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

[Shrikanth Hegde]

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@xxxxxxxxxxxxx>
> > ---
> > 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