Re: [PATCH 0/2] genirq, KVM: arm64: Avoid the irq descriptor lock on vCPU load

[Amir Noohi <amirmnoohi@gmail.com>]

Hi Marc,

On Sat, 24 Oct 2026 12:52:44 +0100, Marc Zyngier <maz@kernel.org> wrote:
> Looking at the test case, this really isn't a representative or
> realistic workload.  This is a contrived setup that has trapping to
> userspace as its only purpose.
>
> I'm not saying that this is useless observation. But it is not
> something that represents something exacerbated by "real" VMs.

Fair enough. I checked with real Linux guests running hackbench on the
same Altra (kvmtool, in-kernel GICv3). With one vCPU per CPU, and in
lighter hackbench runs, it makes no measurable difference, but it does
once busy vCPUs share CPUs.

With four 31-vCPU VMs on 31 CPUs (124 vCPUs), all running
"hackbench -g 31 -f 20 -l 2000", each VM takes 76.5s on mainline and
60.4s with the series, so 21% less time (8 runs per kernel, no
overlap). In a profiled run irq_set_irqchip_state() was called ~2.4M
times/s and 47% of those calls hit contention. With the series the
contention on that path drops to ~7 per second.

So the case that matters is overcommitted hosts with busy vCPUs. I'll
make that the main justification in v2 instead of the exit-to-userspace
test.

Thanks,
Amir