Re: [PATCH v6 05/23] sched/core: Try to use a preferred CPU in is_cpu_allowed

[Yury Norov <yury.norov@gmail.com>]

On Wed, Jul 01, 2026 at 07:46:36PM +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
> intersect or not.
> 
> This takes care of wakeup path optimization for FAIR tasks.
> is_cpu_allowed is called to ensure wakeup happens on preferred CPUs.
> With that, additional checks in available_idle_cpu is not necessary.
> 
> Add a comment on rare case of O(N**2) in select_fallback_rq.
> 
> Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
> ---
> v5->v6:
> - Drop optimization for select_fallback_rq
> - Keep comment on N**2
> 
>  kernel/sched/core.c  | 29 ++++++++++++++++++++++++++++-
>  kernel/sched/sched.h |  9 +++++++++
>  2 files changed, 37 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index a45f7c308329..1fb1c17e8387 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2500,6 +2500,8 @@ static inline bool rq_has_pinned_tasks(struct rq *rq)
>   */
>  static inline bool is_cpu_allowed(struct task_struct *p, int cpu)
>  {
> +	bool task_has_preferred_cpu;
> +
>  	/* When not in the task's cpumask, no point in looking further. */
>  	if (!task_allowed_on_cpu(p, cpu))
>  		return false;
> @@ -2508,9 +2510,30 @@ static inline bool is_cpu_allowed(struct task_struct *p, int cpu)
>  	if (is_migration_disabled(p))
>  		return cpu_online(cpu);
>  
> +	/*
> +	 * 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.
> +	 *
> +	 * If CPU is non-preferred and task can run on other CPUs which are
> +	 * currently preferred, then choose those other CPUs instead.
> +	 * Overhead is minimal when CPU is preferred.
> +	 *
> +	 * 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.
> +	 */

The is_cpu_allowed() is ~20 lines now, and your patch doubles that count.
Can you keep this type of thoughts in commit message? 90% of setups
will disable preferred CPUs, and I guess 99% of developers don't care.

This is the code, not a scientific paper, after all.

> +	task_has_preferred_cpu = !cpu_preferred(cpu) &&
> +				 task_has_preferred_cpus(p);

Maybe it's just me, but the name looks illogical. Because if
'cpu' is preferred, the task indeed has some preferred CPUs.

Maybe 'can_sched_on_preferred' or something like that?

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

The comment on top of the block seems to be applicable to the 2nd
return only, right?

>  
>  	/* KTHREAD_IS_PER_CPU is always allowed. */
>  	if (kthread_is_per_cpu(p))
> @@ -2520,6 +2543,10 @@ static inline bool is_cpu_allowed(struct task_struct *p, int cpu)
>  	if (cpu_dying(cpu))
>  		return false;
>  
> +	/* Try on preferred CPU first if possible*/
> +	if (task_has_preferred_cpu)
> +		return false;

Would it look better if you drop the comment and:
        
        if (need_sched_on_preferred)
                return false;

> +
>  	/* But are allowed during online. */

This comment is the continuation of the cpu_dying() case. With your
change it's not anymore, and it needs to be reworded.

>  	return cpu_online(cpu);
>  }
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 26ae13c86b69..36ae20310891 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -4230,4 +4230,13 @@ DEFINE_CLASS_IS_UNCONDITIONAL(sched_change)
>  
>  #include "ext/ext.h"
>  
> +static inline bool task_has_preferred_cpus(struct task_struct *p)
> +{
> +	/* Only FAIR tasks honor preferred CPU state */
> +	if (unlikely(p->sched_class != &fair_sched_class))
> +		return false;
> +
> +	return cpumask_intersects(p->cpus_ptr, cpu_preferred_mask);
> +}
> +
>  #endif /* _KERNEL_SCHED_SCHED_H */
> -- 
> 2.47.3