cgroup: record ancestor IDs and reimplement cgroup_is_descendant() using it

cgroup_is_descendant() currently walks up the hierarchy and compares
each ancestor to the cgroup in question.  While enough for cgroup core
usages, this can't be used in hot paths to test cgroup membership.
This patch adds cgroup->ancestor_ids[] which records the IDs of all
ancestors including self and cgroup->level for the nesting level.

This allows testing whether a given cgroup is a descendant of another
in three finite steps - testing whether the two belong to the same
hierarchy, whether the descendant candidate is at the same or a higher
level than the ancestor and comparing the recorded ancestor_id at the
matching level.  cgroup_is_descendant() is accordingly reimplmented
and made inline.

Signed-off-by: Tejun Heo <tj@kernel.org>

1 files changed, 10 insertions, 22 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index f1603c153890..3190040792c8 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -459,25 +459,6 @@ struct cgroup_subsys_state *of_css(struct kernfs_open_file *of)
 }
 EXPORT_SYMBOL_GPL(of_css);
 
-/**
- * cgroup_is_descendant - test ancestry
- * @cgrp: the cgroup to be tested
- * @ancestor: possible ancestor of @cgrp
- *
- * Test whether @cgrp is a descendant of @ancestor.  It also returns %true
- * if @cgrp == @ancestor.  This function is safe to call as long as @cgrp
- * and @ancestor are accessible.
- */
-bool cgroup_is_descendant(struct cgroup *cgrp, struct cgroup *ancestor)
-{
-	while (cgrp) {
-		if (cgrp == ancestor)
-			return true;
-		cgrp = cgroup_parent(cgrp);
-	}
-	return false;
-}
-
 static int notify_on_release(const struct cgroup *cgrp)
 {
 	return test_bit(CGRP_NOTIFY_ON_RELEASE, &cgrp->flags);
@@ -1903,6 +1884,7 @@ static int cgroup_setup_root(struct cgroup_root *root, unsigned long ss_mask)
 	if (ret < 0)
 		goto out;
 	root_cgrp->id = ret;
+	root_cgrp->ancestor_ids[0] = ret;
 
 	ret = percpu_ref_init(&root_cgrp->self.refcnt, css_release, 0,
 			      GFP_KERNEL);
@@ -4846,11 +4828,11 @@ err_free_css:
 static int cgroup_mkdir(struct kernfs_node *parent_kn, const char *name,
 			umode_t mode)
 {
-	struct cgroup *parent, *cgrp;
+	struct cgroup *parent, *cgrp, *tcgrp;
 	struct cgroup_root *root;
 	struct cgroup_subsys *ss;
 	struct kernfs_node *kn;
-	int ssid, ret;
+	int level, ssid, ret;
 
 	/* Do not accept '\n' to prevent making /proc/<pid>/cgroup unparsable.
 	 */
@@ -4861,9 +4843,11 @@ static int cgroup_mkdir(struct kernfs_node *parent_kn, const char *name,
 	if (!parent)
 		return -ENODEV;
 	root = parent->root;
+	level = parent->level + 1;
 
 	/* allocate the cgroup and its ID, 0 is reserved for the root */
-	cgrp = kzalloc(sizeof(*cgrp), GFP_KERNEL);
+	cgrp = kzalloc(sizeof(*cgrp) +
+		       sizeof(cgrp->ancestor_ids[0]) * (level + 1), GFP_KERNEL);
 	if (!cgrp) {
 		ret = -ENOMEM;
 		goto out_unlock;
@@ -4887,6 +4871,10 @@ static int cgroup_mkdir(struct kernfs_node *parent_kn, const char *name,
 
 	cgrp->self.parent = &parent->self;
 	cgrp->root = root;
+	cgrp->level = level;
+
+	for (tcgrp = cgrp; tcgrp; tcgrp = cgroup_parent(tcgrp))
+		cgrp->ancestor_ids[tcgrp->level] = tcgrp->id;
 
 	if (notify_on_release(parent))
 		set_bit(CGRP_NOTIFY_ON_RELEASE, &cgrp->flags);