[Devel] [PATCH v2 09/11] memcg: propagate kmem limiting information to children

[Glauber Costa]

The current memcg slab cache management fails to present satisfatory
hierarchical behavior in the following scenario:

-> /cgroups/memory/A/B/C

* kmem limit set at A,
* A and B have no tasks,
* span a new task in in C.

Because kmem_accounted is a boolean that was not set for C, no
accounting would be done. This is, however, not what we expect.

The basic idea, is that when a cgroup is limited, we walk the tree
upwards (something Kame and I already thought about doing for other
purposes), and make sure that we store the information about the parent
being limited in kmem_accounted (that is turned into a bitmap: two
booleans would not be space efficient). The code for that is taken from
sched/core.c. My reasons for not putting it into a common place is to
dodge the type issues that would arise from a common implementation
between memcg and the scheduler - but I think that it should ultimately
happen, so if you want me to do it now, let me know.

We do the reverse operation when a formerly limited cgroup becomes
unlimited.

Signed-off-by: Glauber Costa <glommer at parallels.com>
CC: Christoph Lameter <cl at linux.com>
CC: Pekka Enberg <penberg at cs.helsinki.fi>
CC: Michal Hocko <mhocko at suse.cz>
CC: Kamezawa Hiroyuki <kamezawa.hiroyu at jp.fujitsu.com>
CC: Johannes Weiner <hannes at cmpxchg.org>
CC: Suleiman Souhlal <suleiman at google.com>
---
 mm/memcontrol.c | 88 +++++++++++++++++++++++++++++++++++++++++++++++++++------
 1 file changed, 79 insertions(+), 9 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 3216292..3d30b79 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -295,7 +295,8 @@ struct mem_cgroup {
 	 * Should the accounting and control be hierarchical, per subtree?
 	 */
 	bool use_hierarchy;
-	bool kmem_accounted;
+
+	unsigned long kmem_accounted; /* See KMEM_ACCOUNTED_*, below */
 
 	bool		oom_lock;
 	atomic_t	under_oom;
@@ -348,6 +349,38 @@ struct mem_cgroup {
 #endif
 };
 
+enum {
+	KMEM_ACCOUNTED_THIS, /* accounted by this cgroup itself */
+	KMEM_ACCOUNTED_PARENT, /* accounted by any of its parents. */
+};
+
+#ifdef CONFIG_MEMCG_KMEM
+static bool memcg_kmem_account(struct mem_cgroup *memcg)
+{
+	return !test_and_set_bit(KMEM_ACCOUNTED_THIS, &memcg->kmem_accounted);
+}
+
+static bool memcg_kmem_clear_account(struct mem_cgroup *memcg)
+{
+	return test_and_clear_bit(KMEM_ACCOUNTED_THIS, &memcg->kmem_accounted);
+}
+
+static bool memcg_kmem_is_accounted(struct mem_cgroup *memcg)
+{
+	return test_bit(KMEM_ACCOUNTED_THIS, &memcg->kmem_accounted);
+}
+
+static void memcg_kmem_account_parent(struct mem_cgroup *memcg)
+{
+	set_bit(KMEM_ACCOUNTED_PARENT, &memcg->kmem_accounted);
+}
+
+static void memcg_kmem_clear_account_parent(struct mem_cgroup *memcg)
+{
+	clear_bit(KMEM_ACCOUNTED_PARENT, &memcg->kmem_accounted);
+}
+#endif /* CONFIG_MEMCG_KMEM */
+
 /* Stuffs for move charges at task migration. */
 /*
  * Types of charges to be moved. "move_charge_at_immitgrate" is treated as a
@@ -614,7 +647,7 @@ EXPORT_SYMBOL(__memcg_kmem_free_page);
 
 static void disarm_kmem_keys(struct mem_cgroup *memcg)
 {
-	if (memcg->kmem_accounted)
+	if (test_bit(KMEM_ACCOUNTED_THIS, &memcg->kmem_accounted))
 		static_key_slow_dec(&memcg_kmem_enabled_key);
 }
 #else
@@ -4171,17 +4204,54 @@ static ssize_t mem_cgroup_read(struct cgroup *cont, struct cftype *cft,
 static void memcg_update_kmem_limit(struct mem_cgroup *memcg, u64 val)
 {
 #ifdef CONFIG_MEMCG_KMEM
-	/*
-	 * Once enabled, can't be disabled. We could in theory disable it if we
-	 * haven't yet created any caches, or if we can shrink them all to
-	 * death. But it is not worth the trouble.
-	 */
+	struct mem_cgroup *iter;
+
 	mutex_lock(&set_limit_mutex);
-	if (!memcg->kmem_accounted && val != RESOURCE_MAX) {
+	if ((val != RESOURCE_MAX) && memcg_kmem_account(memcg)) {
+
+		/*
+		 * Once enabled, can't be disabled. We could in theory disable
+		 * it if we haven't yet created any caches, or if we can shrink
+		 * them all to death. But it is not worth the trouble
+		 */
 		static_key_slow_inc(&memcg_kmem_enabled_key);
-		memcg->kmem_accounted = true;
+
+		if (!memcg->use_hierarchy)
+			goto out;
+
+		for_each_mem_cgroup_tree(iter, memcg) {
+			if (iter == memcg)
+				continue;
+			memcg_kmem_account_parent(iter);
+		}
+	} else if ((val == RESOURCE_MAX) && memcg_kmem_clear_account(memcg)) {
+
+		if (!memcg->use_hierarchy)
+			goto out;
+
+		for_each_mem_cgroup_tree(iter, memcg) {
+			struct mem_cgroup *parent;
+
+			if (iter == memcg)
+				continue;
+			/*
+			 * We should only have our parent bit cleared if none
+			 * of our parents are accounted. The transversal order
+			 * of our iter function forces us to always look at the
+			 * parents.
+			 */
+			parent = parent_mem_cgroup(iter);
+			for (; parent != memcg; parent = parent_mem_cgroup(iter))
+				if (memcg_kmem_is_accounted(parent))
+					goto noclear;
+			memcg_kmem_clear_account_parent(iter);
+noclear:
+			continue;
+		}
 	}
+out:
 	mutex_unlock(&set_limit_mutex);
+
 #endif
 }
 
-- 
1.7.11.2