cgroup, rstat: Don't flush subtree root unless necessary

[ Upstream commit b4ff1b44bcd384d22fcbac6ebaf9cc0d33debe50 ]

cgroup_rstat_cpu_pop_updated() is used to traverse the updated cgroups
on flush.  While it was only visiting updated ones in the subtree, it
was visiting @root unconditionally.  We can easily check whether @root
is updated or not by looking at its ->updated_next just as with the
cgroups in the subtree.

* Remove the unnecessary cgroup_parent() test.  The system root cgroup
  is never updated and thus its ->updated_next is always NULL.  No
  need to test whether cgroup_parent() exists in addition to
  ->updated_next.

* Terminate traverse if ->updated_next is NULL.  This can only happen
  for subtree @root and there's no reason to visit it if it's not
  marked updated.

This reduces cpu consumption when reading a lot of rstat backed files.
In a micro benchmark reading stat from ~1600 cgroups, the sys time was
lowered by >40%.
Signed-off-by: NTejun Heo <tj@kernel.org>
Signed-off-by: NSasha Levin <sashal@kernel.org>

kernel/cgroup/rstat.c

@@ -87,7 +87,6 @@ static struct cgroup *cgroup_rstat_cpu_pop_updated(struct cgroup *pos,
 						   struct cgroup *root, int cpu)
 {
 	struct cgroup_rstat_cpu *rstatc;
-	struct cgroup *parent;
 
 	if (pos == root)
 		return NULL;
@@ -115,8 +114,8 @@ static struct cgroup *cgroup_rstat_cpu_pop_updated(struct cgroup *pos,
 	 * However, due to the way we traverse, @pos will be the first
 	 * child in most cases. The only exception is @root.
 	 */
-	parent = cgroup_parent(pos);
-	if (parent && rstatc->updated_next) {
+	if (rstatc->updated_next) {
+		struct cgroup *parent = cgroup_parent(pos);
 		struct cgroup_rstat_cpu *prstatc = cgroup_rstat_cpu(parent, cpu);
 		struct cgroup_rstat_cpu *nrstatc;
 		struct cgroup **nextp;
@@ -140,9 +139,12 @@ static struct cgroup *cgroup_rstat_cpu_pop_updated(struct cgroup *pos,
 		 * updated stat.
 		 */
 		smp_mb();
+
+		return pos;
 	}
 
-	return pos;
+	/* only happens for @root */
+	return NULL;
 }
 
 /* see cgroup_rstat_flush() */