1 files changed, 91 insertions, 62 deletions

diff --git a/kernel/cgroup/rstat.c b/kernel/cgroup/rstat.c
index 1f300bf4dc40..4ec29e6b1d8d 100644
--- a/kernel/cgroup/rstat.c
+++ b/kernel/cgroup/rstat.c
@@ -74,64 +74,109 @@ __bpf_kfunc void cgroup_rstat_updated(struct cgroup *cgrp, int cpu)
 }
 
 /**
- * cgroup_rstat_cpu_pop_updated - iterate and dismantle rstat_cpu updated tree
- * @pos: current position
- * @root: root of the tree to traversal
+ * cgroup_rstat_push_children - push children cgroups into the given list
+ * @head: current head of the list (= subtree root)
+ * @child: first child of the root
  * @cpu: target cpu
+ * Return: A new singly linked list of cgroups to be flush
  *
- * Walks the updated rstat_cpu tree on @cpu from @root.  %NULL @pos starts
- * the traversal and %NULL return indicates the end.  During traversal,
- * each returned cgroup is unlinked from the tree.  Must be called with the
- * matching cgroup_rstat_cpu_lock held.
+ * Iteratively traverse down the cgroup_rstat_cpu updated tree level by
+ * level and push all the parents first before their next level children
+ * into a singly linked list built from the tail backward like "pushing"
+ * cgroups into a stack. The root is pushed by the caller.
+ */
+static struct cgroup *cgroup_rstat_push_children(struct cgroup *head,
+						 struct cgroup *child, int cpu)
+{
+	struct cgroup *chead = child;	/* Head of child cgroup level */
+	struct cgroup *ghead = NULL;	/* Head of grandchild cgroup level */
+	struct cgroup *parent, *grandchild;
+	struct cgroup_rstat_cpu *crstatc;
+
+	child->rstat_flush_next = NULL;
+
+next_level:
+	while (chead) {
+		child = chead;
+		chead = child->rstat_flush_next;
+		parent = cgroup_parent(child);
+
+		/* updated_next is parent cgroup terminated */
+		while (child != parent) {
+			child->rstat_flush_next = head;
+			head = child;
+			crstatc = cgroup_rstat_cpu(child, cpu);
+			grandchild = crstatc->updated_children;
+			if (grandchild != child) {
+				/* Push the grand child to the next level */
+				crstatc->updated_children = child;
+				grandchild->rstat_flush_next = ghead;
+				ghead = grandchild;
+			}
+			child = crstatc->updated_next;
+			crstatc->updated_next = NULL;
+		}
+	}
+
+	if (ghead) {
+		chead = ghead;
+		ghead = NULL;
+		goto next_level;
+	}
+	return head;
+}
+
+/**
+ * cgroup_rstat_updated_list - return a list of updated cgroups to be flushed
+ * @root: root of the cgroup subtree to traverse
+ * @cpu: target cpu
+ * Return: A singly linked list of cgroups to be flushed
+ *
+ * Walks the updated rstat_cpu tree on @cpu from @root.  During traversal,
+ * each returned cgroup is unlinked from the updated tree.
  *
  * The only ordering guarantee is that, for a parent and a child pair
- * covered by a given traversal, if a child is visited, its parent is
- * guaranteed to be visited afterwards.
+ * covered by a given traversal, the child is before its parent in
+ * the list.
+ *
+ * Note that updated_children is self terminated and points to a list of
+ * child cgroups if not empty. Whereas updated_next is like a sibling link
+ * within the children list and terminated by the parent cgroup. An exception
+ * here is the cgroup root whose updated_next can be self terminated.
  */
-static struct cgroup *cgroup_rstat_cpu_pop_updated(struct cgroup *pos,
-						   struct cgroup *root, int cpu)
+static struct cgroup *cgroup_rstat_updated_list(struct cgroup *root, int cpu)
 {
-	struct cgroup_rstat_cpu *rstatc;
-	struct cgroup *parent;
-
-	if (pos == root)
-		return NULL;
+	raw_spinlock_t *cpu_lock = per_cpu_ptr(&cgroup_rstat_cpu_lock, cpu);
+	struct cgroup_rstat_cpu *rstatc = cgroup_rstat_cpu(root, cpu);
+	struct cgroup *head = NULL, *parent, *child;
+	unsigned long flags;
 
