Re: [PATCH 1/2] sched/fair: Prefer fully-idle SMT cores in asym-capacity idle selection

[Andrea Righi]

On Tue, Apr 21, 2026 at 04:52:46PM +0530, K Prateek Nayak wrote:
> Hello Andrea,
> 
> On 4/21/2026 3:05 PM, Andrea Righi wrote:
> > Actually... while preparing the series I realized that in select_idle_capacity()
> > we may end up clearing the has_idle_cores hint even when the failure is due to
> > affinity constraints (no fit CPU in the allowed cpumask), not only when no fully
> > idle core is found in the system and this can lead to false has_idle_cores
> > hints.
> 
> This is also the case with select_idle_cpu() but any core turning
> idle will again reset the indicator so it should be fine for most
> part where there is a lot of blocking + wakeup.

You're right, select_idle_cpu() is also iterating on the allowed CPUs. So,
nevermind, we definitely want to use the has_idle_cores hint.

> 
> > 
> > At this point I'm wondering if it's better to just ignore the has_idle_cores
> > hint completely in the smt+asym-cpu-capacity scenario (which would also simplify
> > the exotic topology cases).
> > 
> > I did some quick tests with this on Vera and I'm getting pretty much the same
> > performance results. Opinions? Am I missing something?
> 
> I don't think so. Generally it is counterproductive to search a lot
> in a busy system but I guess just making the path SMT aware give a
> much better result compared to the baseline that it doesn't matter.
> 
> Can I trouble you to test the SIS_UTIL bailout with your series +
> the topology changes:
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 78f2d2c4e24f..1356bbdbccd4 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7990,6 +7990,7 @@ select_idle_capacity(struct task_struct *p, struct sched_domain *sd, int target)
>  	int fits, best_fits = 0;
>  	int cpu, best_cpu = -1;
>  	struct cpumask *cpus;
> +	int nr = INT_MAX;
>  
>  	cpus = this_cpu_cpumask_var_ptr(select_rq_mask);
>  	cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
> @@ -7998,10 +7999,30 @@ select_idle_capacity(struct task_struct *p, struct sched_domain *sd, int target)
>  	util_min = uclamp_eff_value(p, UCLAMP_MIN);
>  	util_max = uclamp_eff_value(p, UCLAMP_MAX);
>  
> +	if (sched_feat(SIS_UTIL) && sd->shared) {
> +		/*
> +		 * Increment because !--nr is the condition to stop scan.
> +		 *
> +		 * Since "sd" is "sd_llc" for target CPU dereferenced in the
> +		 * caller, it is safe to directly dereference "sd->shared".
> +		 * Topology bits always ensure it assigned for "sd_llc" abd it
> +		 * cannot disappear as long as we have a RCU protected
> +		 * reference to one the associated "sd" here.
> +		 */
> +		nr = READ_ONCE(sd->shared->nr_idle_scan) + 1;
> +		/* overloaded LLC is unlikely to have idle cpu/core */
> +		if (nr == 1)
> +			return -1;
> +	}
> +
>  	for_each_cpu_wrap(cpu, cpus, target) {
>  		bool preferred_core = !prefers_idle_core || is_core_idle(cpu);
>  		unsigned long cpu_cap = capacity_of(cpu);
>  
> +		/* We have found a good enough target. Just use it. */
> +		if (--nr <= 0 && best_fits == -4)
> +			return best_cpu;
> +
>  		if (!choose_idle_cpu(cpu, p))
>  			continue;
>  
> ---
> 
> You can also try "best_fits <= -3" in that last bailout condition and
> see if that help.

Sure, will test in a bit!

Thanks,
-Andrea