sched/fair: Cleanup for SIS_PROP

The sched-domain of this cpu is only used for some heuristics when
SIS_PROP is enabled, and it should be irrelevant whether the local
sd_llc is valid or not, since all we care about is target sd_llc
if !SIS_PROP.

Access the local domain only when there is a need.

Signed-off-by: Abel Wu <wuyun.abel@bytedance.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Mel Gorman <mgorman@suse.de>
Link: https://lore.kernel.org/r/20220907112000.1854-6-wuyun.abel@bytedance.com

Author: Abel-WY

kernel/sched/fair.c

@@ -6396,19 +6396,19 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
 	struct sched_domain_shared *sd_share;
 	struct rq *this_rq = this_rq();
 	int this = smp_processor_id();
-	struct sched_domain *this_sd;
+	struct sched_domain *this_sd = NULL;
 	u64 time = 0;
 
-	this_sd = rcu_dereference(*this_cpu_ptr(&sd_llc));
-	if (!this_sd)
-		return -1;
-
 	cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
 
 	if (sched_feat(SIS_PROP) && !has_idle_core) {
 		u64 avg_cost, avg_idle, span_avg;
 		unsigned long now = jiffies;
 
+		this_sd = rcu_dereference(*this_cpu_ptr(&sd_llc));
+		if (!this_sd)
+			return -1;
+
 		/*
 		 * If we're busy, the assumption that the last idle period
 		 * predicts the future is flawed; age away the remaining
@@ -6462,7 +6462,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
 	if (has_idle_core)
 		set_idle_cores(target, false);
 
-	if (sched_feat(SIS_PROP) && !has_idle_core) {
+	if (sched_feat(SIS_PROP) && this_sd && !has_idle_core) {
 		time = cpu_clock(this) - time;
 
 		/*