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

[Shrikanth Hegde <sshegde@linux.ibm.com>]

On 7/1/26 10:19 PM, Shrikanth Hegde wrote:
> Hi Yury,
> 

[...]

>>
>> 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.
>>
> 
> Ok. I will update the comments and share updated one soon
> as reply to this.
> 

Made it as below. This looks much cleaner and retains the same
functionality. Reduces cache window for kthread too. No double
cost due to earlier returns.

Does this look sane?


/*
  * Per-CPU kthreads are allowed to run on !active && online CPUs, see
  * __set_cpus_allowed_ptr() and select_fallback_rq().
  */
static inline bool is_cpu_allowed(struct task_struct *p, int cpu)
{
	/* When not in the task's cpumask, no point in looking further. */
	if (!task_allowed_on_cpu(p, cpu))
		return false;

	/* migrate_disabled() must be allowed to finish. */
	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)) {
		/* Try to use preferred CPU if task's affinity allows */
		if (task_can_sched_on_preferred(cpu, p))
			return false;
		return cpu_active(cpu);
	}

	/* KTHREAD_IS_PER_CPU is always allowed. */
	if (kthread_is_per_cpu(p))
		return cpu_online(cpu);

	/* Regular kernel threads don't get to stay during offline. */
	if (cpu_dying(cpu))
		return false;

	/* Try to keep unbound kthreads on a preferred CPU if possible. */
	if (task_can_sched_on_preferred(cpu, p))
		return false;

	/* Otherwise, they are allowed to run on online CPU. */
	return cpu_online(cpu);
}

and

static inline bool task_can_sched_on_preferred(int cpu, struct task_struct *p)
{
	if (cpu_preferred(cpu))
		return false;

	/* 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);
}