[PATCH v13 4/7] mm, oom: introduce memory.oom_group

[Roman Gushchin]

The cgroup-aware OOM killer treats leaf memory cgroups as memory
consumption entities and performs the victim selection by comparing
them based on their memory footprint. Then it kills the biggest task
inside the selected memory cgroup.

But there are workloads, which are not tolerant to a such behavior.
Killing a random task may leave the workload in a broken state.

To solve this problem, memory.oom_group knob is introduced.
It will define, whether a memory group should be treated as an
indivisible memory consumer, compared by total memory consumption
with other memory consumers (leaf memory cgroups and other memory
cgroups with memory.oom_group set), and whether all belonging tasks
should be killed if the cgroup is selected.

If set on memcg A, it means that in case of system-wide OOM or
memcg-wide OOM scoped to A or any ancestor cgroup, all tasks,
belonging to the sub-tree of A will be killed. If OOM event is
scoped to a descendant cgroup (A/B, for example), only tasks in
that cgroup can be affected. OOM killer will never touch any tasks
outside of the scope of the OOM event.

Also, tasks with oom_score_adj set to -1000 will not be killed because
this has been a long established way to protect a particular process
from seeing an unexpected SIGKILL from the OOM killer. Ignoring this
user defined configuration might lead to data corruptions or other
misbehavior.

The default value is 0.

Signed-off-by: Roman Gushchin <guro@xxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Vladimir Davydov <vdavydov.dev@xxxxxxxxx>
Cc: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Cc: David Rientjes <rientjes@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: kernel-team@xxxxxx
Cc: cgroups@xxxxxxxxxxxxxxx
Cc: linux-doc@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: linux-mm@xxxxxxxxx
---
 include/linux/memcontrol.h | 17 +++++++++++
 mm/memcontrol.c            | 75 +++++++++++++++++++++++++++++++++++++++++++---
 mm/oom_kill.c              | 47 +++++++++++++++++++++++------
 3 files changed, 126 insertions(+), 13 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index cb4db659a8b5..7b8bcdf6571d 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -203,6 +203,13 @@ struct mem_cgroup {
 	/* OOM-Killer disable */
 	int		oom_kill_disable;
 
+	/*
+	 * Treat the sub-tree as an indivisible memory consumer,
+	 * kill all belonging tasks if the memory cgroup selected
+	 * as OOM victim.
+	 */
+	bool oom_group;
+
 	/* handle for "memory.events" */
 	struct cgroup_file events_file;
 
@@ -490,6 +497,11 @@ bool mem_cgroup_oom_synchronize(bool wait);
 
 bool mem_cgroup_select_oom_victim(struct oom_control *oc);
 
+static inline bool mem_cgroup_oom_group(struct mem_cgroup *memcg)
+{
+	return memcg->oom_group;
+}
+
 #ifdef CONFIG_MEMCG_SWAP
 extern int do_swap_account;
 #endif
@@ -990,6 +1002,11 @@ static inline bool mem_cgroup_select_oom_victim(struct oom_control *oc)
 {
 	return false;
 }
+
+static inline bool mem_cgroup_oom_group(struct mem_cgroup *memcg)
+{
+	return false;
+}
 #endif /* CONFIG_MEMCG */
 
 /* idx can be of type enum memcg_stat_item or node_stat_item */
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 592ffb1c98a7..5d27a4bbd478 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2779,19 +2779,51 @@ static long oom_evaluate_memcg(struct mem_cgroup *memcg,
 
 static void select_victim_memcg(struct mem_cgroup *root, struct oom_control *oc)
 {
-	struct mem_cgroup *iter;
+	struct mem_cgroup *iter, *group = NULL;
+	long group_score = 0;
 
 	oc->chosen_memcg = NULL;
 	oc->chosen_points = 0;
 
+	/*
+	 * If OOM is memcg-wide, and the memcg has the oom_group flag set,
+	 * all tasks belonging to the memcg should be killed.
+	 * So, we mark the memcg as a victim.
+	 */
+	if (oc->memcg && mem_cgroup_oom_group(oc->memcg)) {
+		oc->chosen_memcg = oc->memcg;
+		css_get(&oc->chosen_memcg->css);
+		return;
+	}
+
 	/*
 	 * The oom_score is calculated for leaf memory cgroups (including
 	 * the root memcg).
+	 * Non-leaf oom_group cgroups accumulating score of descendant
+	 * leaf memory cgroups.
 	 */
 	rcu_read_lock();
 	for_each_mem_cgroup_tree(iter, root) {
 		long score;
 
+		/*
+		 * We don't consider non-leaf non-oom_group memory cgroups
+		 * as OOM victims.
+		 */
+		if (memcg_has_children(iter) && iter != root_mem_cgroup &&
+		    !mem_cgroup_oom_group(iter))
+			continue;
+
+		/*
+		 * If group is not set or we've ran out of the group's sub-tree,
+		 * we should set group and reset group_score.
+		 */
+		if (!group || group == root_mem_cgroup ||
+		    !mem_cgroup_is_descendant(iter, group)) {
+			group = iter;
+			group_score = 0;
+		}
+
 		if (memcg_has_children(iter) && iter != root_mem_cgroup)
 			continue;
 
