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Date: Tue, 27 Jul 2010 16:59:38 +0900
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc: "linux-mm@...ck.org" <linux-mm@...ck.org>,
"nishimura@....nes.nec.co.jp" <nishimura@....nes.nec.co.jp>,
"balbir@...ux.vnet.ibm.com" <balbir@...ux.vnet.ibm.com>,
gthelen@...gle.com, m-ikeda@...jp.nec.com,
"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: [RFC][PATCH 5/7][memcg] memcg lockless update of file mapped
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...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@...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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