cgroup: Move populated counters to cgroup_subsys_state

Later patches replace the cgroup-level finish_destroy_work deferral added
by 93618edf7538 ("cgroup: Defer css percpu_ref kill on rmdir until cgroup
is depopulated") with a per-subsys-css deferral. That needs each subsystem
css to track its own populated count. Move the populated counters from
cgroup onto cgroup_subsys_state. cgroup->self is itself a
cgroup_subsys_state and self.parent walks the same chain as cgroup_parent(),
so cgroup_update_populated() generalizes to a single css_update_populated()
taking a css. The cgroup-side bookkeeping runs only when the walk started
from a self css.

Keep nr_populated_{domain,threaded}_children on cgroup. Both sum to
self.nr_populated_children, but staying as dedicated fields to allow readers
like cgroup_can_be_thread_root() unlocked access.

css_set_update_populated() also walks the per-subsys-css chain so each
subsystem css's hierarchical populated count is maintained. No reader
consumes those counts yet.

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

2 files changed, 21 insertions, 14 deletions

diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
index 50a784da7a81..c4929f7bbe5a 100644
--- a/include/linux/cgroup-defs.h
+++ b/include/linux/cgroup-defs.h
@@ -254,6 +254,15 @@ struct cgroup_subsys_state {
 	int nr_descendants;
 
 	/*
+	 * Hierarchical populated state. For cgroup->self, nr_populated_csets
+	 * counts populated csets linked via cgrp_cset_link.
+	 * nr_populated_children counts immediate-child csses whose own
+	 * populated state is nonzero. Protected by css_set_lock.
+	 */
+	int nr_populated_csets;
+	int nr_populated_children;
+
+	/*
 	 * A singly-linked list of css structures to be rstat flushed.
 	 * This is a scratch field to be used exclusively by
 	 * css_rstat_flush().
@@ -504,17 +513,12 @@ struct cgroup {
 	int max_descendants;
 
 	/*
-	 * Each non-empty css_set associated with this cgroup contributes
-	 * one to nr_populated_csets.  The counter is zero iff this cgroup
-	 * doesn't have any tasks.
-	 *
-	 * All children which have non-zero nr_populated_csets and/or
-	 * nr_populated_children of their own contribute one to either
-	 * nr_populated_domain_children or nr_populated_threaded_children
-	 * depending on their type.  Each counter is zero iff all cgroups
-	 * of the type in the subtree proper don't have any tasks.
+	 * Domain/threaded split of self.nr_populated_children: each counts
+	 * immediate-child cgroups whose subtree is populated and sums to
+	 * self.nr_populated_children. Kept as separate fields to allow readers
+	 * like cgroup_can_be_thread_root() unlocked access. Protected by
+	 * css_set_lock; updated by css_update_populated().
 	 */
-	int nr_populated_csets;
 	int nr_populated_domain_children;
 	int nr_populated_threaded_children;
 
diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 9f8bef8f3a60..c2a8c38d8206 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -654,14 +654,17 @@ static inline bool task_under_cgroup_hierarchy(struct task_struct *task,
  */
 static inline bool cgroup_has_tasks(struct cgroup *cgrp)
 {
-	return READ_ONCE(cgrp->nr_populated_csets);
+	return READ_ONCE(cgrp->self.nr_populated_csets);
+}
+
+static inline bool css_is_populated(struct cgroup_subsys_state *css)
+{
+	return READ_ONCE(css->nr_populated_csets) || READ_ONCE(css->nr_populated_children);
 }
 
 static inline bool cgroup_is_populated(struct cgroup *cgrp)
 {
-	return READ_ONCE(cgrp->nr_populated_csets) +
-		READ_ONCE(cgrp->nr_populated_domain_children) +
-		READ_ONCE(cgrp->nr_populated_threaded_children);
+	return css_is_populated(&cgrp->self);
 }
 
 /* returns ino associated with a cgroup */