[PATCH RFC 1/3] cpumask: Honor irq_default_affinity in cpumask_local_spread()

[Yury Norov]

On Wed, Aug 19, 2026 at 04:30:30PM +0200, Florian Bezdeka wrote:
> Many drivers call cpumask_local_spread() to spread IRQs to several
> CPUs, mainly to get best performance and balance CPU load. For
> realtime (PREEMPT_RT) and other cpu-isolating workloads the old
> implementation was triggering an IRQ placement problem. IRQs were
> targeting CPUs that were isolated for those sensitive workloads.
> 
> Userland will tell the kernel about the desired IRQ configuration
> for new interrupts by writing a proper cpumask to
> /proc/irq/default_smp_affinity. This cpu mask has to be honored to
> avoid IRQ noise on isolated cores.
> 
> The default for irq_default_affinity is "all CPUs". So all CPUs will
> be taken into account for spreading when userland did not configure
> something special.
> ---
>  lib/cpumask.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/lib/cpumask.c b/lib/cpumask.c
> index 5adb9874fbd0f5a42ea8cd9e6c3729a70599781f..73e7b60a9201174f83df6067effbe7d1889dcdb9 100644
> --- a/lib/cpumask.c
> +++ b/lib/cpumask.c
> @@ -6,6 +6,7 @@
>  #include <linux/export.h>
>  #include <linux/memblock.h>
>  #include <linux/numa.h>
> +#include <linux/interrupt.h>
>  
>  /* These are not inline because of header tangles. */
>  #ifdef CONFIG_CPUMASK_OFFSTACK
> @@ -81,8 +82,9 @@ void __init free_bootmem_cpumask_var(cpumask_var_t mask)
>   * @i: index number
>   * @node: local numa_node
>   *
> - * Return: online CPU according to a numa aware policy; local cpus are returned
> - * first, followed by non-local ones, then it wraps around.
> + * Return: online CPU according to the default IRQ affinity and a numa aware
> + * policy; local cpus are returned first, followed by non-local ones, then it
> + * wraps around.
>   *
>   * For those who wants to enumerate all CPUs based on their NUMA distances,
>   * i.e. call this function in a loop, like:
> @@ -110,9 +112,9 @@ unsigned int cpumask_local_spread(unsigned int i, int node)

Please don't touch this function. There's ~40 users, and we don't want
to inspect every caller for their intention.

Looking at ionic_get_preferred_cpu(), the cpumask_local_spread() is
used there as a fallback for the affinity IRQ search:

 static int ionic_get_preferred_cpu(struct ionic *ionic,
                                    struct ionic_intr_info *intr)
 {
         int cpu;
 
         cpu = cpumask_first_and(*intr->affinity_mask, cpu_online_mask);
         if (cpu >= nr_cpu_ids)
                 cpu = cpumask_local_spread(0, dev_to_node(ionic->dev));
 
         return cpu;
 }

Your change may affect the logic, seemingly.

Instead, please create something like:

        unsigned int cpumask_spread(struct cpumask cpus,
                                    unsigned int i, int node);

Thanks,
Yury

>  	unsigned int cpu;
>  
>  	/* Wrap: we always want a cpu. */
> -	i %= num_online_cpus();
> +	i %= cpumask_weight(irq_default_affinity);
>  
> -	cpu = sched_numa_find_nth_cpu(cpu_online_mask, i, node);
> +	cpu = sched_numa_find_nth_cpu(irq_default_affinity, i, node);
>  
>  	WARN_ON(cpu >= nr_cpu_ids);
>  	return cpu;
> 
> -- 
> 2.55.0