[Devel] [PATCH rh7] memcg: ignore memory.low for cgroups w/o tasks

[Vladimir Davydov]

It's OK to set memory.low for a cgroup to infinity. This might be useful
if no tasks are supposed to run inside the cgroup itself, but only in
its sub-cgroups (e.g. /machine.slice). In this case protection against
memory pressure originating on upper levels will be guarded solely by
memory.low configuration in sub-cgroups.

However, in the current implementation, in contrast to mainstream,
charges can appear in a cgroup even if there's no tasks in it - they can
be reparented from a dead sub-cgroup. If the cgroup has memory.low set
to inf, such reparented charges will not get reclaimed normally on
memory pressure, resulting in performance degradation in other cgroups.
To avoid that, let's ignore memory.low for cgroups w/o tasks.

https://jira.sw.ru/browse/PSBM-44192

Signed-off-by: Vladimir Davydov <vdavydov at virtuozzo.com>
---
 mm/memcontrol.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index ddd162ae4b3f..6009ff5d1903 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1607,6 +1607,24 @@ bool mem_cgroup_low(struct mem_cgroup *root, struct mem_cgroup *memcg)
 	if (res_counter_read_u64(&memcg->res, RES_USAGE) >= memcg->low)
 		return false;
 
+	/*
+	 * XXX: It's OK to set memory.low for a cgroup to infinity. This might
+	 * be useful if no tasks are supposed to run inside the cgroup itself,
+	 * but only in its sub-cgroups (e.g. /machine.slice). In this case
+	 * protection against memory pressure originating on upper levels will
+	 * be guarded solely by memory.low configuration in sub-cgroups.
+	 *
+	 * However, in the current implementation, in contrast to mainstream,
+	 * charges can appear in a cgroup even if there's no tasks in it - they
+	 * can be reparented from a dead sub-cgroup. If the cgroup has
+	 * memory.low set to inf, such reparented charges will not get
+	 * reclaimed normally on memory pressure, resulting in performance
+	 * degradation in other cgroups. To avoid that, let's ignore memory.low
+	 * for cgroups w/o tasks.
+	 */
+	if (cgroup_task_count(memcg->css.cgroup) == 0)
+		return false;
+
 	while (memcg != root) {
 		memcg = parent_mem_cgroup(memcg);
 		if (!memcg)
-- 
2.1.4