[RFC PATCH v3 3/9] cgroup/pids: Make event counters hierarchical

[Michal Koutný]

The pids.events file should honor the hierarchy, so make the events
propagate from their origin up to the root on the unified hierarchy. The
v1 behavior remains non-hierarchical.

Signed-off-by: Michal Koutný <mkoutny@xxxxxxxx>
---
 Documentation/admin-guide/cgroup-v2.rst |  4 ++-
 kernel/cgroup/pids.c                    | 46 ++++++++++++++++---------
 2 files changed, 33 insertions(+), 17 deletions(-)

diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
index 4f04538d688c..5d4c505cae06 100644
--- a/Documentation/admin-guide/cgroup-v2.rst
+++ b/Documentation/admin-guide/cgroup-v2.rst
@@ -2189,7 +2189,9 @@ PID Interface Files
   pids.events
 	A read-only flat-keyed file which exists on non-root cgroups.  Unless
 	specified otherwise, a value change in this file generates a file modified
-	event. The following entries are defined.
+	event. Fields in this file are hierarchical and the file modified event
+	can be generated due to an event down the hierarchy. The following
+	entries are defined.
 
 	  max
 		The number of times the limit of the cgroup was hit.
diff --git a/kernel/cgroup/pids.c b/kernel/cgroup/pids.c
index 471562609eef..76c0a97e42da 100644
--- a/kernel/cgroup/pids.c
+++ b/kernel/cgroup/pids.c
@@ -238,6 +238,34 @@ static void pids_cancel_attach(struct cgroup_taskset *tset)
 	}
 }
 
+static void pids_event(struct pids_cgroup *pids_forking,
+		       struct pids_cgroup *pids_over_limit)
+{
+	struct pids_cgroup *p = pids_forking;
+	bool limit = false;
+
+	/* Only log the first time limit is hit. */
+	if (atomic64_inc_return(&p->events[PIDCG_MAX_IMPOSED]) == 1) {
+		pr_info("cgroup: fork rejected by pids controller in ");
+		pr_cont_cgroup_path(p->css.cgroup);
+		pr_cont("\n");
+	}
+	/* Events are only notified in pids_forking on v1 */
+	if (!cgroup_subsys_on_dfl(pids_cgrp_subsys))
+		return;
+
+	for (; parent_pids(p); p = parent_pids(p)) {
+		atomic64_inc(&p->events[PIDCG_MAX_IMPOSED]);
+
+		if (p == pids_over_limit)
+			limit = true;
+		if (limit)
+			atomic64_inc(&p->events[PIDCG_MAX]);
+
+		cgroup_file_notify(&p->events_file);
+	}
+}
+
 /*
  * task_css_check(true) in pids_can_fork() and pids_cancel_fork() relies
  * on cgroup_threadgroup_change_begin() held by the copy_process().
@@ -254,23 +282,9 @@ static int pids_can_fork(struct task_struct *task, struct css_set *cset)
 		css = task_css_check(current, pids_cgrp_id, true);
 	pids = css_pids(css);
 	err = pids_try_charge(pids, 1, &pids_over_limit);
-	if (err) {
-		/* compatibility on v1 where events were notified in leaves. */
-		if (!cgroup_subsys_on_dfl(pids_cgrp_subsys))
-			pids_over_limit = pids;
-
-		/* Only log the first time limit is hit. */
-		if (atomic64_inc_return(&pids->events[PIDCG_MAX_IMPOSED]) == 1) {
-			pr_info("cgroup: fork rejected by pids controller in ");
-			pr_cont_cgroup_path(pids->css.cgroup);
-			pr_cont("\n");
-		}
-		atomic64_inc(&pids_over_limit->events[PIDCG_MAX]);
+	if (err)
+		pids_event(pids, pids_over_limit);
 
-		cgroup_file_notify(&pids->events_file);
-		if (pids_over_limit != pids)
-			cgroup_file_notify(&pids_over_limit->events_file);
-	}
 	return err;
 }
 
-- 
2.44.0