[PATCH 1/4 -mm][memcg] quick ID lookup in memcg

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

Now, memory cgroup has an ID per cgroup and make use of it at
 - hierarchy walk,
 - swap recording.

This patch is for making more use of it. The final purpose is
to replace page_cgroup->mem_cgroup's pointer to an unsigned short.

This patch caches a pointer of memcg in an array. By this, we
don't have to call css_lookup() which requires radix-hash walk.
This saves some amount of memory footprint at lookup memcg via id.

Changelog: 20100804
 - fixed description in init/Kconfig

Changelog: 20100730
 - fixed rcu_read_unlock() placement.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
---
 init/Kconfig    |   10 ++++++++++
 mm/memcontrol.c |   48 ++++++++++++++++++++++++++++++++++--------------
 2 files changed, 44 insertions(+), 14 deletions(-)

Index: mmotm-0727/mm/memcontrol.c
===================================================================
--- mmotm-0727.orig/mm/memcontrol.c
+++ mmotm-0727/mm/memcontrol.c
@@ -292,6 +292,30 @@ static bool move_file(void)
 					&mc.to->move_charge_at_immigrate);
 }
 
+/* 0 is unused */
+static atomic_t mem_cgroup_num;
+#define NR_MEMCG_GROUPS (CONFIG_MEM_CGROUP_MAX_GROUPS + 1)
+static struct mem_cgroup *mem_cgroups[NR_MEMCG_GROUPS] __read_mostly;
+
+static struct mem_cgroup *id_to_memcg(unsigned short id)
+{
+	/*
+	 * This array is set to NULL when mem_cgroup is freed.
+	 * IOW, there are no more references && rcu_synchronized().
+	 * This lookup-caching is safe.
+	 */
+	if (unlikely(!mem_cgroups[id])) {
+		struct cgroup_subsys_state *css;
+
+		rcu_read_lock();
+		css = css_lookup(&mem_cgroup_subsys, id);
+		rcu_read_unlock();
+		if (!css)
+			return NULL;
+		mem_cgroups[id] = container_of(css, struct mem_cgroup, css);
+	}
+	return mem_cgroups[id];
+}
 /*
  * Maximum loops in mem_cgroup_hierarchical_reclaim(), used for soft
  * limit reclaim to prevent infinite loops, if they ever occur.
@@ -1824,18 +1848,7 @@ static void mem_cgroup_cancel_charge(str
  * it's concern. (dropping refcnt from swap can be called against removed
  * memcg.)
  */
-static struct mem_cgroup *mem_cgroup_lookup(unsigned short id)
-{
-	struct cgroup_subsys_state *css;
 
-	/* ID 0 is unused ID */
-	if (!id)
-		return NULL;
-	css = css_lookup(&mem_cgroup_subsys, id);
-	if (!css)
-		return NULL;
-	return container_of(css, struct mem_cgroup, css);
-}
 
 struct mem_cgroup *try_get_mem_cgroup_from_page(struct page *page)
 {
@@ -1856,7 +1869,7 @@ struct mem_cgroup *try_get_mem_cgroup_fr
 		ent.val = page_private(page);
 		id = lookup_swap_cgroup(ent);
 		rcu_read_lock();
-		mem = mem_cgroup_lookup(id);
+		mem = id_to_memcg(id);
 		if (mem && !css_tryget(&mem->css))
 			mem = NULL;
 		rcu_read_unlock();
@@ -2208,7 +2221,7 @@ __mem_cgroup_commit_charge_swapin(struct
 
 		id = swap_cgroup_record(ent, 0);
 		rcu_read_lock();
-		memcg = mem_cgroup_lookup(id);
+		memcg = id_to_memcg(id);
 		if (memcg) {
 			/*
 			 * This recorded memcg can be obsolete one. So, avoid
@@ -2472,7 +2485,7 @@ void mem_cgroup_uncharge_swap(swp_entry_
 
 	id = swap_cgroup_record(ent, 0);
 	rcu_read_lock();
-	memcg = mem_cgroup_lookup(id);
+	memcg = id_to_memcg(id);
 	if (memcg) {
 		/*
 		 * We uncharge this because swap is freed.
@@ -3988,6 +4001,9 @@ static struct mem_cgroup *mem_cgroup_all
 	struct mem_cgroup *mem;
 	int size = sizeof(struct mem_cgroup);
 
+	if (atomic_read(&mem_cgroup_num) == NR_MEMCG_GROUPS)
+		return NULL;
+
 	/* Can be very big if MAX_NUMNODES is very big */
 	if (size < PAGE_SIZE)
 		mem = kmalloc(size, GFP_KERNEL);
@@ -4025,7 +4041,10 @@ static void __mem_cgroup_free(struct mem
 	int node;
 
 	mem_cgroup_remove_from_trees(mem);
+	/* No more lookup against this ID */
+	mem_cgroups[css_id(&mem->css)] = NULL;
 	free_css_id(&mem_cgroup_subsys, &mem->css);
+	atomic_dec(&mem_cgroup_num);
 
 	for_each_node_state(node, N_POSSIBLE)
 		free_mem_cgroup_per_zone_info(mem, node);
@@ -4162,6 +4181,7 @@ mem_cgroup_create(struct cgroup_subsys *
 	atomic_set(&mem->refcnt, 1);
 	mem->move_charge_at_immigrate = 0;
 	mutex_init(&mem->thresholds_lock);
+	atomic_inc(&mem_cgroup_num);
 	return &mem->css;
 free_out:
 	__mem_cgroup_free(mem);
Index: mmotm-0727/init/Kconfig
===================================================================
--- mmotm-0727.orig/init/Kconfig
+++ mmotm-0727/init/Kconfig
@@ -594,6 +594,16 @@ config CGROUP_MEM_RES_CTLR_SWAP
 	  Now, memory usage of swap_cgroup is 2 bytes per entry. If swap page
 	  size is 4096bytes, 512k per 1Gbytes of swap.
 
+config MEM_CGROUP_MAX_GROUPS
+	int "Maximum number of memory cgroups on a system"
+	range 1 65535
+	default 8192 if 64BIT
+	default 2048 if 32BIT
+	help
+	  Memory cgroup has limitation of the number of groups created.
+	  Please select your favorite value. The more you allow, the more
+	  memory(a pointer per group) will be consumed.
+
 menuconfig CGROUP_SCHED
 	bool "Group CPU scheduler"
 	depends on EXPERIMENTAL && CGROUPS

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