[PATCH 5/5] sched/fair: Add SIS_UTIL support to select_idle_capacity()

[Andrea Righi]

From: K Prateek Nayak <kprateek.nayak@xxxxxxx>

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.

Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Cc: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
Co-developed-by: Andrea Righi <arighi@xxxxxxxxxx>
Signed-off-by: Andrea Righi <arighi@xxxxxxxxxx>
Signed-off-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
---
 kernel/sched/fair.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 2ddba8bd27e59..494149f14d98f 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8084,6 +8084,7 @@ select_idle_capacity(struct task_struct *p, struct sched_domain *sd, int target)
 	int fits, best_fits = ASYM_IDLE_COMPLETE_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);
@@ -8092,10 +8093,28 @@ 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 (!choose_idle_cpu(cpu, p))
 			continue;
 
-- 
2.54.0