[PATCH 4/5] 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 and allows an early return once the
scan limit is reached, if we already picked a sufficiently strong
idle-core candidate (best_fits == -4).

Signed-off-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
---
 kernel/sched/fair.c | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 9bd9dc6e0882e..6b67049f04c3e 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8002,6 +8002,7 @@ select_idle_capacity(struct task_struct *p, struct sched_domain *sd, int target)
 	int fits, best_fits = 0;
 	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);
@@ -8010,10 +8011,30 @@ 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) {
+		/*
+		 * Increment because !--nr is the condition to stop scan.
+		 *
+		 * Since "sd" is "sd_llc" for target CPU dereferenced in the
+		 * caller, it is safe to directly dereference "sd->shared".
+		 * Topology bits always ensure it assigned for "sd_llc" and it
+		 * cannot disappear as long as we have a RCU protected
+		 * reference to one the associated "sd" here.
+		 */
+		nr = READ_ONCE(sd->shared->nr_idle_scan) + 1;
+		/* overloaded LLC is unlikely to have idle cpu/core */
+		if (nr == 1)
+			return -1;
+	}
+
 	for_each_cpu_wrap(cpu, cpus, target) {
 		bool preferred_core = !prefers_idle_core || is_core_idle(cpu);
 		unsigned long cpu_cap = capacity_of(cpu);
 
+		/* We have found a good enough target. Just use it. */
+		if (--nr <= 0 && best_fits == -4)
+			return best_cpu;
+
 		if (!choose_idle_cpu(cpu, p))
 			continue;
 
-- 
2.54.0