[PATCH v2] sched: update_top_cache_domain only at the times of building sched domain.

[Rakib Mullick <rakib.mullick@gmail.com>]

Currently, update_top_cache_domain() is called whenever schedule domain is built or destroyed. But, the following
callpath shows that they're at the same callpath and can be avoided update_top_cache_domain() while destroying schedule
domain and update only at the times of building schedule domains.

	partition_sched_domains()
		detach_destroy_domain()
			cpu_attach_domain()
				update_top_cache_domain()
		build_sched_domains()
			cpu_attach_domain()
				update_top_cache_domain()

Changes since v1: use sd to determine when to skip, courtesy PeterZ

Signed-off-by: Rakib Mullick <rakib.mullick@gmail.com>
---

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index b7c32cb..387fb66 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5138,7 +5138,8 @@ cpu_attach_domain(struct sched_domain *sd, struct root_domain *rd, int cpu)
 	rcu_assign_pointer(rq->sd, sd);
 	destroy_sched_domains(tmp, cpu);
 
-	update_top_cache_domain(cpu);
+	if (sd)
+		update_top_cache_domain(cpu);
 }
 
 /* cpus with isolated domains */