Re: [PATCH v6 03/23] cpumask: Introduce cpu_preferred_mask

[Yury Norov <yury.norov@gmail.com>]

On Wed, Jul 01, 2026 at 07:46:34PM +0530, Shrikanth Hegde wrote:
> Provide cpu_preferred_mask infrastructure. Define get/set macros
> which could be used to get/set CPU state as preferred.
> 
> Values are set/clear by the new driver called steal_monitor.
> It periodically samples the steal time and decides preferred CPU state.
> 
> A CPU is set to preferred when it becomes active. Later it may be
> marked as non-preferred depending on steal time values with
> steal_monitor being enabled.
> 
> Always maintain design construct of preferred is subset of active.
> i.e. preferred ⊆ active ⊆ online ⊆ present ⊆ possible
> 
> With PREFERRED_CPU=n, set is nop and get returns active state.
> 
> Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
> ---
> v5->v6:
> - Make it nop for PREFERRED_CPU=n
> - Make it EXPORT_SYMBOL_GPL
> 
>  include/linux/cpumask.h | 27 ++++++++++++++++++++++++++-
>  kernel/cpu.c            |  6 ++++++
>  kernel/sched/core.c     |  5 +++++
>  3 files changed, 37 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
> index d3cda0544954..c97271c063ce 100644
> --- a/include/linux/cpumask.h
> +++ b/include/linux/cpumask.h
> @@ -122,12 +122,20 @@ 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
> +
>  #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;
> @@ -1164,6 +1172,13 @@ void init_cpu_possible(const struct cpumask *src);
>  #define set_cpu_active(cpu, active)	assign_cpu((cpu), &__cpu_active_mask, (active))
>  #define set_cpu_dying(cpu, dying)	assign_cpu((cpu), &__cpu_dying_mask, (dying))
>  
> +#ifdef CONFIG_PREFERRED_CPU
> +#define set_cpu_preferred(cpu, preferred) assign_cpu((cpu), &__cpu_preferred_mask, (preferred))
> +#else
> +/* Don't edit active state when the feature is off */

And that makes a random reader thinking like why in the world he mentions
active state here?

Can you move this in commit message, or drop entirely?

> +#define set_cpu_preferred(cpu, preferred) {}
> +#endif
> +
>  void set_cpu_online(unsigned int cpu, bool online);
>  void set_cpu_possible(unsigned int cpu, bool possible);
>  
> @@ -1258,7 +1273,12 @@ static __always_inline bool cpu_dying(unsigned int cpu)
>  	return cpumask_test_cpu(cpu, cpu_dying_mask);
>  }
>  
> -#else
> +static __always_inline bool cpu_preferred(unsigned int cpu)
> +{
> +	return cpumask_test_cpu(cpu, cpu_preferred_mask);
> +}
> +
> +#else	/* NR_CPUS <= 1 */

NR_CPUS can't be less than 1, I guess.

>  
>  #define num_online_cpus()	1U
>  #define num_possible_cpus()	1U
> @@ -1296,6 +1316,11 @@ static __always_inline bool cpu_dying(unsigned int cpu)
>  	return false;
>  }
>  
> +static __always_inline bool cpu_preferred(unsigned int cpu)
> +{
> +	return cpu == 0;
> +}
> +
>  #endif /* NR_CPUS > 1 */
>  
>  #define cpu_is_offline(cpu)	unlikely(!cpu_online(cpu))
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index b3c8553d7bd6..376d297a6292 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -3103,6 +3103,11 @@ 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_GPL(__cpu_preferred_mask);
> +#endif
> +
>  void init_cpu_present(const struct cpumask *src)
>  {
>  	cpumask_copy(&__cpu_present_mask, src);
> @@ -3160,6 +3165,7 @@ 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);
>  	set_cpu_present(cpu, true);
>  	set_cpu_possible(cpu, true);
>  
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 2e7cde033a31..a45f7c308329 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -8690,6 +8690,9 @@ 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;
>  }
>  
> @@ -8703,6 +8706,8 @@ 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
> -- 
> 2.47.3