Re: [PATCH v3 06/20] sched/core: allow only preferred CPUs in is_cpu_allowed

[K Prateek Nayak]

Hello Shrikanth,

On 5/14/2026 8:51 PM, Shrikanth Hegde wrote:
> +	/*
> +	 * This is essential to maintain user affinities when preferred
> +	 * CPUs change. A task pinned on non-preferred CPU should continue
> +	 * to run there, since this is non-user triggered.
> +	 *
> +	 * For majority of the cases this would still keep select_fallback_rq
> +	 * as O(N). task_has_preferred_cpus which is O(N) is called only if
> +	 * !cpu_preferred. Then task running there is expected to move out.
> +	 * So subsequent it should run on preferred CPU. This becomes O(N**2)
> +	 * only for tasks pinned only non preferred CPUs. That is rare case.
> +	 */
> +	task_has_preferred_cpu = !cpu_preferred(cpu) && task_has_preferred_cpus(p);

For this O(N**2) bit, can't we cache the result of cpumask_intersect()
in the task_struct similar to "p->sched_task_hot" when entering
select_fallback_rq() and clear it at the end?

> +
>  	/* Non kernel threads are not allowed during either online or offline. */
> -	if (!(p->flags & PF_KTHREAD))
> -		return cpu_active(cpu);
> +	if (!(p->flags & PF_KTHREAD)) {
> +		if (!cpu_active(cpu))
> +			return false;
> +		if (task_has_preferred_cpu)
> +			return false;

Shouldn't this return true at the end here for
cpu_active() && !task_has_preferred_cpu?

Essentially, this:

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 77a172f31862..3bf7e3c4d518 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2489,6 +2489,7 @@ static inline bool is_cpu_allowed(struct task_struct *p, int cpu)
 			return false;
 		if (task_has_preferred_cpu)
 			return false;
+		return true;
 	}
 
 	/* KTHREAD_IS_PER_CPU is always allowed. */
---

Otherwise, it starts evaluating kthread bits for user tasks.

> +	}
>  
>  	/* KTHREAD_IS_PER_CPU is always allowed. */
>  	if (kthread_is_per_cpu(p))

-- 
Thanks and Regards,
Prateek