sched/fair: Add SIS_UTIL support to select_idle_capacity()

BugLink: https://bugs.launchpad.net/bugs/2150671

Add to select_idle_capacity() the same SIS_UTIL-controlled idle-scan
mechanism, already used by select_idle_cpu(): when sched_feat(SIS_UTIL)
is enabled and the LLC domain has sched_domain_shared data, derive the
per-attempt scan limit from sd->shared->nr_idle_scan.

That bounds the walk on large LLCs: once nr_idle_scan is exhausted,
return the best CPU seen so far. The early exit is gated on
!has_idle_core so an active idle-core search (SMT with idle cores
reported by test_idle_cores()) isn't cut short before it gets a chance
to find one.

Co-developed-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: K Prateek Nayak <kprateek.nayak@amd.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://patch.msgid.link/20260509180955.1840064-6-arighi@nvidia.com
(backported from commit 61ea17a linux-next)
[ arighi: choose_idle_cpu() not available in v6.18 ]
Signed-off-by: Andrea Righi <arighi@nvidia.com>
Acked-by: Seth Forshee <sforshee@nvidia.com>
Acked-by: Nirmoy Das <nirmoyd@nvidia.com>
Acked-by: Matthew R. Ochs <mochs@nvidia.com>
Signed-off-by: Seth Forshee <sforshee@nvidia.com>

Author: kudureranganath

kernel/sched/fair.c

@@ -7871,6 +7871,7 @@ select_idle_capacity(struct task_struct *p, struct sched_domain *sd, int target)
 	int fits, best_fits = ASYM_IDLE_THREAD_MISFIT;
 	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);
@@ -7879,10 +7880,28 @@ 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) {
+		/*
+		 * Same nr_idle_scan hint as select_idle_cpu(), nr only limits
+		 * the scan when not preferring an idle core.
+		 */
+		nr = READ_ONCE(sd->shared->nr_idle_scan) + 1;
+		/* overloaded domain is unlikely to have idle cpu/core */
+		if (nr == 1)
+			return -1;
+	}
+
 	for_each_cpu_wrap(cpu, cpus, target) {
 		bool preferred_core = !has_idle_core || is_core_idle(cpu);
 		unsigned long cpu_cap = capacity_of(cpu);
 
+		/*
+		 * Stop when the nr_idle_scan is exhausted (mirrors
+		 * select_idle_cpu() logic).
+		 */
+		if (!has_idle_core && --nr <= 0)
+			return best_cpu;
+
 		if (!available_idle_cpu(cpu) && !sched_idle_cpu(cpu))
 			continue;