Re: [PATCH] memcg: use ID in page_cgroup

[Greg Thelen <gthelen@google.com>]

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;