@@ -2813,9 +2845,11 @@ static void select_victim_memcg(struct mem_cgroup *root, struct oom_control *oc)
 			break;
 		}
 
-		if (score > oc->chosen_points) {
-			oc->chosen_points = score;
-			oc->chosen_memcg = iter;
+		group_score += score;
+
+		if (group_score > oc->chosen_points) {
+			oc->chosen_points = group_score;
+			oc->chosen_memcg = group;
 		}
 	}
 
@@ -5440,6 +5474,33 @@ static ssize_t memory_max_write(struct kernfs_open_file *of,
 	return nbytes;
 }
 
+static int memory_oom_group_show(struct seq_file *m, void *v)
+{
+	struct mem_cgroup *memcg = mem_cgroup_from_css(seq_css(m));
+	bool oom_group = memcg->oom_group;
+
+	seq_printf(m, "%d\n", oom_group);
+
+	return 0;
+}
+
+static ssize_t memory_oom_group_write(struct kernfs_open_file *of,
+				      char *buf, size_t nbytes,
+				      loff_t off)
+{
+	struct mem_cgroup *memcg = mem_cgroup_from_css(of_css(of));
+	int oom_group;
+	int err;
+
+	err = kstrtoint(strstrip(buf), 0, &oom_group);
+	if (err)
+		return err;
+
+	memcg->oom_group = oom_group;
+
+	return nbytes;
+}
+
 static int memory_events_show(struct seq_file *m, void *v)
 {
 	struct mem_cgroup *memcg = mem_cgroup_from_css(seq_css(m));
@@ -5559,6 +5620,12 @@ static struct cftype memory_files[] = {
 		.seq_show = memory_max_show,
 		.write = memory_max_write,
 	},
+	{
+		.name = "oom_group",
+		.flags = CFTYPE_NOT_ON_ROOT | CFTYPE_NS_DELEGATABLE,
+		.seq_show = memory_oom_group_show,
+		.write = memory_oom_group_write,
+	},
 	{
 		.name = "events",
 		.flags = CFTYPE_NOT_ON_ROOT,
diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index bcfa92f29407..4678468bae17 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -820,6 +820,17 @@ static void __oom_kill_process(struct task_struct *victim)
 	struct mm_struct *mm;
 	bool can_oom_reap = true;
 
+	/*
+	 * __oom_kill_process() is used to kill all tasks belonging to
+	 * the selected memory cgroup, so we should check that we're not
+	 * trying to kill an unkillable task.
+	 */
+	if (is_global_init(victim) || (victim->flags & PF_KTHREAD) ||
+	    victim->signal->oom_score_adj == OOM_SCORE_ADJ_MIN) {
+		put_task_struct(victim);
+		return;
+	}
+
 	p = find_lock_task_mm(victim);
 	if (!p) {
 		put_task_struct(victim);
@@ -956,21 +967,39 @@ static void oom_kill_process(struct oom_control *oc, const char *message)
 	__oom_kill_process(victim);
 }
 
-static bool oom_kill_memcg_victim(struct oom_control *oc)
+static int oom_kill_memcg_member(struct task_struct *task, void *unused)
 {
+	get_task_struct(task);
+	__oom_kill_process(task);
+	return 0;
+}
 
+static bool oom_kill_memcg_victim(struct oom_control *oc)
+{
 	if (oc->chosen_memcg == NULL || oc->chosen_memcg == INFLIGHT_VICTIM)
 		return oc->chosen_memcg;
 
-	/* Kill a task in the chosen memcg with the biggest memory footprint */
-	oc->chosen_points = 0;
-	oc->chosen_task = NULL;
-	mem_cgroup_scan_tasks(oc->chosen_memcg, oom_evaluate_task, oc);
-
-	if (oc->chosen_task == NULL || oc->chosen_task == INFLIGHT_VICTIM)
-		goto out;
+	/*
+	 * If memory.oom_group is set, kill all tasks belonging to the sub-tree
+	 * of the chosen memory cgroup, otherwise kill the task with the biggest
+	 * memory footprint.
+	 */
+	if (mem_cgroup_oom_group(oc->chosen_memcg)) {
+		mem_cgroup_scan_tasks(oc->chosen_memcg, oom_kill_memcg_member,
+				      NULL);
+		/* We have one or more terminating processes at this point. */
+		oc->chosen_task = INFLIGHT_VICTIM;
+	} else {
+		oc->chosen_points = 0;
+		oc->chosen_task = NULL;
+		mem_cgroup_scan_tasks(oc->chosen_memcg, oom_evaluate_task, oc);
+
+		if (oc->chosen_task == NULL ||
+		    oc->chosen_task == INFLIGHT_VICTIM)
+			goto out;
 
-	__oom_kill_process(oc->chosen_task);
+		__oom_kill_process(oc->chosen_task);
+	}
 
 out:
 	mem_cgroup_put(oc->chosen_memcg);
-- 
2.14.3