Re: [PATCH v2 03/17] cpumask: Introduce cpu_preferred_mask

[Yury Norov <ynorov@nvidia.com>]

On Wed, Apr 08, 2026 at 12:49:36AM +0530, Shrikanth Hegde wrote:
> This patch does
> - Declare and Define cpu_preferred_mask.
> - Get/Set helpers for it.
> 
> Values are set/clear by the scheduler by detecting the steal time values.
> 
> A CPU is set to preferred when it comes online. Later it may be
> marked as non-preferred depending on steal time values with 
> STEAL_MONITOR enabled.
> 
> Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
> ---
>  include/linux/cpumask.h | 22 ++++++++++++++++++++++
>  kernel/cpu.c            |  6 ++++++
>  kernel/sched/core.c     |  5 +++++
>  3 files changed, 33 insertions(+)
> 
> diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
> index 80211900f373..80c5cc13b8ad 100644
> --- a/include/linux/cpumask.h
> +++ b/include/linux/cpumask.h
> @@ -1296,6 +1296,28 @@ static __always_inline bool cpu_dying(unsigned int cpu)
>  
>  #endif /* NR_CPUS > 1 */
>  
> +/*
> + * All related wrappers kept together to avoid too many ifdefs
> + * See Documentation/scheduler/sched-arch.rst for details
> + */
> +#ifdef CONFIG_PARAVIRT
> +extern struct cpumask __cpu_preferred_mask;
> +#define cpu_preferred_mask    ((const struct cpumask *)&__cpu_preferred_mask)
> +#define set_cpu_preferred(cpu, preferred) assign_cpu((cpu), &__cpu_preferred_mask, (preferred))
> +
> +static __always_inline bool cpu_preferred(unsigned int cpu)
> +{
> +	return cpumask_test_cpu(cpu, cpu_preferred_mask);
> +}
> +#else
> +static __always_inline bool cpu_preferred(unsigned int cpu)
> +{
> +	return true;
> +}

This doesn't look consistent, probably not correct. What if
I pass an offline CPU here? Is it still preferred?

Later you say that preferred CPU is online + STEAL-approved one.
So in non-paravirtualized case, I believe, you should consider
that only online CPUs are preferred. What about dying CPUs? Can
they be preferred too?

At least, please run cpumask_check() on the argument.

There's a top-comment describing all the system cpumasks. Except for
cpu_dying, it's nice and complete. Can you describe your new creature
there?

Finally, I don't think that __cpu_preferred_mask should depend on 
PARAVIRT config. Consider cpu_present_mask. It mirrors cpu_possible_mask
if hotplug is disabled, but it's still a real mask even in that case.
The way you're doing it, you spread CONFIG_PARAVIRT ifdefery pretty
much anywhere where people might want to use this new mask for anything
except for testing a bit.

Thanks,
Yury

> +static __always_inline void set_cpu_preferred(unsigned int cpu, bool preferred) { }
> +#endif
> +
>  #define cpu_is_offline(cpu)	unlikely(!cpu_online(cpu))
>  
>  #if NR_CPUS <= BITS_PER_LONG
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index bc4f7a9ba64e..2d4d037680d4 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -3137,6 +3137,12 @@ void set_cpu_online(unsigned int cpu, bool online)
>  		if (cpumask_test_and_clear_cpu(cpu, &__cpu_online_mask))
>  			atomic_dec(&__num_online_cpus);
>  	}
> +
> +	/*
> +	 * An online CPU is by default assumed to be preferred
> +	 * Unitl STEAL_MONITOR changes it
> +	 */
> +	set_cpu_preferred(cpu, online);
>  }
>  
>  /*
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index f351296922ac..7ea05a7a717b 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -11228,3 +11228,8 @@ void sched_change_end(struct sched_change_ctx *ctx)
>  		p->sched_class->prio_changed(rq, p, ctx->prio);
>  	}
>  }
> +
> +#ifdef CONFIG_PARAVIRT
> +struct cpumask __cpu_preferred_mask __read_mostly;
> +EXPORT_SYMBOL(__cpu_preferred_mask);
> +#endif
> -- 
> 2.47.3