[RFC PATCH 7/9 v2] cgroup: remove unneeded rcu list

[JP Kobryn <inwardvessel@gmail.com>]

Since the cgroup_subsystem_state now owns the rstat tree, the list management
previously done on the cgroup to keep track of which subsystem states are
participating in rstat is no longer needed.

Signed-off-by: JP Kobryn <inwardvessel@gmail.com>
---
 include/linux/cgroup-defs.h | 11 -----------
 kernel/cgroup/cgroup.c      | 20 +-------------------
 kernel/cgroup/rstat.c       |  9 +--------
 3 files changed, 2 insertions(+), 38 deletions(-)

diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
index 4d87519ff023..836260c422a0 100644
--- a/include/linux/cgroup-defs.h
+++ b/include/linux/cgroup-defs.h
@@ -182,14 +182,6 @@ struct cgroup_subsys_state {
 
 	/* per-cpu recursive resource statistics */
 	struct cgroup_rstat_cpu __percpu *rstat_cpu;
-	struct list_head rstat_css_list;
-
-	/*
-	 * Add padding to separate the read mostly rstat_cpu and
-	 * rstat_css_list into a different cacheline from the following
-	 * rstat_flush_next and *bstat fields which can have frequent updates.
-	 */
-	CACHELINE_PADDING(_pad_);
 
 	/*
 	 * A singly-linked list of cgroup structures to be rstat flushed.
@@ -198,9 +190,6 @@ struct cgroup_subsys_state {
 	 */
 	struct cgroup_subsys_state *rstat_flush_next;
 
-	/* flush target list anchored at cgrp->rstat_css_list */
-	struct list_head rstat_css_node;
-
 	/*
 	 * PI: Subsys-unique ID.  0 is unused and root is always 1.  The
 	 * matching css can be looked up using css_from_id().
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 96a2d15fe5e9..a36ed3995c6f 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -1826,7 +1826,6 @@ int rebind_subsystems(struct cgroup_root *dst_root, u16 ss_mask)
 		struct cgroup_root *src_root = ss->root;
 		struct cgroup *scgrp = &src_root->cgrp;
 		struct cgroup_subsys_state *css = cgroup_css(scgrp, ss);
-		struct cgroup_subsys_state *dcss = cgroup_css(dcgrp, ss);
 		struct css_set *cset, *cset_pos;
 		struct css_task_iter *it;
 
@@ -1864,13 +1863,6 @@ int rebind_subsystems(struct cgroup_root *dst_root, u16 ss_mask)
 		}
 		spin_unlock_irq(&css_set_lock);
 
-		if (ss->css_rstat_flush) {
-			list_del_rcu(&css->rstat_css_node);
-			synchronize_rcu();
-			list_add_rcu(&css->rstat_css_node,
-				     &dcss->rstat_css_list);
-		}
-
 		/* default hierarchy doesn't enable controllers by default */
 		dst_root->subsys_mask |= 1 << ssid;
 		if (dst_root == &cgrp_dfl_root) {
@@ -5491,11 +5483,7 @@ static void css_release_work_fn(struct work_struct *work)
 	if (ss) {
 		struct cgroup *parent_cgrp;
 
-		/* css release path */
-		if (!list_empty(&css->rstat_css_node)) {
-			cgroup_rstat_flush(css);
-			list_del_rcu(&css->rstat_css_node);
-		}
+		cgroup_rstat_flush(css);
 
 		cgroup_idr_replace(&ss->css_idr, NULL, css->id);
 		if (ss->css_released)
@@ -5569,8 +5557,6 @@ static void init_and_link_css(struct cgroup_subsys_state *css,
 	css->id = -1;
 	INIT_LIST_HEAD(&css->sibling);
 	INIT_LIST_HEAD(&css->children);
-	INIT_LIST_HEAD(&css->rstat_css_list);
-	INIT_LIST_HEAD(&css->rstat_css_node);
 	css->serial_nr = css_serial_nr_next++;
 	atomic_set(&css->online_cnt, 0);
 
@@ -5579,9 +5565,6 @@ static void init_and_link_css(struct cgroup_subsys_state *css,
 		css_get(css->parent);
 	}
 
-	if (ss->css_rstat_flush)
-		list_add_rcu(&css->rstat_css_node, &css->rstat_css_list);
-
 	BUG_ON(cgroup_css(cgrp, ss));
 }
 
@@ -5687,7 +5670,6 @@ static struct cgroup_subsys_state *css_create(struct cgroup *cgrp,
 err_list_del:
 	list_del_rcu(&css->sibling);
 err_free_css:
-	list_del_rcu(&css->rstat_css_node);
 	INIT_RCU_WORK(&css->destroy_rwork, css_free_rwork_fn);
 	queue_rcu_work(cgroup_destroy_wq, &css->destroy_rwork);
 	return ERR_PTR(err);
diff --git a/kernel/cgroup/rstat.c b/kernel/cgroup/rstat.c
index 92a46b960be1..c52e8429c75d 100644
--- a/kernel/cgroup/rstat.c
+++ b/kernel/cgroup/rstat.c
@@ -347,15 +347,8 @@ static void cgroup_rstat_flush_locked(struct cgroup_subsys_state *css)
 		struct cgroup_subsys_state *pos = cgroup_rstat_updated_list(css, cpu);
 
 		for (; pos; pos = pos->rstat_flush_next) {
-			struct cgroup_subsys_state *css_iter;
-
 			bpf_rstat_flush(pos->cgroup, cgroup_parent(pos->cgroup), cpu);
-
-			rcu_read_lock();
-			list_for_each_entry_rcu(css_iter, &pos->rstat_css_list,
-						rstat_css_node)
-				css_iter->ss->css_rstat_flush(css_iter, cpu);
-			rcu_read_unlock();
+			pos->ss->css_rstat_flush(pos, cpu);
 		}
 
 		/* play nice and yield if necessary */
-- 
2.47.1