Re: [PATCH v2 05/17] sched/core: allow only preferred CPUs in is_cpu_allowed

[Yury Norov <ynorov@nvidia.com>]

On Wed, Apr 08, 2026 at 12:49:38AM +0530, Shrikanth Hegde wrote:
> 
> When possible, choose a preferred CPUs to pick.
> 
> Push task mechanism uses stopper thread which going to call
> select_fallback_rq and use this mechanism to pick only a preferred CPU.
> 
> When task is affined only to non-preferred CPUs it should continue to
> run there. Detect that by checking if cpus_ptr and cpu_preferred_mask
> interesect or not.
> 
> Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
> ---
>  kernel/sched/core.c  | 17 ++++++++++++++---
>  kernel/sched/sched.h | 12 ++++++++++++
>  2 files changed, 26 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 7ea05a7a717b..336e7c694eb7 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2463,9 +2463,16 @@ static inline bool is_cpu_allowed(struct task_struct *p, int cpu)
>  	if (is_migration_disabled(p))
>  		return cpu_online(cpu);
>  
> -	/* Non kernel threads are not allowed during either online or offline. */
> -	if (!(p->flags & PF_KTHREAD))
> -		return cpu_active(cpu);
> +	/*
> +	 * Non kernel threads are not allowed during either online or offline.
> +	 * Ensure it is a preferred CPU to avoid further contention
> +	 */
> +	if (!(p->flags & PF_KTHREAD)) {
> +		if (!cpu_active(cpu))
> +			return false;
> +		if (!cpu_preferred(cpu) && task_can_run_on_preferred_cpu(p))
> +			return false;
> +	}
>  
>  	/* KTHREAD_IS_PER_CPU is always allowed. */
>  	if (kthread_is_per_cpu(p))
> @@ -2475,6 +2482,10 @@ static inline bool is_cpu_allowed(struct task_struct *p, int cpu)
>  	if (cpu_dying(cpu))
>  		return false;
>  
> +	/* Try on preferred CPU first */
> +	if (!cpu_preferred(cpu) && task_can_run_on_preferred_cpu(p))
> +		return false;

You repeat this for the 2nd time. The cpu_preferred() call should go
inside task_can_run_on_preferred_cpu().

And can you please pick some shorter name?

> +
>  	/* But are allowed during online. */
>  	return cpu_online(cpu);
>  }
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 88e0c93b9e21..7271af2ca64f 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -4130,4 +4130,16 @@ DEFINE_CLASS_IS_UNCONDITIONAL(sched_change)
>  
>  #include "ext.h"
>  
> +#ifdef CONFIG_PARAVIRT
> +static inline bool task_can_run_on_preferred_cpu(struct task_struct *p)
> +{
> +	return cpumask_intersects(p->cpus_ptr, cpu_preferred_mask);

This makes is_cpu_allowed() O(N). Even if CONFIG_PARAVIRT is enabled,
I think some people would prefer to avoid this. Also, select_fallback_rq()
calls it in a loop, and this makes it O(N^2).


       /* Any allowed, online CPU? */
       for_each_cpu(dest_cpu, p->cpus_ptr) {
               if (!is_cpu_allowed(p, dest_cpu))
                       continue;

               goto out;
       }

You can keep it O(N):
       for_each_cpu_and(dest_cpu, p->cpus_ptr, cpu_preferred_mask) {
                ...
       }

Not sure how critical that path is, but this looks suspicious.

> +}
> +#else
> +static inline bool task_can_run_on_preferred_cpu(struct task_struct *p)
> +{
> +	return true;
> +}
> +#endif

Same comment as in patch 3. I believe, it's worth to declare cpu_preferred_mask
unrelated to CONFIG_PARAVIRT, so that you'll not have to spread this
ifdefery around. 

> +
>  #endif /* _KERNEL_SCHED_SCHED_H */
> -- 
> 2.47.3