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

[K Prateek Nayak]

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)

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