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Subject[PATCH] memcg: use ID in page_cgroup

I have an idea to remove page_cgroup->page pointer, 8bytes reduction per page.
But it will be after this work.
==
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

Now, addresses of memory cgroup can be calculated by their ID without complex.
This patch relplaces pc->mem_cgroup from a pointer to a unsigned short.
On 64bit architecture, this offers us more 6bytes room per page_cgroup.
Use 2bytes for blkio-cgroup's page tracking. More 4bytes will be used for
some light-weight concurrent access.

We may able to move this id onto flags field but ...go step by step.

Changelog: 20100811
- using new rcu APIs, as rcu_dereference_check() etc.
Changelog: 20100804
- added comments to page_cgroup.h
Changelog: 20100730
- fixed some garbage added by debug code in early stage

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
---
include/linux/page_cgroup.h | 6 ++++-
mm/memcontrol.c | 52 +++++++++++++++++++++++++++-----------------
mm/page_cgroup.c | 2 -
3 files changed, 38 insertions(+), 22 deletions(-)
Index: mmotm-0811/include/linux/page_cgroup.h
===================================================================
--- mmotm-0811.orig/include/linux/page_cgroup.h
+++ mmotm-0811/include/linux/page_cgroup.h
@@ -9,10 +9,14 @@
* page_cgroup helps us identify information about the cgroup
* All page cgroups are allocated at boot or memory hotplug event,
* then the page cgroup for pfn always exists.
+ *
+ * TODO: It seems ID for cgroup can be packed into "flags". But there will
+ * be race between assigning ID <-> set/clear flags. Please be careful.
*/
struct page_cgroup {
unsigned long flags;
- struct mem_cgroup *mem_cgroup;
+ unsigned short mem_cgroup; /* ID of assigned memory cgroup */
+ unsigned short blk_cgroup; /* Not Used..but will be. */
struct page *page;
struct list_head lru; /* per cgroup LRU list */
};
Index: mmotm-0811/mm/page_cgroup.c
===================================================================
--- mmotm-0811.orig/mm/page_cgroup.c
+++ mmotm-0811/mm/page_cgroup.c
@@ -15,7 +15,7 @@ static void __meminit
__init_page_cgroup(struct page_cgroup *pc, unsigned long pfn)
{
pc->flags = 0;
- pc->mem_cgroup = NULL;
+ pc->mem_cgroup = 0;
pc->page = pfn_to_page(pfn);
INIT_LIST_HEAD(&pc->lru);
}
Index: mmotm-0811/mm/memcontrol.c
===================================================================
--- mmotm-0811.orig/mm/memcontrol.c
+++ mmotm-0811/mm/memcontrol.c
@@ -300,12 +300,13 @@ static atomic_t mem_cgroup_num;
#define NR_MEMCG_GROUPS (CONFIG_MEM_CGROUP_MAX_GROUPS + 1)
static struct mem_cgroup *mem_cgroups[NR_MEMCG_GROUPS] __read_mostly;

-/* Must be called under rcu_read_lock */
-static struct mem_cgroup *id_to_memcg(unsigned short id)
+/* Must be called under rcu_read_lock, set safe==true if under lock */
+static struct mem_cgroup *id_to_memcg(unsigned short id, bool safe)
{
struct mem_cgroup *ret;
/* see mem_cgroup_free() */
- ret = rcu_dereference_check(mem_cgroups[id], rch_read_lock_held());
+ ret = rcu_dereference_check(mem_cgroups[id],
+ rch_read_lock_held() || safe);
if (likely(ret && ret->valid))
return ret;
return NULL;
@@ -381,7 +382,12 @@ struct cgroup_subsys_state *mem_cgroup_c
static struct mem_cgroup_per_zone *
page_cgroup_zoneinfo(struct page_cgroup *pc)
{
- struct mem_cgroup *mem = pc->mem_cgroup;
+ /*
+ * The caller should guarantee this "pc" is under lock. In typical
+ * case, this function is called by lru function with zone->lru_lock.
+ * It is a safe access.
+ */
+ struct mem_cgroup *mem = id_to_memcg(pc->mem_cgroup, true);
int nid = page_cgroup_nid(pc);
int zid = page_cgroup_zid(pc);

@@ -723,6 +729,11 @@ static inline bool mem_cgroup_is_root(st
return (mem == root_mem_cgroup);
}

+static inline bool mem_cgroup_is_rootid(unsigned short id)
+{
+ return (id == 1);
+}
+
/*
* Following LRU functions are allowed to be used without PCG_LOCK.
* Operations are called by routine of global LRU independently from memcg.
@@ -755,7 +766,7 @@ void mem_cgroup_del_lru_list(struct page
*/
mz = page_cgroup_zoneinfo(pc);
MEM_CGROUP_ZSTAT(mz, lru) -= 1;
- if (mem_cgroup_is_root(pc->mem_cgroup))
+ if (mem_cgroup_is_rootid(pc->mem_cgroup))
return;
VM_BUG_ON(list_empty(&pc->lru));
list_del_init(&pc->lru);
@@ -782,7 +793,7 @@ void mem_cgroup_rotate_lru_list(struct p
*/
smp_rmb();
/* unused or root page is not rotated. */
- if (!PageCgroupUsed(pc) || mem_cgroup_is_root(pc->mem_cgroup))
+ if (!PageCgroupUsed(pc) || mem_cgroup_is_rootid(pc->mem_cgroup))
return;
mz = page_cgroup_zoneinfo(pc);
list_move(&pc->lru, &mz->lists[lru]);
@@ -808,7 +819,7 @@ void mem_cgroup_add_lru_list(struct page
mz = page_cgroup_zoneinfo(pc);
MEM_CGROUP_ZSTAT(mz, lru) += 1;
SetPageCgroupAcctLRU(pc);
- if (mem_cgroup_is_root(pc->mem_cgroup))
+ if (mem_cgroup_is_rootid(pc->mem_cgroup))
return;
list_add(&pc->lru, &mz->lists[lru]);
}
@@ -1497,7 +1508,7 @@ void mem_cgroup_update_file_mapped(struc
return;

