linux-kernel - Re: [PATCH 2/3] arm64: smp: Implement cpus_has_pending

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Message-ID: <87ms5pzkxa.ffs@tglx>
Date: Fri, 17 Oct 2025 16:01:53 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: Ulf Hansson <ulf.hansson@...aro.org>, "Rafael J . Wysocki"
 <rafael@...nel.org>, Catalin Marinas <catalin.marinas@....com>, Will
 Deacon <will@...nel.org>, Mark Rutland <mark.rutland@....com>
Cc: Maulik Shah <quic_mkshah@...cinc.com>, Sudeep Holla
 <sudeep.holla@....com>, Daniel Lezcano <daniel.lezcano@...aro.org>,
 Vincent Guittot <vincent.guittot@...aro.org>, linux-pm@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, Ulf
 Hansson <ulf.hansson@...aro.org>
Subject: Re: [PATCH 2/3] arm64: smp: Implement cpus_has_pending_ipi()

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