cgroup: use cgrp->kn->id as the cgroup ID

cgroup ID is currently allocated using a dedicated per-hierarchy idr
and used internally and exposed through tracepoints and bpf.  This is
confusing because there are tracepoints and other interfaces which use
the cgroupfs ino as IDs.

The preceding changes made kn->id exposed as ino as 64bit ino on
supported archs or ino+gen (low 32bits as ino, high gen).  There's no
reason for cgroup to use different IDs.  The kernfs IDs are unique and
userland can easily discover them and map them back to paths using
standard file operations.

This patch replaces cgroup IDs with kernfs IDs.

* cgroup_id() is added and all cgroup ID users are converted to use it.

* kernfs_node creation is moved to earlier during cgroup init so that
  cgroup_id() is available during init.

* While at it, s/cgroup/cgrp/ in psi helpers for consistency.

* Fallback ID value is changed to 1 to be consistent with root cgroup
  ID.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Namhyung Kim <namhyung@kernel.org>

3 files changed, 12 insertions, 28 deletions

diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
index 4904b1ebd1ff..63097cb243cb 100644
--- a/include/linux/cgroup-defs.h
+++ b/include/linux/cgroup-defs.h
@@ -355,16 +355,6 @@ struct cgroup {
 	unsigned long flags;		/* "unsigned long" so bitops work */
 
 	/*
-	 * idr allocated in-hierarchy ID.
-	 *
-	 * ID 0 is not used, the ID of the root cgroup is always 1, and a
-	 * new cgroup will be assigned with a smallest available ID.
-	 *
-	 * Allocating/Removing ID must be protected by cgroup_mutex.
-	 */
-	int id;
-
-	/*
 	 * The depth this cgroup is at.  The root is at depth zero and each
 	 * step down the hierarchy increments the level.  This along with
 	 * ancestor_ids[] can determine whether a given cgroup is a
@@ -488,7 +478,7 @@ struct cgroup {
 	struct cgroup_freezer_state freezer;
 
 	/* ids of the ancestors at each level including self */
-	int ancestor_ids[];
+	u64 ancestor_ids[];
 };
 
 /*
@@ -509,7 +499,7 @@ struct cgroup_root {
 	struct cgroup cgrp;
 
 	/* for cgrp->ancestor_ids[0] */
-	int cgrp_ancestor_id_storage;
+	u64 cgrp_ancestor_id_storage;
 
 	/* Number of cgroups in the hierarchy, used only for /proc/cgroups */
 	atomic_t nr_cgrps;
@@ -520,9 +510,6 @@ struct cgroup_root {
 	/* Hierarchy-specific flags */
 	unsigned int flags;
 
-	/* IDs for cgroups in this hierarchy */
-	struct idr cgroup_idr;
-
 	/* The path to use for release notifications. */
 	char release_agent_path[PATH_MAX];
 
diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 815fff49d555..d7ddebd0cdec 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -304,6 +304,11 @@ void css_task_iter_end(struct css_task_iter *it);
  * Inline functions.
  */
 
+static inline u64 cgroup_id(struct cgroup *cgrp)
+{
+	return cgrp->kn->id;
+}
+
 /**
  * css_get - obtain a reference on the specified css
  * @css: target css
@@ -565,7 +570,7 @@ static inline bool cgroup_is_descendant(struct cgroup *cgrp,
 {
 	if (cgrp->root != ancestor->root || cgrp->level < ancestor->level)
 		return false;
-	return cgrp->ancestor_ids[ancestor->level] == ancestor->id;
+	return cgrp->ancestor_ids[ancestor->level] == cgroup_id(ancestor);
 }
 
 /**
@@ -687,17 +692,13 @@ static inline void cgroup_kthread_ready(void)
 	current->no_cgroup_migration = 0;
 }
 
-static inline u64 cgroup_get_kernfs_id(struct cgroup *cgrp)
-{
-	return cgrp->kn->id;
-}
-
 void cgroup_path_from_kernfs_id(u64 id, char *buf, size_t buflen);
 #else /* !CONFIG_CGROUPS */
 
 struct cgroup_subsys_state;
 struct cgroup;
 
+static inline u64 cgroup_id(struct cgroup *cgrp) { return 1; }
 static inline void css_get(struct cgroup_subsys_state *css) {}
 static inline void css_put(struct cgroup_subsys_state *css) {}
 static inline int cgroup_attach_task_all(struct task_struct *from,
@@ -717,10 +718,6 @@ static inline int cgroup_init_early(void) { return 0; }
 static inline int cgroup_init(void) { return 0; }
 static inline void cgroup_init_kthreadd(void) {}
 static inline void cgroup_kthread_ready(void) {}
-static inline union u64 cgroup_get_kernfs_id(struct cgroup *cgrp)
-{
-	return 0;
-}
 
 static inline struct cgroup *cgroup_parent(struct cgroup *cgrp)
 {
diff --git a/include/trace/events/cgroup.h b/include/trace/events/cgroup.h
index a566cc521476..7f42a3de59e6 100644
--- a/include/trace/events/cgroup.h
+++ b/include/trace/events/cgroup.h
@@ -66,7 +66,7 @@ DECLARE_EVENT_CLASS(cgroup,
 
 	TP_fast_assign(
 		__entry->root = cgrp->root->hierarchy_id;
-		__entry->id = cgrp->id;
+		__entry->id = cgroup_id(cgrp);
 		__entry->level = cgrp->level;
 		__assign_str(path, path);
 	),
@@ -135,7 +135,7 @@ DECLARE_EVENT_CLASS(cgroup_migrate,
 
 	TP_fast_assign(
 		__entry->dst_root = dst_cgrp->root->hierarchy_id;
-		__entry->dst_id = dst_cgrp->id;
+		__entry->dst_id = cgroup_id(dst_cgrp);
 		__entry->dst_level = dst_cgrp->level;
 		__assign_str(dst_path, path);
 		__entry->pid = task->pid;
@@ -179,7 +179,7 @@ DECLARE_EVENT_CLASS(cgroup_event,
 
 	TP_fast_assign(
 		__entry->root = cgrp->root->hierarchy_id;
-		__entry->id = cgrp->id;
+		__entry->id = cgroup_id(cgrp);
 		__entry->level = cgrp->level;
 		__assign_str(path, path);
 		__entry->val = val;