 	/*
-	 * We're gonna walk down to the first leaf and visit/remove it.  We
-	 * can pick whatever unvisited node as the starting point.
+	 * The _irqsave() is needed because cgroup_rstat_lock is
+	 * spinlock_t which is a sleeping lock on PREEMPT_RT. Acquiring
+	 * this lock with the _irq() suffix only disables interrupts on
+	 * a non-PREEMPT_RT kernel. The raw_spinlock_t below disables
+	 * interrupts on both configurations. The _irqsave() ensures
+	 * that interrupts are always disabled and later restored.
 	 */
-	if (!pos) {
-		pos = root;
-		/* return NULL if this subtree is not on-list */
-		if (!cgroup_rstat_cpu(pos, cpu)->updated_next)
-			return NULL;
-	} else {
-		pos = cgroup_parent(pos);
-	}
+	raw_spin_lock_irqsave(cpu_lock, flags);
 
-	/* walk down to the first leaf */
-	while (true) {
-		rstatc = cgroup_rstat_cpu(pos, cpu);
-		if (rstatc->updated_children == pos)
-			break;
-		pos = rstatc->updated_children;
-	}
+	/* Return NULL if this subtree is not on-list */
+	if (!rstatc->updated_next)
+		goto unlock_ret;
 
 	/*
-	 * Unlink @pos from the tree.  As the updated_children list is
+	 * Unlink @root from its parent. As the updated_children list is
 	 * singly linked, we have to walk it to find the removal point.
-	 * However, due to the way we traverse, @pos will be the first
-	 * child in most cases. The only exception is @root.
 	 */
-	parent = cgroup_parent(pos);
+	parent = cgroup_parent(root);
 	if (parent) {
 		struct cgroup_rstat_cpu *prstatc;
 		struct cgroup **nextp;
 
 		prstatc = cgroup_rstat_cpu(parent, cpu);
 		nextp = &prstatc->updated_children;
-		while (*nextp != pos) {
+		while (*nextp != root) {
 			struct cgroup_rstat_cpu *nrstatc;
 
 			nrstatc = cgroup_rstat_cpu(*nextp, cpu);
@@ -142,31 +187,15 @@ static struct cgroup *cgroup_rstat_cpu_pop_updated(struct cgroup *pos,
 	}
 
 	rstatc->updated_next = NULL;
-	return pos;
-}
 
-/* Return a list of updated cgroups to be flushed */
-static struct cgroup *cgroup_rstat_updated_list(struct cgroup *root, int cpu)
-{
-	raw_spinlock_t *cpu_lock = per_cpu_ptr(&cgroup_rstat_cpu_lock, cpu);
-	struct cgroup *head, *tail, *next;
-	unsigned long flags;
-
-	/*
-	 * The _irqsave() is needed because cgroup_rstat_lock is
-	 * spinlock_t which is a sleeping lock on PREEMPT_RT. Acquiring
-	 * this lock with the _irq() suffix only disables interrupts on
-	 * a non-PREEMPT_RT kernel. The raw_spinlock_t below disables
-	 * interrupts on both configurations. The _irqsave() ensures
-	 * that interrupts are always disabled and later restored.
-	 */
-	raw_spin_lock_irqsave(cpu_lock, flags);
-	head = tail = cgroup_rstat_cpu_pop_updated(NULL, root, cpu);
-	while (tail) {
-		next = cgroup_rstat_cpu_pop_updated(tail, root, cpu);
-		tail->rstat_flush_next = next;
-		tail = next;
-	}
+	/* Push @root to the list first before pushing the children */
+	head = root;
+	root->rstat_flush_next = NULL;
+	child = rstatc->updated_children;
+	rstatc->updated_children = root;
+	if (child != root)
+		head = cgroup_rstat_push_children(head, child, cpu);
+unlock_ret:
 	raw_spin_unlock_irqrestore(cpu_lock, flags);
 	return head;
 }