[RFC][PATCH 3/4] Memory controller soft limit organize cgroups

[Balbir Singh]

From: Balbir Singh <balbir@xxxxxxxxxxxxxxxxxx>

This patch introduces a RB-Tree for storing memory cgroups that are over their
soft limit. The overall goal is to

1. Add a memory cgroup to the RB-Tree when the soft limit is exceeded.
   We are careful about updates, updates take place only after a particular
   time interval has passed
2. We remove the node from the RB-Tree when the usage goes below the soft
   limit

The next set of patches will exploit the RB-Tree to get the group that is
over its soft limit by the largest amount and reclaim from it, when we
face memory contention.

Signed-off-by: Balbir Singh <balbir@xxxxxxxxxxxxxxxxxx>
---

 mm/memcontrol.c |   78 ++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 78 insertions(+)

diff -puN mm/memcontrol.c~memcg-organize-over-soft-limit-groups mm/memcontrol.c
--- a/mm/memcontrol.c~memcg-organize-over-soft-limit-groups
+++ a/mm/memcontrol.c
@@ -28,6 +28,7 @@
 #include <linux/bit_spinlock.h>
 #include <linux/rcupdate.h>
 #include <linux/mutex.h>
+#include <linux/rbtree.h>
 #include <linux/slab.h>
 #include <linux/swap.h>
 #include <linux/spinlock.h>
@@ -119,6 +120,13 @@ struct mem_cgroup_lru_info {
 };
 
 /*
+ * Cgroups above their limits are maintained in a RB-Tree, independent of
+ * their hierarchy representation
+ */
+static struct rb_root mem_cgroup_soft_limit_exceeded_groups;
+static DEFINE_MUTEX(memcg_soft_limit_tree_mutex);
+
+/*
  * The memory controller data structure. The memory controller controls both
  * page cache and RSS per cgroup. We would eventually like to provide
  * statistics based on the statistics developed by Rik Van Riel for clock-pro,
@@ -166,12 +174,18 @@ struct mem_cgroup {
 
 	unsigned int	swappiness;
 
+	struct rb_node mem_cgroup_node;
+	unsigned long long usage_in_excess;
+	unsigned long last_tree_update;
+
 	/*
 	 * statistics. This must be placed at the end of memcg.
 	 */
 	struct mem_cgroup_stat stat;
 };
 
+#define	MEM_CGROUP_TREE_UPDATE_INTERVAL		(HZ)
+
 enum charge_type {
 	MEM_CGROUP_CHARGE_TYPE_CACHE = 0,
 	MEM_CGROUP_CHARGE_TYPE_MAPPED,
@@ -203,6 +217,39 @@ pcg_default_flags[NR_CHARGE_TYPE] = {
 static void mem_cgroup_get(struct mem_cgroup *mem);
 static void mem_cgroup_put(struct mem_cgroup *mem);
 
+static void mem_cgroup_insert_exceeded(struct mem_cgroup *mem)
+{
+	struct rb_node **p = &mem_cgroup_soft_limit_exceeded_groups.rb_node;
+	struct rb_node *parent = NULL;
+	struct mem_cgroup *mem_node;
+
+	mutex_lock(&memcg_soft_limit_tree_mutex);
+	while (*p) {
+		parent = *p;
+		mem_node = rb_entry(parent, struct mem_cgroup, mem_cgroup_node);
+		if (mem->usage_in_excess < mem_node->usage_in_excess)
+			p = &(*p)->rb_left;
+		/*
+		 * We can't avoid mem cgroups that are over their soft
+		 * limit by the same amount
+		 */
+		else if (mem->usage_in_excess >= mem_node->usage_in_excess)
+			p = &(*p)->rb_right;
+	}
+	rb_link_node(&mem->mem_cgroup_node, parent, p);
+	rb_insert_color(&mem->mem_cgroup_node,
+			&mem_cgroup_soft_limit_exceeded_groups);
+	mem->last_tree_update = jiffies;
+	mutex_unlock(&memcg_soft_limit_tree_mutex);
+}
+
+static void mem_cgroup_remove_exceeded(struct mem_cgroup *mem)
+{
+	mutex_lock(&memcg_soft_limit_tree_mutex);
+	rb_erase(&mem->mem_cgroup_node, &mem_cgroup_soft_limit_exceeded_groups);
+	mutex_unlock(&memcg_soft_limit_tree_mutex);
+}
+
 static void mem_cgroup_charge_statistics(struct mem_cgroup *mem,
 					 struct page_cgroup *pc,
 					 bool charge)
@@ -917,6 +964,10 @@ static void __mem_cgroup_commit_charge(s
 				     struct page_cgroup *pc,
 				     enum charge_type ctype)
 {
+	unsigned long long prev_usage_in_excess, new_usage_in_excess;
+	bool updated_tree = false;
+	unsigned long next_update;
+
 	/* try_charge() can return NULL to *memcg, taking care of it. */
 	if (!mem)
 		return;
@@ -937,6 +988,30 @@ static void __mem_cgroup_commit_charge(s
 	mem_cgroup_charge_statistics(mem, pc, true);
 
 	unlock_page_cgroup(pc);
+
+	mem_cgroup_get(mem);
+	prev_usage_in_excess = mem->usage_in_excess;
+	new_usage_in_excess = res_counter_soft_limit_excess(&mem->res);
+
+	next_update = mem->last_tree_update + MEM_CGROUP_TREE_UPDATE_INTERVAL;
+	if (new_usage_in_excess && time_after(jiffies, next_update)) {
+		if (prev_usage_in_excess)
+			mem_cgroup_remove_exceeded(mem);
+		mem_cgroup_insert_exceeded(mem);
+		updated_tree = true;
+	} else if (prev_usage_in_excess && !new_usage_in_excess) {
+		mem_cgroup_remove_exceeded(mem);
+		updated_tree = true;
+	}
+
+	if (updated_tree) {
+		mutex_lock(&memcg_soft_limit_tree_mutex);
+		mem->last_tree_update = jiffies;
+		mem->usage_in_excess = new_usage_in_excess;
+		mutex_unlock(&memcg_soft_limit_tree_mutex);
+	}
+	mem_cgroup_put(mem);
+
 }
 
 /**
@@ -2218,6 +2293,7 @@ mem_cgroup_create(struct cgroup_subsys *
 	if (cont->parent == NULL) {
 		enable_swap_cgroup();
 		parent = NULL;
+		mem_cgroup_soft_limit_exceeded_groups = RB_ROOT;
 	} else {
 		parent = mem_cgroup_from_cont(cont->parent);
 		mem->use_hierarchy = parent->use_hierarchy;
@@ -2231,6 +2307,8 @@ mem_cgroup_create(struct cgroup_subsys *
 		res_counter_init(&mem->memsw, NULL);
 	}
 	mem->last_scanned_child = NULL;
+	mem->usage_in_excess = 0;
+	mem->last_tree_update = 0;	/* Yes, time begins at 0 here */
 	spin_lock_init(&mem->reclaim_param_lock);
 
 	if (parent)
_

-- 
	Balbir
--
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