Re: [PATCH v5 04/24] cpumask: Introduce cpu_preferred_mask

[Peter Zijlstra]

On Thu, Jun 25, 2026 at 06:16:28PM +0530, Shrikanth Hegde wrote:

> diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
> index 80211900f373..5a643d608ea6 100644
> --- a/include/linux/cpumask.h
> +++ b/include/linux/cpumask.h
> @@ -120,12 +120,20 @@ extern struct cpumask __cpu_enabled_mask;
>  extern struct cpumask __cpu_present_mask;
>  extern struct cpumask __cpu_active_mask;
>  extern struct cpumask __cpu_dying_mask;
> +
> +#ifdef CONFIG_PREFERRED_CPU
> +extern struct cpumask __cpu_preferred_mask;
> +#else
> +#define __cpu_preferred_mask __cpu_active_mask
> +#endif

This is cure, but does it not result in set_cpu_preferred() changing
active mask, and it that not somewhat unexpected behaviour?

>  #define cpu_possible_mask ((const struct cpumask *)&__cpu_possible_mask)
>  #define cpu_online_mask   ((const struct cpumask *)&__cpu_online_mask)
>  #define cpu_enabled_mask   ((const struct cpumask *)&__cpu_enabled_mask)
>  #define cpu_present_mask  ((const struct cpumask *)&__cpu_present_mask)
>  #define cpu_active_mask   ((const struct cpumask *)&__cpu_active_mask)
>  #define cpu_dying_mask    ((const struct cpumask *)&__cpu_dying_mask)
> +#define cpu_preferred_mask ((const struct cpumask *)&__cpu_preferred_mask)
>  
>  extern atomic_t __num_online_cpus;
>  extern unsigned int __num_possible_cpus;

> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index bc4f7a9ba64e..d623a9c5554a 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -3107,6 +3107,11 @@ EXPORT_SYMBOL(__cpu_dying_mask);
>  atomic_t __num_online_cpus __read_mostly;
>  EXPORT_SYMBOL(__num_online_cpus);
>  
> +#ifdef CONFIG_PREFERRED_CPU
> +struct cpumask __cpu_preferred_mask __read_mostly;
> +EXPORT_SYMBOL(__cpu_preferred_mask);
> +#endif

Precedent is definitely towards !GPL exports for this, but could we get
away with making this one GPL?


> @@ -3164,6 +3169,7 @@ void __init boot_cpu_init(void)
>  	/* Mark the boot cpu "present", "online" etc for SMP and UP case */
>  	set_cpu_online(cpu, true);
>  	set_cpu_active(cpu, true);
> +	set_cpu_preferred(cpu, true);

This sets active twice, which is harmless, but wasteful...

>  	set_cpu_present(cpu, true);
>  	set_cpu_possible(cpu, true);
>  
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 2f4530eb543f..9e16946c9d62 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -8685,6 +8685,9 @@ int sched_cpu_activate(unsigned int cpu)
>  	 */
>  	sched_set_rq_online(rq, cpu);
>  
> +	/* preferred is subset of active and follows its state */
> +	set_cpu_preferred(cpu, true);
> +
>  	return 0;
>  }
>  
> @@ -8698,6 +8701,8 @@ int sched_cpu_deactivate(unsigned int cpu)
>  	if (ret)
>  		return ret;
>  
> +	set_cpu_preferred(cpu, false);
> +
>  	/*
>  	 * Remove CPU from nohz.idle_cpus_mask to prevent participating in
>  	 * load balancing when not active

But this one clears active earlier, is that not a problem?

Perhaps it is best if the modifier is a no-op when preferred mask does
not exist?