lock_page_cgroup(pc);
- mem = pc->mem_cgroup;
+ mem = id_to_memcg(pc->mem_cgroup, true);
if (!mem || !PageCgroupUsed(pc))
goto done;

@@ -1885,14 +1896,14 @@ struct mem_cgroup *try_get_mem_cgroup_fr
pc = lookup_page_cgroup(page);
lock_page_cgroup(pc);
if (PageCgroupUsed(pc)) {
- mem = pc->mem_cgroup;
+ mem = id_to_memcg(pc->mem_cgroup, true);
if (mem && !css_tryget(&mem->css))
mem = NULL;
} else if (PageSwapCache(page)) {
ent.val = page_private(page);
id = lookup_swap_cgroup(ent);
rcu_read_lock();
- mem = id_to_memcg(id);
+ mem = id_to_memcg(id, false);
if (mem && !css_tryget(&mem->css))
mem = NULL;
rcu_read_unlock();
@@ -1921,7 +1932,7 @@ static void __mem_cgroup_commit_charge(s
return;
}

- pc->mem_cgroup = mem;
+ pc->mem_cgroup = css_id(&mem->css);
/*
* We access a page_cgroup asynchronously without lock_page_cgroup().
* Especially when a page_cgroup is taken from a page, pc->mem_cgroup
@@ -1979,7 +1990,7 @@ static void __mem_cgroup_move_account(st
VM_BUG_ON(PageLRU(pc->page));
VM_BUG_ON(!PageCgroupLocked(pc));
VM_BUG_ON(!PageCgroupUsed(pc));
- VM_BUG_ON(pc->mem_cgroup != from);
+ VM_BUG_ON(id_to_memcg(pc->mem_cgroup, true) != from);

if (PageCgroupFileMapped(pc)) {
/* Update mapped_file data for mem_cgroup */
@@ -1994,7 +2005,7 @@ static void __mem_cgroup_move_account(st
mem_cgroup_cancel_charge(from);

/* caller should have done css_get */
- pc->mem_cgroup = to;
+ pc->mem_cgroup = css_id(&to->css);
mem_cgroup_charge_statistics(to, pc, true);
/*
* We charges against "to" which may not have any tasks. Then, "to"
@@ -2014,11 +2025,11 @@ static int mem_cgroup_move_account(struc
{
int ret = -EINVAL;
lock_page_cgroup(pc);
- if (PageCgroupUsed(pc) && pc->mem_cgroup == from) {
+ if (PageCgroupUsed(pc) && id_to_memcg(pc->mem_cgroup, true) == from) {
__mem_cgroup_move_account(pc, from, to, uncharge);
ret = 0;
}
- unlock_page_cgroup(pc);
+ rcu_read_unlock();
/*
* check events
*/
@@ -2244,7 +2255,7 @@ __mem_cgroup_commit_charge_swapin(struct

id = swap_cgroup_record(ent, 0);
rcu_read_lock();
- memcg = id_to_memcg(id);
+ memcg = id_to_memcg(id, false);
if (memcg) {
/*
* This recorded memcg can be obsolete one. So, avoid
@@ -2354,7 +2365,7 @@ __mem_cgroup_uncharge_common(struct page

lock_page_cgroup(pc);

- mem = pc->mem_cgroup;
+ mem = id_to_memcg(pc->mem_cgroup, true);

if (!PageCgroupUsed(pc))
goto unlock_out;
@@ -2509,7 +2520,7 @@ void mem_cgroup_uncharge_swap(swp_entry_

id = swap_cgroup_record(ent, 0);
rcu_read_lock();
- memcg = id_to_memcg(id);
+ memcg = id_to_memcg(id, false);
if (memcg) {
/*
* We uncharge this because swap is freed.
@@ -2600,7 +2611,7 @@ int mem_cgroup_prepare_migration(struct
pc = lookup_page_cgroup(page);
lock_page_cgroup(pc);
if (PageCgroupUsed(pc)) {
- mem = pc->mem_cgroup;
+ mem = id_to_memcg(pc->mem_cgroup, true);
css_get(&mem->css);
/*
* At migrating an anonymous page, its mapcount goes down
@@ -4442,7 +4453,8 @@ static int is_target_pte_for_mc(struct v
* mem_cgroup_move_account() checks the pc is valid or not under
* the lock.
*/
- if (PageCgroupUsed(pc) && pc->mem_cgroup == mc.from) {
+ if (PageCgroupUsed(pc) &&
+ id_to_memcg(pc->mem_cgroup, true) == mc.from) {
ret = MC_TARGET_PAGE;
if (target)
target->page = page;


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