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Subject[PATCH] memcg: avoid lock in updating file_mapped (Was fix race in file_mapped accouting flag management

Very sorry, subject was wrong..(reposting).

==
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() for guarantee
pc->mem_cgroup is not overwritten while we make use of it.

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.
(See comment in the patch)

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.

Following is a perf data of a process which mmap()/munmap 32MB of file cache
in a minute.

Before patch:
28.25% mmap mmap [.] main
22.64% mmap [kernel.kallsyms] [k] page_fault
9.96% mmap [kernel.kallsyms] [k] mem_cgroup_update_file_mapped
3.67% mmap [kernel.kallsyms] [k] filemap_fault
3.50% mmap [kernel.kallsyms] [k] unmap_vmas
2.99% mmap [kernel.kallsyms] [k] __do_fault
2.76% mmap [kernel.kallsyms] [k] find_get_page

After patch:
30.00% mmap mmap [.] main
23.78% mmap [kernel.kallsyms] [k] page_fault
5.52% mmap [kernel.kallsyms] [k] mem_cgroup_update_file_mapped
3.81% mmap [kernel.kallsyms] [k] unmap_vmas
3.26% mmap [kernel.kallsyms] [k] find_get_page
3.18% mmap [kernel.kallsyms] [k] __do_fault
3.03% mmap [kernel.kallsyms] [k] filemap_fault
2.40% mmap [kernel.kallsyms] [k] handle_mm_fault
2.40% mmap [kernel.kallsyms] [k] do_page_fault

This patch reduces memcg's cost to some extent.
(mem_cgroup_update_file_mapped is called by both of map/unmap)

Note: It seems some more improvements are required..but no idea.
maybe removing set/unset flag is required.

Changelog: 20100913
- decoupled with ID patches.
- updated comments.

Changelog: 20100901
- changes id_to_memcg(pc, true) to be id_to_memcg(pc, false)
in update_file_mapped()
- updated comments on lock rule of update_file_mapped()
Changelog: 20100825
- added a comment about mc.lock
- fixed bad lock.
Changelog: 20100804
- added a comment for possible optimization hint.
Changelog: 20100730
- some cleanup.
Changelog: 20100729
- replaced __this_cpu_xxx() with this_cpu_xxx
(because we don't call spinlock)
- added VM_BUG_ON().

Reviewed-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
---
mm/memcontrol.c | 99 ++++++++++++++++++++++++++++++++++++++++++++++++--------
1 file changed, 85 insertions(+), 14 deletions(-)
Index: lockless-update/mm/memcontrol.c
===================================================================
--- lockless-update.orig/mm/memcontrol.c
+++ lockless-update/mm/memcontrol.c
@@ -90,6 +90,7 @@ 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, /* someone is moving account between groups */

MEM_CGROUP_STAT_NSTATS,
};
@@ -1051,7 +1052,46 @@ 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;
+ /* Because this is for moving account, reuse mc.lock */
+ 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;
+ spin_lock(&mc.lock);
+ for_each_possible_cpu(cpu)
+ per_cpu(mem->stat->count[MEM_CGROUP_ON_MOVE], cpu) -= 1;
+ spin_unlock(&mc.lock);
+}
+/*
+ * 2 routines for checking "mem" is under move_account() or not.
+ *
+ * mem_cgroup_stealed() - checking a cgroup is mc.from or not. This is used
+ * for avoiding race in accounting. If true,
+ * pc->mem_cgroup may be overwritten.
+ *
+ * mem_cgroup_under_move() - checking a cgroup is mc.from or mc.to or
+ * under hierarchy of moving cgroups. This is for
+ * waiting at hith-memory prressure caused by "move".
+ */
+
+static bool mem_cgroup_stealed(struct mem_cgroup *mem)
+{
+ VM_BUG_ON(!rcu_read_lock_held());
+ return this_cpu_read(mem->stat->count[MEM_CGROUP_ON_MOVE]) > 0;
+}

static bool mem_cgroup_under_move(struct mem_cgroup *mem)
{
@@ -1462,35 +1502,62 @@ bool mem_cgroup_handle_oom(struct mem_cg
/*
* Currently used to update mapped file statistics, but the routine can be
* generalized to update other statistics as well.
+ *
+ * Notes: Race condition
+ *
+ * We usually use page_cgroup_lock() for accessing page_cgroup member but
+ * it tends to be costly. But considering some conditions, we doesn't need
+ * to do so _always_.
+ *
+ * Considering "charge", lock_page_cgroup() is not required because all
+ * file-stat operations happen after a page is attached to radix-tree. There
+ * are no race with "charge".
+ *
+ * Considering "uncharge", we know that memcg doesn't clear pc->mem_cgroup
+ * at "uncharge" intentionally. So, we always see valid pc->mem_cgroup even
+ * if there are race with "uncharge". Statistics itself is properly handled
+ * by flags.
+ *
+ * Considering "move", this is an only case we see a race. To make the race
+ * small, we check MEM_CGROUP_ON_MOVE percpu value and detect there are
+ * possibility of race condition. If there is, we take a lock.
*/
void mem_cgroup_update_file_mapped(struct page *page, int val)
{
struct mem_cgroup *mem;
- struct page_cgroup *pc;
+ struct page_cgroup *pc = lookup_page_cgroup(page);
+ bool need_unlock = false;

- pc = lookup_page_cgroup(page);
if (unlikely(!pc))
return;

- lock_page_cgroup(pc);
+ rcu_read_lock();
mem = pc->mem_cgroup;
- if (!mem || !PageCgroupUsed(pc))
- goto done;
-
- /*
- * Preemption is already disabled. We can use __this_cpu_xxx
- */
+ if (unlikely(!mem || !PageCgroupUsed(pc)))
+ goto out;
+ /* pc->mem_cgroup is unstable ? */
+ if (unlikely(mem_cgroup_stealed(mem))) {
+ /* take a lock against to access pc->mem_cgroup */
+ lock_page_cgroup(pc);
+ need_unlock = true;
+ mem = pc->mem_cgroup;
+ if (!mem || !PageCgroupUsed(pc))
+ goto out;
+ }
if (val > 0) {
- __this_cpu_inc(mem->stat->count[MEM_CGROUP_STAT_FILE_MAPPED]);
+ this_cpu_inc(mem->stat->count[MEM_CGROUP_STAT_FILE_MAPPED]);
SetPageCgroupFileMapped(pc);
} else {
- __this_cpu_dec(mem->stat->count[MEM_CGROUP_STAT_FILE_MAPPED]);
+ this_cpu_dec(mem->stat->count[MEM_CGROUP_STAT_FILE_MAPPED]);
if (!page_mapped(page)) /* for race between dec->inc counter */
ClearPageCgroupFileMapped(pc);
}

-done:
- unlock_page_cgroup(pc);
+out:
+ if (unlikely(need_unlock))
+ unlock_page_cgroup(pc);
+ rcu_read_unlock();
+ return;
}

/*
@@ -3039,6 +3106,7 @@ 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;
@@ -3052,6 +3120,7 @@ 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)
@@ -4510,6 +4579,7 @@ 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);
@@ -4540,6 +4610,7 @@ 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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