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Subject[PATCH 3/6] memcg: remove PCG_MOVE_LOCK flag from page_cgroup
From 62a96c625be0c30fc5828d88685b6873ed254060 Mon Sep 17 00:00:00 2001
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Date: Thu, 2 Feb 2012 11:49:59 +0900
Subject: [PATCH 3/6] memcg: remove PCG_MOVE_LOCK flag from page_cgroup.

PCG_MOVE_LOCK is used for bit spinlock to avoid race between overwriting
pc->mem_cgroup and page statistics accounting per memcg.
This lock helps to avoid the race but the race is very rare because moving
tasks between cgroup is not a usual job.
So, it seems using 1bit per page is too costly.

This patch changes this lock as per-memcg spinlock and removes PCG_MOVE_LOCK.

If smaller lock is required, we'll be able to add some hashes but
I'd like to start from this.

Changelog:
- fixed to pass memcg as an argument rather than page_cgroup.
and renamed from move_lock_page_cgroup() to move_lock_mem_cgroup()

Acked-by: Greg Thelen <gthelen@google.com>
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
---
include/linux/page_cgroup.h | 19 -------------------
mm/memcontrol.c | 42 ++++++++++++++++++++++++++++++++----------
2 files changed, 32 insertions(+), 29 deletions(-)

diff --git a/include/linux/page_cgroup.h b/include/linux/page_cgroup.h
index 1060292..7a3af74 100644
--- a/include/linux/page_cgroup.h
+++ b/include/linux/page_cgroup.h
@@ -7,7 +7,6 @@ enum {
PCG_USED, /* this object is in use. */
PCG_MIGRATION, /* under page migration */
/* flags for mem_cgroup and file and I/O status */
- PCG_MOVE_LOCK, /* For race between move_account v.s. following bits */
PCG_FILE_MAPPED, /* page is accounted as "mapped" */
__NR_PCG_FLAGS,
};
@@ -89,24 +88,6 @@ static inline void unlock_page_cgroup(struct page_cgroup *pc)
bit_spin_unlock(PCG_LOCK, &pc->flags);
}

-static inline void move_lock_page_cgroup(struct page_cgroup *pc,
- unsigned long *flags)
-{
- /*
- * We know updates to pc->flags of page cache's stats are from both of
- * usual context or IRQ context. Disable IRQ to avoid deadlock.
- */
- local_irq_save(*flags);
- bit_spin_lock(PCG_MOVE_LOCK, &pc->flags);
-}
-
-static inline void move_unlock_page_cgroup(struct page_cgroup *pc,
- unsigned long *flags)
-{
- bit_spin_unlock(PCG_MOVE_LOCK, &pc->flags);
- local_irq_restore(*flags);
-}
-
#else /* CONFIG_CGROUP_MEM_RES_CTLR */
struct page_cgroup;

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 9c94389..7065028 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -280,6 +280,8 @@ struct mem_cgroup {
* set > 0 if pages under this cgroup are moving to other cgroup.
*/
atomic_t moving_account;
+ /* taken only while moving_account > 0 */
+ spinlock_t move_lock;
/*
* percpu counter.
*/
@@ -1343,6 +1345,24 @@ static bool mem_cgroup_wait_acct_move(struct mem_cgroup *memcg)
return false;
}

+/*
+ * Take this lock when
+ * - a code tries to modify page's memcg while it's USED.
+ * - a code tries to modify page state accounting in a memcg.
+ * see mem_cgroup_stealed(), too.
+ */
+static void move_lock_mem_cgroup(struct mem_cgroup *memcg,
+ unsigned long *flags)
+{
+ spin_lock_irqsave(&memcg->move_lock, *flags);
+}
+
+static void move_unlock_mem_cgroup(struct mem_cgroup *memcg,
+ unsigned long *flags)
+{
+ spin_unlock_irqrestore(&memcg->move_lock, *flags);
+}
+
/**
* mem_cgroup_print_oom_info: Called from OOM with tasklist_lock held in read mode.
* @memcg: The memory cgroup that went over limit
@@ -1867,7 +1887,7 @@ void mem_cgroup_update_page_stat(struct page *page,

if (mem_cgroup_disabled())
return;
-
+again:
rcu_read_lock();
memcg = pc->mem_cgroup;
if (unlikely(!memcg || !PageCgroupUsed(pc)))
@@ -1875,11 +1895,13 @@ void mem_cgroup_update_page_stat(struct page *page,
/* pc->mem_cgroup is unstable ? */
if (unlikely(mem_cgroup_stealed(memcg))) {
/* take a lock against to access pc->mem_cgroup */
- move_lock_page_cgroup(pc, &flags);
+ move_lock_mem_cgroup(memcg, &flags);
+ if (memcg != pc->mem_cgroup || !PageCgroupUsed(pc)) {
+ move_unlock_mem_cgroup(memcg, &flags);
+ rcu_read_unlock();
+ goto again;
+ }
need_unlock = true;
- memcg = pc->mem_cgroup;
- if (!memcg || !PageCgroupUsed(pc))
- goto out;
}

switch (idx) {
@@ -1898,7 +1920,7 @@ void mem_cgroup_update_page_stat(struct page *page,

out:
if (unlikely(need_unlock))
- move_unlock_page_cgroup(pc, &flags);
+ move_unlock_mem_cgroup(memcg, &flags);
rcu_read_unlock();
}

@@ -2440,8 +2462,7 @@ static void __mem_cgroup_commit_charge(struct mem_cgroup *memcg,

#ifdef CONFIG_TRANSPARENT_HUGEPAGE

-#define PCGF_NOCOPY_AT_SPLIT ((1 << PCG_LOCK) | (1 << PCG_MOVE_LOCK) |\
- (1 << PCG_MIGRATION))
+#define PCGF_NOCOPY_AT_SPLIT ((1 << PCG_LOCK) | (1 << PCG_MIGRATION))
/*
* Because tail pages are not marked as "used", set it. We're under
* zone->lru_lock, 'splitting on pmd' and compound_lock.
@@ -2512,7 +2533,7 @@ static int mem_cgroup_move_account(struct page *page,
if (!PageCgroupUsed(pc) || pc->mem_cgroup != from)
goto unlock;

- move_lock_page_cgroup(pc, &flags);
+ move_lock_mem_cgroup(from, &flags);

if (PageCgroupFileMapped(pc)) {
/* Update mapped_file data for mem_cgroup */
@@ -2536,7 +2557,7 @@ static int mem_cgroup_move_account(struct page *page,
* guaranteed that "to" is never removed. So, we don't check rmdir
* status here.
*/
- move_unlock_page_cgroup(pc, &flags);
+ move_unlock_mem_cgroup(from, &flags);
ret = 0;
unlock:
unlock_page_cgroup(pc);
@@ -4928,6 +4949,7 @@ mem_cgroup_create(struct cgroup *cont)
atomic_set(&memcg->refcnt, 1);
memcg->move_charge_at_immigrate = 0;
mutex_init(&memcg->thresholds_lock);
+ spin_lock_init(&memcg->move_lock);
return &memcg->css;
free_out:
__mem_cgroup_free(memcg);
--
1.7.4.1



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