cgroup: add css_parent()

Currently, controllers have to explicitly follow the cgroup hierarchy
to find the parent of a given css.  cgroup is moving towards using
cgroup_subsys_state as the main controller interface construct, so
let's provide a way to climb the hierarchy using just csses.

This patch implements css_parent() which, given a css, returns its
parent.  The function is guarnateed to valid non-NULL parent css as
long as the target css is not at the top of the hierarchy.

freezer, cpuset, cpu, cpuacct, hugetlb, memory, net_cls and devices
are converted to use css_parent() instead of accessing cgroup->parent
directly.

* __parent_ca() is dropped from cpuacct and its usage is replaced with
  parent_ca().  The only difference between the two was NULL test on
  cgroup->parent which is now embedded in css_parent() making the
  distinction moot.  Note that eventually a css->parent field will be
  added to css and the NULL check in css_parent() will go away.

This patch shouldn't cause any behavior differences.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

1 files changed, 3 insertions, 6 deletions

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 5bccb0277129..7a10742b389a 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7114,13 +7114,10 @@ static struct cgroup_subsys_state *cpu_cgroup_css_alloc(struct cgroup *cgrp)
 static int cpu_cgroup_css_online(struct cgroup *cgrp)
 {
         struct task_group *tg = cgroup_tg(cgrp);
-        struct task_group *parent;
+        struct task_group *parent = css_tg(css_parent(&tg->css));
 
-        if (!cgrp->parent)
-                return 0;
-
-        parent = cgroup_tg(cgrp->parent);
-        sched_online_group(tg, parent);
+        if (parent)
+                sched_online_group(tg, parent);
         return 0;
 }