linux-kernel - Re: [PATCH v2] KVM: arm64: Get rid of userspace_irqchip_in

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Message-ID: <87iktat2y8.wl-maz@kernel.org>
Date: Wed, 30 Oct 2024 08:22:39 +0000
From: Marc Zyngier <maz@...nel.org>
To: Raghavendra Rao Ananta <rananta@...gle.com>
Cc: Oliver Upton <oliver.upton@...ux.dev>,
	linux-arm-kernel@...ts.infradead.org,
	kvmarm@...ts.linux.dev,
	linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org,
	stable@...r.kernel.org,
	syzbot <syzkaller@...glegroups.com>
Subject: Re: [PATCH v2] KVM: arm64: Get rid of userspace_irqchip_in_use

On Wed, 30 Oct 2024 00:16:48 +0000,
Raghavendra Rao Ananta <rananta@...gle.com> wrote:
> 
> On Tue, Oct 29, 2024 at 11:47 AM Marc Zyngier <maz@...nel.org> wrote:
> >
> > On Tue, 29 Oct 2024 17:06:09 +0000,
> > Raghavendra Rao Ananta <rananta@...gle.com> wrote:
> > >
> > > On Tue, Oct 29, 2024 at 9:27 AM Marc Zyngier <maz@...nel.org> wrote:
> > > >
> > > > On Mon, 28 Oct 2024 23:45:33 +0000,
> > > > Raghavendra Rao Ananta <rananta@...gle.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. 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.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.