From: K Prateek Nayak <kprateek.nayak@amd.com>
Date: Thu, 12 Mar 2026 04:44:34 +0000 (+0000)
Subject: sched/fair: Simplify SIS_UTIL handling in select_idle_cpu()
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=fe7171d0d5dfbe189e41db99580ebacafc3c09ce;p=thirdparty%2Fkernel%2Flinux.git

sched/fair: Simplify SIS_UTIL handling in select_idle_cpu()

Use the "sd_llc" passed to select_idle_cpu() to obtain the
"sd_llc_shared" instead of dereferencing the per-CPU variable.

Since "sd->shared" is always reclaimed at the same time as "sd" via
call_rcu() and update_top_cache_domain() always ensures a valid
"sd->shared" assignment when "sd_llc" is present, "sd_llc->shared" can
always be dereferenced without needing an additional check.

While at it move the cpumask_and() operation after the SIS_UTIL bailout
check to avoid unnecessarily computing the cpumask.

Signed-off-by: K Prateek Nayak <kprateek.nayak@amd.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Chen Yu <yu.c.chen@intel.com>
Reviewed-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Tested-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Link: https://patch.msgid.link/20260312044434.1974-10-kprateek.nayak@amd.com
---

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 85c22f0f8de80..0a35a82e47920 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7876,21 +7876,26 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
 {
 	struct cpumask *cpus = this_cpu_cpumask_var_ptr(select_rq_mask);
 	int i, cpu, idle_cpu = -1, nr = INT_MAX;
-	struct sched_domain_shared *sd_share;
-
-	cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
 
 	if (sched_feat(SIS_UTIL)) {
-		sd_share = rcu_dereference_all(per_cpu(sd_llc_shared, target));
-		if (sd_share) {
-			/* because !--nr is the condition to stop scan */
-			nr = READ_ONCE(sd_share->nr_idle_scan) + 1;
-			/* overloaded LLC is unlikely to have idle cpu/core */
-			if (nr == 1)
-				return -1;
-		}
+		/*
+		 * 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" abd 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;
 	}
 
+	if (!cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr))
+		return -1;
+
 	if (static_branch_unlikely(&sched_cluster_active)) {
 		struct sched_group *sg = sd->groups;