[PATCH 3/5] cgroup: Move populated counters to cgroup_subsys_state

[Tejun Heo]

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@xxxxxxxxxx>
---
 include/linux/cgroup-defs.h | 24 ++++++----
 include/linux/cgroup.h      | 11 +++--
 kernel/cgroup/cgroup.c      | 95 +++++++++++++++++++++----------------
 3 files changed, 76 insertions(+), 54 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
@@ -253,6 +253,15 @@ 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
@@ -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 */
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index d1395784871a..dd4ea9d83100 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -756,65 +756,70 @@ static bool css_set_populated(struct css_set *cset)
 }
 
 /**
- * cgroup_update_populated - update the populated count of a cgroup
- * @cgrp: the target cgroup
- * @populated: inc or dec populated count
- *
- * One of the css_sets associated with @cgrp is either getting its first
- * task or losing the last.  Update @cgrp->nr_populated_* accordingly.  The
- * count is propagated towards root so that a given cgroup's
- * nr_populated_children is zero iff none of its descendants contain any
- * tasks.
- *
- * @cgrp's interface file "cgroup.populated" is zero if both
- * @cgrp->nr_populated_csets and @cgrp->nr_populated_children are zero and
- * 1 otherwise.  When the sum changes from or to zero, userland is notified
- * that the content of the interface file has changed.  This can be used to
- * detect when @cgrp and its descendants become populated or empty.
+ * css_update_populated - update the populated state of a css and ancestors
+ * @css: leaf css whose own populated count is changing
+ * @populated: inc or dec
+ *
+ * One of the css_sets pinned by @css is getting its first task or losing the
+ * last. Propagate the transition up the parent chain so that a css's
+ * nr_populated_children is zero iff none of its descendants contain any tasks.
+ *
+ * For a cgroup->self walk, also runs cgroup-side bookkeeping at each level:
+ * domain/threaded child split, deferred-destroy trigger, and notification via
+ * "cgroup.populated" (zero iff cgrp->self has neither populated csets nor
+ * populated children; userland is notified on transitions).
  */
-static void cgroup_update_populated(struct cgroup *cgrp, bool populated)
+static void css_update_populated(struct cgroup_subsys_state *css, bool populated)
 {
-	struct cgroup *child = NULL;
+	struct cgroup_subsys_state *child = NULL;
 	int adj = populated ? 1 : -1;
 
 	lockdep_assert_held(&css_set_lock);
 
 	do {
-		bool was_populated = cgroup_is_populated(cgrp);
+		/* non-NULL only on the cgroup->self walk */
+		struct cgroup *cgrp = css_is_self(css) ? css->cgroup : NULL;
+		bool was_populated = css_is_populated(css);
 
 		if (!child) {
-			WRITE_ONCE(cgrp->nr_populated_csets,
-				   cgrp->nr_populated_csets + adj);
+			WRITE_ONCE(css->nr_populated_csets,
+				   css->nr_populated_csets + adj);
 		} else {
-			if (cgroup_is_threaded(child))
-				WRITE_ONCE(cgrp->nr_populated_threaded_children,
-					   cgrp->nr_populated_threaded_children + adj);
-			else
-				WRITE_ONCE(cgrp->nr_populated_domain_children,
-					   cgrp->nr_populated_domain_children + adj);
+			WRITE_ONCE(css->nr_populated_children,
+				   css->nr_populated_children + adj);
+			if (cgrp) {
+				if (cgroup_is_threaded(child->cgroup))
+					WRITE_ONCE(cgrp->nr_populated_threaded_children,
+						   cgrp->nr_populated_threaded_children + adj);
+				else
+					WRITE_ONCE(cgrp->nr_populated_domain_children,
+						   cgrp->nr_populated_domain_children + adj);
+			}
 		}
 
-		if (was_populated == cgroup_is_populated(cgrp))
+		if (was_populated == css_is_populated(css))
 			break;
 
 		/*
 		 * Subtree just emptied below an offlined cgrp. Fire deferred
 		 * destroy. The transition is one-shot.
 		 */
-		if (was_populated && !css_is_online(&cgrp->self)) {
+		if (cgrp && was_populated && !css_is_online(css)) {
 			cgroup_get(cgrp);
 			WARN_ON_ONCE(!queue_work(cgroup_offline_wq,
 						 &cgrp->finish_destroy_work));
 		}
 
-		cgroup1_check_for_release(cgrp);
-		TRACE_CGROUP_PATH(notify_populated, cgrp,
-				  cgroup_is_populated(cgrp));
-		cgroup_file_notify(&cgrp->events_file);
+		if (cgrp) {
+			cgroup1_check_for_release(cgrp);
+			TRACE_CGROUP_PATH(notify_populated, cgrp,
+					  cgroup_is_populated(cgrp));
+			cgroup_file_notify(&cgrp->events_file);
+		}
 
-		child = cgrp;
-		cgrp = cgroup_parent(cgrp);
-	} while (cgrp);
+		child = css;
+		css = css->parent;
+	} while (css);
 }
 
 /**
@@ -822,17 +827,27 @@ static void cgroup_update_populated(struct cgroup *cgrp, bool populated)
  * @cset: target css_set
  * @populated: whether @cset is populated or depopulated
  *
- * @cset is either getting the first task or losing the last.  Update the
- * populated counters of all associated cgroups accordingly.
+ * @cset is either getting the first task or losing the last. Update the
+ * populated counters along each linked cgroup's self chain and each
+ * subsystem css that @cset pins.
  */
 static void css_set_update_populated(struct css_set *cset, bool populated)
 {
 	struct cgrp_cset_link *link;
+	struct cgroup_subsys *ss;
+	int ssid;
 
 	lockdep_assert_held(&css_set_lock);
 
 	list_for_each_entry(link, &cset->cgrp_links, cgrp_link)
-		cgroup_update_populated(link->cgrp, populated);
+		css_update_populated(&link->cgrp->self, populated);
+
+	for_each_subsys(ss, ssid) {
+		struct cgroup_subsys_state *css = cset->subsys[ssid];
+
+		if (css)
+			css_update_populated(css, populated);
+	}
 }
 
 /*
@@ -2190,7 +2205,7 @@ int cgroup_setup_root(struct cgroup_root *root, u32 ss_mask)
 	hash_for_each(css_set_table, i, cset, hlist) {
 		link_css_set(&tmp_links, cset, root_cgrp);
 		if (css_set_populated(cset))
-			cgroup_update_populated(root_cgrp, true);
+			css_update_populated(&root_cgrp->self, true);
 	}
 	spin_unlock_irq(&css_set_lock);
 
@@ -6145,7 +6160,7 @@ static void kill_css_finish(struct cgroup_subsys_state *css)
  *
  * - cgroup_finish_destroy(): kicks the percpu_ref kill via kill_css_finish() on
  *   each subsystem css. Fires once @cgrp's subtree is fully drained, either
- *   inline here or from cgroup_update_populated().
+ *   inline here or from css_update_populated().
  *
  * - The percpu_ref kill chain: css_killed_ref_fn -> css_killed_work_fn ->
  *   ->css_offline() -> release/free.
-- 
2.54.0