Re: [PATCH v4 10/20] sched/core: Push current task from non preferred CPU

[Shrikanth Hegde]

On 6/18/26 9:39 AM, K Prateek Nayak wrote:
> Hello Shrikanth,
>
> On 6/17/2026 11:11 PM, Shrikanth Hegde wrote:
> > +static int sched_non_preferred_cpu_push_stop(void *arg)
> > +{
> > +	struct task_struct *p = arg;
> > +	struct rq *rq = this_rq();
> > +	struct rq_flags rf;
> > +	int cpu;
> > +
> > +	/* sanity check */
> > +	if (cpu_preferred(rq->cpu))
> > +		return 0;
>
> I think this early return path should also clear "push_task_work_done"
> indicator, otherwise, we will fail to schedule the stopper on this CPU
> next time.

Ok.

> Also, we may need to add a context_unsafe_alias(rq) call here to keep
> the context analysis bits happy similar to migration_cpu_stop().

Let me see that.

> > +
> > +	raw_spin_lock_irq(&p->pi_lock);
> > +	rq_lock(rq, &rf);
> > +	rq->push_task_work_done = 0;
> > +
> > +	update_rq_clock(rq);
> > +
> > +	if (task_rq(p) == rq && task_on_rq_queued(p)) {
> > +		cpu = select_fallback_rq(rq->cpu, p);
>
> Do we need a task_has_preferred_cpus() sanity check here?
>
> If the affinity changed before the stopper grabbed the p->pi_lock, and
> there are no preferred CPUs to run on anymore, might as well keep the
> task here instead of migrating it away.

No I think.

If affinity changed before stopper started running,
subsequent select_fallback_rq will return the CPU it can run.
If it has no choice but a non-preferred CPU, it will return a non-preferred CPU too.

> > +		rq = __migrate_task(rq, &rf, p, cpu);
> > +	}
> > +
> > +	rq_unlock(rq, &rf);
> > +	raw_spin_unlock_irq(&p->pi_lock);
> > +	put_task_struct(p);
> > +
> > +	return 0;
> > +}