cgroup: enable refcnting for root csses

Currently, css_get(), css_tryget() and css_tryget_online() are noops
for root csses as an optimization; however, we're planning to use css
refcnts to track of cgroup lifetime too and root cgroups also need to
be reference counted.  Since css has been converted to percpu_refcnt,
the overhead of refcnting is miniscule and this optimization isn't too
meaningful anymore.  Furthermore, controllers which optimize the root
cgroup often never even invoke these functions in their hot paths.

This patch enables refcnting for root csses too.  This makes CSS_ROOT
flag unused and removes it.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

include/linux/cgroup.h

@@ -77,7 +77,6 @@ struct cgroup_subsys_state {
 
 /* bits in struct cgroup_subsys_state flags field */
 enum {
-	CSS_ROOT	= (1 << 0), /* this CSS is the root of the subsystem */
 	CSS_ONLINE	= (1 << 1), /* between ->css_online() and ->css_offline() */
 };
 
@@ -89,9 +88,7 @@ enum {
  */
 static inline void css_get(struct cgroup_subsys_state *css)
 {
-	/* We don't need to reference count the root state */
-	if (!(css->flags & CSS_ROOT))
-		percpu_ref_get(&css->refcnt);
+	percpu_ref_get(&css->refcnt);
 }
 
 /**
@@ -106,8 +103,6 @@ static inline void css_get(struct cgroup_subsys_state *css)
  */
 static inline bool css_tryget_online(struct cgroup_subsys_state *css)
 {
-	if (css->flags & CSS_ROOT)
-		return true;
 	return percpu_ref_tryget_live(&css->refcnt);
 }
 
@@ -119,8 +114,7 @@ static inline bool css_tryget_online(struct cgroup_subsys_state *css)
  */
 static inline void css_put(struct cgroup_subsys_state *css)
 {
-	if (!(css->flags & CSS_ROOT))
-		percpu_ref_put(&css->refcnt);
+	percpu_ref_put(&css->refcnt);
 }
 
 /* bits in struct cgroup flags field */

kernel/cgroup.c

@@ -4158,8 +4158,6 @@ static void init_and_link_css(struct cgroup_subsys_state *css,
 	if (cgrp->parent) {
 		css->parent = cgroup_css(cgrp->parent, ss);
 		css_get(css->parent);
-	} else {
-		css->flags |= CSS_ROOT;
 	}
 
 	BUG_ON(cgroup_css(cgrp, ss));
@@ -4582,9 +4580,10 @@ static void __init cgroup_init_subsys(struct cgroup_subsys *ss, bool early)
 	BUG_ON(IS_ERR(css));
 	init_and_link_css(css, ss, &cgrp_dfl_root.cgrp);
 	if (early) {
-		/* idr_alloc() can't be called safely during early init */
+		/* allocation can't be done safely during early init */
 		css->id = 1;
 	} else {
+		BUG_ON(percpu_ref_init(&css->refcnt, css_release));
 		css->id = cgroup_idr_alloc(&ss->css_idr, css, 1, 2, GFP_KERNEL);
 		BUG_ON(css->id < 0);
 	}
@@ -4671,6 +4670,7 @@ int __init cgroup_init(void)
 			struct cgroup_subsys_state *css =
 				init_css_set.subsys[ss->id];
 
+			BUG_ON(percpu_ref_init(&css->refcnt, css_release));
 			css->id = cgroup_idr_alloc(&ss->css_idr, css, 1, 2,
 						   GFP_KERNEL);
 			BUG_ON(css->id < 0);