[PATCH v3 04/11] memcg: add lock to synchronize page accounting and migration

Greg Thelen
2010-10-19T00:41:27Z

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

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

Introduce a new bit spin lock, PCG_MOVE_LOCK, to synchronize
the page accounting and migration code. This reworks the
locking scheme of _update_stat() and _move_account() by
adding new lock bit PCG_MOVE_LOCK, which is always taken
under IRQ disable.

1. If pages are being migrated from a memcg, then updates to
that memcg page statistics are protected by grabbing
PCG_MOVE_LOCK using move_lock_page_cgroup(). In an
upcoming commit, memcg dirty page accounting will be
updating memcg page accounting (specifically: num
writeback pages) from IRQ context (softirq). Avoid a
deadlocking nested spin lock attempt by disabling irq on
the local processor when grabbing the PCG_MOVE_LOCK.

2. lock for update_page_stat is used only for avoiding race
with move_account(). So, IRQ awareness of
lock_page_cgroup() itself is not a problem. The problem
is between mem_cgroup_update_page_stat() and
mem_cgroup_move_account_page().

Trade-off:
* Changing lock_page_cgroup() to always disable IRQ (or
local_bh) has some impacts on performance and I think
it's bad to disable IRQ when it's not necessary.
* adding a new lock makes move_account() slower. Score is
here.

Performance Impact: moving a 8G anon process.

Before:
real 0m0.792s
user 0m0.000s
sys 0m0.780s

After:
real 0m0.854s
user 0m0.000s
sys 0m0.842s

This score is bad but planned patches for optimization can reduce
this impact.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Greg Thelen <gthelen@google.com>
---
include/linux/page_cgroup.h | 31 ++++++++++++++++++++++++++++---
mm/memcontrol.c | 9 +++++++--
2 files changed, 35 insertions(+), 5 deletions(-)

diff --git a/include/linux/page_cgroup.h b/include/linux/page_cgroup.h
index b59c298..509452e 100644
--- a/include/linux/page_cgroup.h
+++ b/include/linux/page_cgroup.h
@@ -35,15 +35,18 @@ struct page_cgroup *lookup_page_cgroup(struct page *page);

enum {
/* flags for mem_cgroup */
- PCG_LOCK, /* page cgroup is locked */
+ PCG_LOCK, /* Lock for pc->mem_cgroup and following bits. */
PCG_CACHE, /* charged as cache */
PCG_USED, /* this object is in use. */
- PCG_ACCT_LRU, /* page has been accounted for */
+ 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" */
PCG_FILE_DIRTY, /* page is dirty */
PCG_FILE_WRITEBACK, /* page is under writeback */
PCG_FILE_UNSTABLE_NFS, /* page is NFS unstable */
- PCG_MIGRATION, /* under page migration */
+ /* No lock in page_cgroup */
+ PCG_ACCT_LRU, /* page has been accounted for (under lru_lock) */
};

#define TESTPCGFLAG(uname, lname) \
@@ -119,6 +122,10 @@ static inline enum zone_type page_cgroup_zid(struct page_cgroup *pc)

static inline void lock_page_cgroup(struct page_cgroup *pc)
{
+ /*
+ * Don't take this lock in IRQ context.
+ * This lock is for pc->mem_cgroup, USED, CACHE, MIGRATION
+ */
bit_spin_lock(PCG_LOCK, &pc->flags);
}

@@ -127,6 +134,24 @@ 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 369879a..697f7b8 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1615,6 +1615,7 @@ void mem_cgroup_update_page_stat(struct page *page,
struct mem_cgroup *mem;
struct page_cgroup *pc = lookup_page_cgroup(page);
bool need_unlock = false;
+ unsigned long uninitialized_var(flags);

if (unlikely(!pc))
return;
@@ -1626,7 +1627,7 @@ void mem_cgroup_update_page_stat(struct page *page,
/* pc->mem_cgroup is unstable ? */
if (unlikely(mem_cgroup_stealed(mem))) {
/* take a lock against to access pc->mem_cgroup */
- lock_page_cgroup(pc);
+ move_lock_page_cgroup(pc, &flags);
need_unlock = true;
mem = pc->mem_cgroup;
if (!mem || !PageCgroupUsed(pc))
@@ -1649,7 +1650,7 @@ void mem_cgroup_update_page_stat(struct page *page,

out:
if (unlikely(need_unlock))
- unlock_page_cgroup(pc);
+ move_unlock_page_cgroup(pc, &flags);
rcu_read_unlock();
return;
}
@@ -2203,9 +2204,13 @@ static int mem_cgroup_move_account(struct page_cgroup *pc,
struct mem_cgroup *from, struct mem_cgroup *to, bool uncharge)
{
int ret = -EINVAL;
+ unsigned long flags;
+
lock_page_cgroup(pc);
if (PageCgroupUsed(pc) && pc->mem_cgroup == from) {
+ move_lock_page_cgroup(pc, &flags);
__mem_cgroup_move_account(pc, from, to, uncharge);
+ move_unlock_page_cgroup(pc, &flags);
ret = 0;
}
unlock_page_cgroup(pc);
--
1.7.1