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SubjectRe: [PATCH] memcg: use ID in page_cgroup
Date
KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> writes:

> 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));

Should this be VM_BUG_ON(!rcu_read_lock_held())? I suspect that
mem_cgroup_move_account() should grab rcu read lock (see my comment below).

> 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);

Should this be rcu_read_lock() instead of lock_page_cgroup()?

> - 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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