Re: [PATCH 1/4] sched/fair: Prefer fully-idle SMT cores in asym-capacity idle selection

[Andrea Righi <arighi@nvidia.com>]

Hi Prateek,

On Mon, Mar 30, 2026 at 03:47:07PM +0530, K Prateek Nayak wrote:
> Hello Andrea,
> 
> On 3/27/2026 10:09 PM, Andrea Righi wrote:
> >> My naive eyes say it should be equivalent of what you have but maybe
> >> I'm wrong?
> > 
> > It seems correct to my naive eyes as well. Will test this out to make sure.
> 
> So I found one small problem with fits > 0 && !preferred_core where even
> though it is an ideal target, we don't end up preferring it because of
> the larger "fits" value.
> 
> Here is an updated diff:
> 
>   (Only build tested)

I'm getting worse performance with this one (but better than mainline).
I'm trying to understand why.

BTW, we also need to fix asym_fits_cpu() to do something like this:

	return (!sched_smt_active() || is_core_idle(cpu)) &&
	       (util_fits_cpu(util, util_min, util_max, cpu) > 0);

...or we'd return early from select_idle_sibling() with busy SMT cores.

Thanks,
-Andrea

> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 226509231e67..580218656865 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7949,6 +7949,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
>  static int
>  select_idle_capacity(struct task_struct *p, struct sched_domain *sd, int target)
>  {
> +	bool prefers_idle_core = sched_smt_active() && test_idle_cores(target);
>  	unsigned long task_util, util_min, util_max, best_cap = 0;
>  	int fits, best_fits = 0;
>  	int cpu, best_cpu = -1;
> @@ -7962,6 +7963,7 @@ select_idle_capacity(struct task_struct *p, struct sched_domain *sd, int target)
>  	util_max = uclamp_eff_value(p, UCLAMP_MAX);
>  
>  	for_each_cpu_wrap(cpu, cpus, target) {
> +		bool preferred_core = !prefers_idle_core || is_core_idle(cpu);
>  		unsigned long cpu_cap = capacity_of(cpu);
>  
>  		if (!choose_idle_cpu(cpu, p))
> @@ -7970,7 +7972,7 @@ select_idle_capacity(struct task_struct *p, struct sched_domain *sd, int target)
>  		fits = util_fits_cpu(task_util, util_min, util_max, cpu);
>  
>  		/* This CPU fits with all requirements */
> -		if (fits > 0)
> +		if (fits > 0 && preferred_core)
>  			return cpu;
>  		/*
>  		 * Only the min performance hint (i.e. uclamp_min) doesn't fit.
> @@ -7978,9 +7980,30 @@ select_idle_capacity(struct task_struct *p, struct sched_domain *sd, int target)
>  		 */
>  		else if (fits < 0)
>  			cpu_cap = get_actual_cpu_capacity(cpu);
> +		/*
> +		 * fits > 0 implies we are not on a preferred core
> +		 * but the util fits CPU capacity. Set fits to -2 so
> +		 * the effective range becomes [-2, 0] where:
> +		 *    0 - does not fit
> +		 *   -1 - fits with the exception of UCLAMP_MIN
> +		 *   -2 - fits with the exception of preferred_core
> +		 */
> +		else if (fits > 0)
> +			fits = -2;
> +
> +		/*
> +		 * If we are on an preferred core, translate the range of fits
> +		 * of [-1, 0] to [-4, -3]. This ensures that an idle core
> +		 * is always given priority over (partially) busy core.
> +		 *
> +		 * A fully fitting idle core would have returned early and hence
> +		 * fits > 0 for preferred_core need not be dealt with.
> +		 */
> +		if (preferred_core)
> +			fits -= 3;
>  
>  		/*
> -		 * First, select CPU which fits better (-1 being better than 0).
> +		 * First, select CPU which fits better (lower is more preferred).
>  		 * Then, select the one with best capacity at same level.
>  		 */
>  		if ((fits < best_fits) ||
> ---
> 
> Sorry for the oversight but this should now be equivalent to your
> Patch 1. I'll let Vincent comment if he prefers this to the original
> or not :-)
> 
> -- 
> Thanks and Regards,
> Prateek
>