cgroup: support to enable nmi-safe css_rstat_updated

Add necessary infrastructure to enable the nmi-safe execution of
css_rstat_updated(). Currently css_rstat_updated() takes a per-cpu
per-css raw spinlock to add the given css in the per-cpu per-css update
tree. However the kernel can not spin in nmi context, so we need to
remove the spinning on the raw spinlock in css_rstat_updated().

To support lockless css_rstat_updated(), let's add necessary data
structures in the css and ss structures.

Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
Tested-by: JP Kobryn <inwardvessel@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

Author: shakeelb

include/linux/cgroup-defs.h

@@ -384,6 +384,9 @@ struct css_rstat_cpu {
 	 */
 	struct cgroup_subsys_state *updated_children;
 	struct cgroup_subsys_state *updated_next;	/* NULL if not on the list */
+
+	struct llist_node lnode;		/* lockless list for update */
+	struct cgroup_subsys_state *owner;	/* back pointer */
 };
 
 /*
@@ -822,6 +825,7 @@ struct cgroup_subsys {
 
 	spinlock_t rstat_ss_lock;
 	raw_spinlock_t __percpu *rstat_ss_cpu_lock;
+	struct llist_head __percpu *lhead; /* lockless update list head */
 };
 
 extern struct percpu_rw_semaphore cgroup_threadgroup_rwsem;

kernel/cgroup/rstat.c

@@ -11,6 +11,7 @@
 
 static DEFINE_SPINLOCK(rstat_base_lock);
 static DEFINE_PER_CPU(raw_spinlock_t, rstat_base_cpu_lock);
+static DEFINE_PER_CPU(struct llist_head, rstat_backlog_list);
 
 static void cgroup_base_stat_flush(struct cgroup *cgrp, int cpu);
 
@@ -45,6 +46,13 @@ static spinlock_t *ss_rstat_lock(struct cgroup_subsys *ss)
 	return &rstat_base_lock;
 }
 
+static inline struct llist_head *ss_lhead_cpu(struct cgroup_subsys *ss, int cpu)
+{
+	if (ss)
+		return per_cpu_ptr(ss->lhead, cpu);
+	return per_cpu_ptr(&rstat_backlog_list, cpu);
+}
+
 static raw_spinlock_t *ss_rstat_cpu_lock(struct cgroup_subsys *ss, int cpu)
 {
 	if (ss)
@@ -456,7 +464,8 @@ int css_rstat_init(struct cgroup_subsys_state *css)
 	for_each_possible_cpu(cpu) {
 		struct css_rstat_cpu *rstatc = css_rstat_cpu(css, cpu);
 
-		rstatc->updated_children = css;
+		rstatc->owner = rstatc->updated_children = css;
+		init_llist_node(&rstatc->lnode);
 
 		if (is_self) {
 			struct cgroup_rstat_base_cpu *rstatbc;
@@ -525,9 +534,19 @@ int __init ss_rstat_init(struct cgroup_subsys *ss)
 	}
 #endif
 
+	if (ss) {
+		ss->lhead = alloc_percpu(struct llist_head);
+		if (!ss->lhead) {
+			free_percpu(ss->rstat_ss_cpu_lock);
+			return -ENOMEM;
+		}
+	}
+
 	spin_lock_init(ss_rstat_lock(ss));
-	for_each_possible_cpu(cpu)
+	for_each_possible_cpu(cpu) {
 		raw_spin_lock_init(ss_rstat_cpu_lock(ss, cpu));
+		init_llist_head(ss_lhead_cpu(ss, cpu));
+	}
 
 	return 0;
 }