Re: [PATCH v2] sched/fair: Prefer waker CPU for reciprocal sync wakeups

[K Prateek Nayak]

Hello Shubhang,

On 7/23/2026 4:20 AM, Shubhang Kaushik (Ampere) wrote:
> +/*
> + * For reciprocal WF_SYNC handoffs, prefer the waker CPU when it has no
> + * other runnable fair task.
> + */
> +static bool prefer_sync_pair_cpu(struct task_struct *p, int cpu)
> +{
> +	struct rq *rq = cpu_rq(cpu);
> +
> +	if ((rq->nr_running - cfs_h_nr_delayed(rq)) != 1)
> +		return false;
> +
> +	if (!cpumask_test_cpu(cpu, p->cpus_ptr))
> +		return false;

want_affine already covers the affinity test now that you've moved it
into the SD_WAKE_AFFINE branch.

> +
> +	if (sched_asym_cpucap_active()) {
> +		sync_entity_load_avg(&p->se);
> +		if (!task_fits_cpu(p, cpu))
> +			return false;
> +	}
> +
> +	return true;

Reads a lot like WA_IDLE. Couldn't this simply be an early returns
in select_idle_sibling() if we passed wake_flags there?

> +}
> +
>  /*
>   * Try and locate an idle core/thread in the LLC cache domain.
>   */
> @@ -9579,6 +9602,11 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
>  		 */
>  		if (want_affine && (tmp->flags & SD_WAKE_AFFINE) &&
>  		    cpumask_test_cpu(prev_cpu, sched_domain_span(tmp))) {
> +			if (sync &&
> +			    READ_ONCE(p->last_wakee) == current &&

No need for READ_ONCE(), Last wakee is recorded when "p" has last
woken up a task and given we are now doing a wakeup for "p", it
is completely off the rq and p->last_wakee is stable.

> +			    prefer_sync_pair_cpu(p, cpu))
> +				return cpu;
> +
>  			if (cpu != prev_cpu)
>  				new_cpu = wake_affine(tmp, p, cpu, prev_cpu, sync);
>  
> 
> ---
> base-commit: 248951ddc14de84de3910f9b13f51491a8cd91df
> change-id: 20260721-b4-sched-sync-wakeup-04d40cbeb1da
> 
> Best regards,

-- 
Thanks and Regards,
Prateek