Re: [PATCH 2/3] arm64: smp: Implement cpus_has_pending_ipi()

[Thomas Gleixner <tglx@linutronix.de>]

On Fri, Oct 03 2025 at 17:02, Ulf Hansson wrote:
> Note, the implementation is intentionally lightweight and doesn't use
> any

By some definition of lightweight.

>  static void smp_cross_call(const struct cpumask *target, unsigned int ipinr)
>  {
> +	unsigned int cpu;
> +
> +	for_each_cpu(cpu, target)
> +		per_cpu(pending_ipi, cpu) = true;

Iterating over a full cpumask on a big system is not necessarily
considered lightweight. And that comes on top of the loop in
smp_call_function_many_cond() plus the potential loop in
arm64_send_ipi()...

None of this is actually needed. If you want a lightweight racy check
whether there is an IPI en route to a set of CPUs then you can simply do
that in kernel/smp.c:

bool smp_pending_ipis_crystalball(mask)
{
	for_each_cpu(cpu, mask) {
                if (!llist_empty(per_cpu_ptr(&call_single_queue, cpu)))
                	return true;
        }
        return false;
}

No?

Thanks,

        tglx