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>

2 files changed, 5 insertions, 15 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;
 }
 
diff --git a/kernel/sched/cpuacct.c b/kernel/sched/cpuacct.c
index 8ccfa10cc89f..f6926a149a71 100644
--- a/kernel/sched/cpuacct.c
+++ b/kernel/sched/cpuacct.c
@@ -50,16 +50,9 @@ static inline struct cpuacct *task_ca(struct task_struct *tsk)
         return css_ca(task_css(tsk, cpuacct_subsys_id));
 }
 
-static inline struct cpuacct *__parent_ca(struct cpuacct *ca)
-{
-        return cgroup_ca(ca->css.cgroup->parent);
-}
-
 static inline struct cpuacct *parent_ca(struct cpuacct *ca)
 {
-        if (!ca->css.cgroup->parent)
-                return NULL;
-        return cgroup_ca(ca->css.cgroup->parent);
+        return css_ca(css_parent(&ca->css));
 }
 
 static DEFINE_PER_CPU(u64, root_cpuacct_cpuusage);
@@ -284,7 +277,7 @@ void cpuacct_account_field(struct task_struct *p, int index, u64 val)
         while (ca != &root_cpuacct) {
                 kcpustat = this_cpu_ptr(ca->cpustat);
                 kcpustat->cpustat[index] += val;
-                ca = __parent_ca(ca);
+                ca = parent_ca(ca);
         }
         rcu_read_unlock();
 }