at kernel.org> wrote:
>
> On Wed, 30 Oct 2024 00:16:48 +0000,
> Raghavendra Rao Ananta <rananta at google.com> wrote:
> >
> > On Tue, Oct 29, 2024 at 11:47 AM Marc Zyngier <maz at kernel.org> wrote:
> > >
> > > On Tue, 29 Oct 2024 17:06:09 +0000,
> > > Raghavendra Rao Ananta <rananta at google.com> wrote:
> > > >
> > > > On Tue, Oct 29, 2024 at 9:27 AM Marc Zyngier <maz at kernel.org> wrote:
> > > > >
> > > > > On Mon, 28 Oct 2024 23:45:33 +0000,
> > > > > Raghavendra Rao Ananta <rananta at google.com> wrote:
> > > > > >
> > > > > Did you have a chance to check whether this had any negative impact on
> > > > > actual workloads? Since the entry/exit code is a bit of a hot spot,
> > > > > I'd like to make sure we're not penalising the common case (I only
> > > > > wrote this patch while waiting in an airport, and didn't test it at
> > > > > all).
> > > > >
> > > > I ran the kvm selftests, kvm-unit-tests and booted a linux guest to
> > > > test the change and noticed no failures.
> > > > Any specific test you want to try out?
> > >
> > > My question is not about failures (I didn't expect any), but
> > > specifically about *performance*, and whether checking the flag
> > > without a static key can lead to any performance drop on the hot path.
> > >
> > > Can you please run an exit-heavy workload (such as hackbench, for
> > > example), and report any significant delta you could measure?
> >
> > Oh, I see. I ran hackbench and micro-bench from kvm-unit-tests (which
> > also causes a lot of entry/exits), on Ampere Altra with kernel at
> > v6.12-rc1, and see no significant difference in perf.
>
> Thanks for running this stuff.
>
> > timer_10ms                          231040.0                          902.0
> > timer_10ms                         234120.0                            914.0
>
> This seems to be the only case were we are adversely affected by this
> change.
Hmm, I'm not sure how much we want to trust this comparison. For
instance, I just ran micro-bench again a few more times and here are
the outcomes of timer_10ms for each try with the patch:

Tries                                             total ns
               avg ns
-----------------------------------------------------------------------------------
1_timer_10ms                             231840.0                          905.0
2_timer_10ms                             234560.0                          916.0
3_timer_10ms                             227440.0                          888.0
4_timer_10ms                             236640.0                          924.0
5_timer_10ms                             231200.0                          903.0

Here's a few on the baseline:

Tries                                             total ns
               avg ns
-----------------------------------------------------------------------------------
1_timer_10ms                             231080.0                          902.0
2_timer_10ms                             238040.0                          929.0
3_timer_10ms                             231680.0                          905.0
4_timer_10ms                             229280.0                          895.0
5_timer_10ms                             228520.0                          892.0


> In the grand scheme of thins, that's noise. But this gives us
> a clear line of sight for the removal of the in-kernel interrupts back
> to userspace.
Sorry, I didn't follow you completely on this part.

Thank you.
Raghavendra