Re: [PATCH v7 02/12] cpumask: Introduce cpu_preferred_mask

[Yury Norov <ynorov@nvidia.com>]

On Fri, Jul 10, 2026 at 03:26:38AM +0530, Shrikanth Hegde wrote:
> Provide cpu_preferred_mask infrastructure. Define get/set macros
> which could be used to get/set CPU state as preferred.
> 
> PREFERRED_CPU config will be selected by the driver which handles
> steal time values. It is going to set/clear preferred CPU state.
> This driver will be called steal_monitor and it is introduced in
> subsequent patches. It periodically samples the steal time and
> decides on 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, ensure set_cpu_preferred is a nop and get
> method returns the active state in that case.
> 
> Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
> ---
> v6->v7:
> - removed CONFIG_PREFERRED_CPU as user option.
> - Use do { } while (0) for nop 
> 
>  include/linux/cpumask.h | 24 ++++++++++++++++++++++++
>  kernel/Kconfig.preempt  |  3 +++
>  kernel/cpu.c            |  6 ++++++
>  kernel/sched/core.c     |  5 +++++
>  4 files changed, 38 insertions(+)
> 
> diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
> index d3cda0544954..34d08a3d80e1 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 +1172,12 @@ 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
> +#define set_cpu_preferred(cpu, preferred) do { } while (0)
> +#endif
> +
>  void set_cpu_online(unsigned int cpu, bool online);
>  void set_cpu_possible(unsigned int cpu, bool possible);
>  
> @@ -1258,6 +1272,11 @@ static __always_inline bool cpu_dying(unsigned int cpu)
>  	return cpumask_test_cpu(cpu, cpu_dying_mask);
>  }
>  
> +static __always_inline bool cpu_preferred(unsigned int cpu)
> +{
> +	return cpumask_test_cpu(cpu, cpu_preferred_mask);
> +}
> +
>  #else
>  
>  #define num_online_cpus()	1U
> @@ -1296,6 +1315,11 @@ 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/Kconfig.preempt b/kernel/Kconfig.preempt
> index 88c594c6d7fc..ed02e4431230 100644
> --- a/kernel/Kconfig.preempt
> +++ b/kernel/Kconfig.preempt
> @@ -192,3 +192,6 @@ config SCHED_CLASS_EXT
>  	  For more information:
>  	    Documentation/scheduler/sched-ext.rst
>  	    https://github.com/sched-ext/scx
> +
> +config PREFERRED_CPU
> +	bool

This still should depend on PARAVIRT and SMP. And maybe to enforce it
even stronger, your driver should fail to build if PREFERRED_CPU is
disabled. Imagine a scenario when someone makes PREFERRED_CPU
depending on some other config, but doesn't modify your driver. That
way you'll build the STEAL_MONITOR successfully, but because
PREFERRED_CPU is off, you'll end up with non-working functionality at
best, or corrupted cpu_active_mask at worst.

Also, the name 'steal monitor' implies monitoring, while in fact
you're actively affecting the scheduling process.

Maybe 'steal governor'?

Thanks,
Yury