cpuset: don't pass empty cpumasks to partition_sched_domains()

I create lots of empty cpusets(empty cpumasks) and turn off the
"sched_load_balance" in top cpuset.

I found that all these empty cpumasks are passed to
partition_sched_domains() in rebuild_sched_domains(), it's very
time-consuming for partition_sched_domains() and it's not need.

It also reduce memory consumed and some works in rebuild_sched_domains()
too.
Signed-off-by: NLai Jiangshan <laijs@cn.fujitsu.com>
Acked-by: NPaul Menage <menage@google.com>
Cc: Paul Jackson <pj@sgi.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

kernel/cpuset.c

@@ -610,8 +610,13 @@ void rebuild_sched_domains(void)
 	while (__kfifo_get(q, (void *)&cp, sizeof(cp))) {
 		struct cgroup *cont;
 		struct cpuset *child;   /* scans child cpusets of cp */
+
+		if (cpus_empty(cp->cpus_allowed))
+			continue;
+
 		if (is_sched_load_balance(cp))
 			csa[csn++] = cp;
+
 		list_for_each_entry(cont, &cp->css.cgroup->children, sibling) {
 			child = cgroup_cs(cont);
 			__kfifo_put(q, (void *)&child, sizeof(cp));