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

[Marc Zyngier <maz@kernel.org>]

On Fri, 03 Oct 2025 16:02:44 +0100,
Ulf Hansson <ulf.hansson@linaro.org> wrote:
> 
> To add support for keeping track of whether there may be a pending IPI
> scheduled for a CPU or a group of CPUs, let's implement
> cpus_has_pending_ipi() for arm64.
> 
> Note, the implementation is intentionally lightweight and doesn't use any
> additional lock. This is good enough for cpuidle based decisions.
> 
> Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
> ---
>  arch/arm64/kernel/smp.c | 20 ++++++++++++++++++++
>  1 file changed, 20 insertions(+)
> 
> diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
> index 68cea3a4a35c..dd1acfa91d44 100644
> --- a/arch/arm64/kernel/smp.c
> +++ b/arch/arm64/kernel/smp.c
> @@ -55,6 +55,8 @@
>  
>  #include <trace/events/ipi.h>
>  
> +static DEFINE_PER_CPU(bool, pending_ipi);
> +
>  /*
>   * as from 2.5, kernels no longer have an init_tasks structure
>   * so we need some other way of telling a new secondary core
> @@ -1012,6 +1014,8 @@ static void do_handle_IPI(int ipinr)
>  
>  	if ((unsigned)ipinr < NR_IPI)
>  		trace_ipi_exit(ipi_types[ipinr]);
> +
> +	per_cpu(pending_ipi, cpu) = false;
>  }
>  
>  static irqreturn_t ipi_handler(int irq, void *data)
> @@ -1024,10 +1028,26 @@ static irqreturn_t ipi_handler(int irq, void *data)
>  
>  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;
> +

Why isn't all of this part of the core IRQ management? We already
track things like timers, I assume for similar reasons. If IPIs have
to be singled out, I'd rather this is done in common code, and not on
a per architecture basis.

>  	trace_ipi_raise(target, ipi_types[ipinr]);
>  	arm64_send_ipi(target, ipinr);
>  }
>  
> +bool cpus_has_pending_ipi(const struct cpumask *mask)
> +{
> +	unsigned int cpu;
> +
> +	for_each_cpu(cpu, mask) {
> +		if (per_cpu(pending_ipi, cpu))
> +			return true;
> +	}
> +	return false;
> +}
> +

The lack of memory barriers makes me wonder how reliable this is.
Maybe this is relying on the IPIs themselves acting as such, but
that's extremely racy no matter how you look at it.

	M.

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