Re: [PATCH v2 0/8] x86, sched, mm: Speed up CPU hotplug on large systems

[Pasha Tatashin]

On Mon, Oct 19, 2026 at 2:22 PM Luck, Tony <tony.luck@xxxxxxxxx> wrote:
>
> > Benchmark workload (mean of 10 runs per step):
> >
> >  # echo 1 > /sys/kernel/rcu_expedited
> >  # for i in {1..N}; do echo 0 > /sys/devices/system/cpu/cpu$i/online; done
>
> Side question. Have you had issues with this step failing on large systems with "smart" I/O
> devices that support many interrupt vectors?
>
> Some validation folks inside Intel had some tests that looped taking all CPUs offline and
> bringing them back. They complained starting around Sapphire Rapids this their test
> would get down to just CPU 0 and CPU 1 and fail to take CPU 1 offline because there
> were too many interrupts to fit into just a single CPUs available vectors.
>
> It was a while ago, so maybe something changed in interrupt code to handle this.

Hi Tony,

I didn't encounter that issue while collecting the benchmark data.

For the Live Update work I'm currently doing, I leave 8 CPUs on the
host to perform kexec while the AP CPUs are removed from the
scheduler. In the future, I plan to leave only one host CPU and have
the remaining CPUs manage the VMs in Caretaker mode.

Best,
Pasha

>
> -Tony
>
>
>