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

[Shrikanth Hegde <sshegde@linux.ibm.com>]

Hi Yury,

On 7/13/26 8:23 PM, Yury Norov wrote:
> 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.
> 

Sorry, i may not understand all the intricacies of kconfigs.
But, Since driver selects PREFERRED_CPU, and PREFERRED_CPU can't be enabled
individually, driver again can't depend on PREFERRED_CPU right?

As per previous discussion, it is probably better that driver selects PREFERRED_CPU.
Keeping them both independent and selectable brings too many variations.
No?


I guess you meant below.

In kernel/Kconfig.preempt:
config PREFERRED_CPU
	bool
	depends on SMP && PARAVIRT

Driver's Kconfig (this is there already)
config VIRT_STEAL_GOVERNOR
	tristate "Virtual Steal Time Governor"
	depends on SMP && PARAVIRT
	select PREFERRED_CPU


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

Make sense. Will do.

> Thanks,
> Yury