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 4:28 PM, Andrea Righi wrote:
> On Fri, Mar 27, 2026 at 04:14:57PM +0530, K Prateek Nayak wrote:
>> Hello Andrea,
>>
>> On 3/26/2026 8:32 PM, Andrea Righi wrote:
>>>             /* This CPU fits with all requirements */
>>> -           if (fits > 0)
>>> -                   return cpu;
>>> +           if (fits > 0) {
>>> +                   if (prefer_idle_cores && on_idle_core)
>>> +                           return cpu;
>>> +                   if (!prefer_idle_cores)
>>> +                           return cpu;
>>
>> nit.
>>
>> Can the above two be re-wittern as:
>>
>>     if (!prefer_idle_cores || on_idle_core)
>>         return cpu;
>>
>> since they are equivalent.
> 
> Oh yes, indeed.

Also, can we just rewrite this Patch as:

  (Includes feedback from Vincent; Only build tested)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 700d0f145ca6..cffd5649b54e 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7946,6 +7946,7 @@ 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;
@@ -7959,6 +7960,7 @@ 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))
@@ -7967,7 +7969,7 @@ 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.
@@ -7976,6 +7978,14 @@ select_idle_capacity(struct task_struct *p, struct sched_domain *sd, int target)
 		else if (fits < 0)
 			cpu_cap = get_actual_cpu_capacity(cpu);
 
+		/*
+		 * If we are on an preferred core, translate the range of fits
+		 * from [-1, 1] to [-4, -2]. This ensures that an idle core
+		 * is always given priority over (paritally) busy core.
+		 */
+		if (preferred_core)
+			fits -= 3;
+
 		/*
 		 * First, select CPU which fits better (-1 being better than 0).
 		 * Then, select the one with best capacity at same level.
---

My naive eyes say it should be equivalent of what you have but maybe
I'm wrong?

-- 
Thanks and Regards,
Prateek