[RFC][PATCH 5/7][memcg] memcg lockless update of file mapped

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

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

At accounting file events per memory cgroup, we need to find memory cgroup
via page_cgroup->mem_cgroup. Now, we use lock_page_cgroup().

But, considering the context which page-cgroup for files are accessed,
we can use alternative light-weight mutual execusion in the most case.
At handling file-caches, the only race we have to take care of is "moving"
account, IOW, overwriting page_cgroup->mem_cgroup. Because file status
update is done while the page-cache is in stable state, we don't have to
take care of race with charge/uncharge.

Unlike charge/uncharge, "move" happens not so frequently. It happens only when
rmdir() and task-moving (with a special settings.)
This patch adds a race-checker for file-cache-status accounting v.s. account
moving. The new per-cpu-per-memcg counter MEM_CGROUP_ON_MOVE is added.
The routine for account move 
  1. Increment it before start moving
  2. Call synchronize_rcu()
  3. Decrement it after the end of moving.
By this, file-status-counting routine can check it needs to call
lock_page_cgroup(). In most case, I doesn't need to call it.

Note: update_file_mapped is safe against charge/uncharge even if it's
not under address_space->tree_lock or lock_page(). Because it's under
page_table_lock(), anyone can't unmap it...then, anyone can't uncharge().



Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
---
 mm/memcontrol.c |   64 ++++++++++++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 60 insertions(+), 4 deletions(-)

Index: mmotm-0719/mm/memcontrol.c
===================================================================
--- mmotm-0719.orig/mm/memcontrol.c
+++ mmotm-0719/mm/memcontrol.c
@@ -89,6 +89,7 @@ enum mem_cgroup_stat_index {
 	MEM_CGROUP_STAT_PGPGOUT_COUNT,	/* # of pages paged out */
 	MEM_CGROUP_STAT_SWAPOUT, /* # of pages, swapped out */
 	MEM_CGROUP_EVENTS,	/* incremented at every  pagein/pageout */
+	MEM_CGROUP_ON_MOVE,   /* A check for locking move account/status */
 
 	MEM_CGROUP_STAT_NSTATS,
 };
@@ -1071,7 +1072,48 @@ static unsigned int get_swappiness(struc
 	return swappiness;
 }
 
-/* A routine for testing mem is not under move_account */
+static void mem_cgroup_start_move(struct mem_cgroup *mem)
+{
+	int cpu;
+	/* for fast checking in mem_cgroup_update_file_stat() etc..*/
+	spin_lock(&mc.lock);
+	for_each_possible_cpu(cpu)
+		per_cpu(mem->stat->count[MEM_CGROUP_ON_MOVE], cpu) += 1;
+	spin_unlock(&mc.lock);
+
+	synchronize_rcu();
+}
+
+static void mem_cgroup_end_move(struct mem_cgroup *mem)
+{
+	int cpu;
+
+	if (!mem)
+		return;
+	/* for fast checking in mem_cgroup_update_file_stat() etc..*/
+	spin_lock(&mc.lock);
+	for_each_possible_cpu(cpu)
+		per_cpu(mem->stat->count[MEM_CGROUP_ON_MOVE], cpu) -= 1;
+	spin_unlock(&mc.lock);
+}
+
+/*
+ * mem_cgroup_is_moved -- checking a cgroup is mc.from target or not.
+ *                          used for avoiding race.
+ * mem_cgroup_under_move -- checking a cgroup is mc.from or mc.to or
+ *			    under hierarchy of them. used for waiting at
+ *			    memory pressure.
+ * Result of is_moved can be trusted until the end of rcu_read_unlock().
+ * The caller must do
+ *	rcu_read_lock();
+ *	result = mem_cgroup_is_moved();
+ *	.....make use of result here....
+ *	rcu_read_unlock();
+ */
+static bool mem_cgroup_is_moved(struct mem_cgroup *mem)
+{
+	return this_cpu_read(mem->stat->count[MEM_CGROUP_ON_MOVE]) > 0;
+}
 
 static bool mem_cgroup_under_move(struct mem_cgroup *mem)
 {
@@ -1470,13 +1512,21 @@ void mem_cgroup_update_file_mapped(struc
 {
 	struct mem_cgroup *mem;
 	struct page_cgroup *pc;
+	bool need_lock = false;
 
 	pc = lookup_page_cgroup(page);
 	if (unlikely(!pc))
 		return;
-
-	lock_page_cgroup(pc);
+	rcu_read_lock();
 	mem = id_to_mem(pc->mem_cgroup);
+	if (!mem)
+		goto done;
+	need_lock = mem_cgroup_is_moved(mem);
+	if (need_lock) {
+		/* need to serialize with move_account */
+		lock_page_cgroup(pc);
+		mem = id_to_mem(pc->mem_cgroup);
+	}
 	if (!mem || !PageCgroupUsed(pc))
 		goto done;
 
@@ -1492,7 +1542,9 @@ void mem_cgroup_update_file_mapped(struc
 	}
 
 done:
-	unlock_page_cgroup(pc);
+	if (need_lock)
+		unlock_page_cgroup(pc);
+	rcu_read_unlock();
 }
 
 /*
@@ -3024,6 +3076,7 @@ move_account:
 		lru_add_drain_all();
 		drain_all_stock_sync();
 		ret = 0;
+		mem_cgroup_start_move(mem);
 		for_each_node_state(node, N_HIGH_MEMORY) {
 			for (zid = 0; !ret && zid < MAX_NR_ZONES; zid++) {
 				enum lru_list l;
@@ -3037,6 +3090,7 @@ move_account:
 			if (ret)
 				break;
 		}
+		mem_cgroup_end_move(mem);
 		memcg_oom_recover(mem);
 		/* it seems parent cgroup doesn't have enough mem */
 		if (ret == -ENOMEM)
@@ -4503,6 +4557,7 @@ static void mem_cgroup_clear_mc(void)
 	mc.to = NULL;
 	mc.moving_task = NULL;
 	spin_unlock(&mc.lock);
+	mem_cgroup_end_move(from);
 	memcg_oom_recover(from);
 	memcg_oom_recover(to);
 	wake_up_all(&mc.waitq);
@@ -4533,6 +4588,7 @@ static int mem_cgroup_can_attach(struct 
 			VM_BUG_ON(mc.moved_charge);
 			VM_BUG_ON(mc.moved_swap);
 			VM_BUG_ON(mc.moving_task);
+			mem_cgroup_start_move(from);
 			spin_lock(&mc.lock);
 			mc.from = from;
 			mc.to = mem;

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