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

Andrea Righi
2026-04-28T14:44:49Z

From: K Prateek Nayak <kprateek.nayak@amd.com>

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 == ASYM_IDLE_CORE_UCLAMP_MISFIT).

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>
---
kernel/sched/fair.c | 19 +++++++++++++++++++
1 file changed, 19 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index a1f4d70f6b3d9..1cde3a9b1e0f5 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8018,6 +8018,7 @@ 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);
@@ -8026,10 +8027,28 @@ 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 = !prefers_idle_core || is_core_idle(cpu);
unsigned long cpu_cap = capacity_of(cpu);

+ /*
+ * Good-enough early exit (mirrors select_idle_cpu() logic).
+ */
+ if (!prefers_idle_core &&
+ --nr <= 0 && best_fits == ASYM_IDLE_CORE_UCLAMP_MISFIT)
+ return best_cpu;
+
if (!choose_idle_cpu(cpu, p))
continue;

--
2.54.0