cpuset: remove cpuset->parent

cgroup already tracks the hierarchy.  Follow cgroup->parent to find
the parent and drop cpuset->parent.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Michal Hocko <mhocko@suse.cz>
Acked-by: Li Zefan <lizefan@huawei.com>

kernel/cpuset.c

@@ -87,8 +87,6 @@ struct cpuset {
 	cpumask_var_t cpus_allowed;	/* CPUs allowed to tasks in cpuset */
 	nodemask_t mems_allowed;	/* Memory Nodes allowed to tasks */
 
-	struct cpuset *parent;		/* my parent */
-
 	struct fmeter fmeter;		/* memory_pressure filter */
 
 	/*
@@ -120,6 +118,15 @@ static inline struct cpuset *task_cs(struct task_struct *task)
 			    struct cpuset, css);
 }
 
+static inline struct cpuset *parent_cs(const struct cpuset *cs)
+{
+	struct cgroup *pcgrp = cs->css.cgroup->parent;
+
+	if (pcgrp)
+		return cgroup_cs(pcgrp);
+	return NULL;
+}
+
 #ifdef CONFIG_NUMA
 static inline bool task_has_mempolicy(struct task_struct *task)
 {
@@ -323,7 +330,7 @@ static void guarantee_online_cpus(const struct cpuset *cs,
 				  struct cpumask *pmask)
 {
 	while (cs && !cpumask_intersects(cs->cpus_allowed, cpu_online_mask))
-		cs = cs->parent;
+		cs = parent_cs(cs);
 	if (cs)
 		cpumask_and(pmask, cs->cpus_allowed, cpu_online_mask);
 	else
@@ -348,7 +355,7 @@ static void guarantee_online_mems(const struct cpuset *cs, nodemask_t *pmask)
 {
 	while (cs && !nodes_intersects(cs->mems_allowed,
 					node_states[N_MEMORY]))
-		cs = cs->parent;
+		cs = parent_cs(cs);
 	if (cs)
 		nodes_and(*pmask, cs->mems_allowed,
 					node_states[N_MEMORY]);
@@ -461,7 +468,7 @@ static int validate_change(const struct cpuset *cur, const struct cpuset *trial)
 	if (cur == &top_cpuset)
 		goto out;
 
-	par = cur->parent;
+	par = parent_cs(cur);
 
 	/* We must be a subset of our parent cpuset */
 	ret = -EACCES;
@@ -1866,15 +1873,14 @@ static struct cgroup_subsys_state *cpuset_css_alloc(struct cgroup *cont)
 	fmeter_init(&cs->fmeter);
 	INIT_WORK(&cs->hotplug_work, cpuset_propagate_hotplug_workfn);
 	cs->relax_domain_level = -1;
-	cs->parent = cgroup_cs(cont->parent);
 
 	return &cs->css;
 }
 
 static int cpuset_css_online(struct cgroup *cgrp)
 {
 	struct cpuset *cs = cgroup_cs(cgrp);
-	struct cpuset *parent = cs->parent;
+	struct cpuset *parent = parent_cs(cs);
 	struct cpuset *tmp_cs;
 	struct cgroup *pos_cg;
 
@@ -2058,10 +2064,10 @@ static void remove_tasks_in_empty_cpuset(struct cpuset *cs)
 	 * Find its next-highest non-empty parent, (top cpuset
 	 * has online cpus, so can't be empty).
 	 */
-	parent = cs->parent;
+	parent = parent_cs(cs);
 	while (cpumask_empty(parent->cpus_allowed) ||
 			nodes_empty(parent->mems_allowed))
-		parent = parent->parent;
+		parent = parent_cs(parent);
 
 	move_member_tasks_to_cpuset(cs, parent);
 }
@@ -2373,8 +2379,8 @@ int cpuset_nodemask_valid_mems_allowed(nodemask_t *nodemask)
  */
 static const struct cpuset *nearest_hardwall_ancestor(const struct cpuset *cs)
 {
-	while (!(is_mem_exclusive(cs) || is_mem_hardwall(cs)) && cs->parent)
-		cs = cs->parent;
+	while (!(is_mem_exclusive(cs) || is_mem_hardwall(cs)) && parent_cs(cs))
+		cs = parent_cs(cs);
 	return cs;
 }