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Message-Id: <20100824165108.dd986751.kamezawa.hiroyu@jp.fujitsu.com>
Date: Tue, 24 Aug 2010 16:51:08 +0900
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To: Greg Thelen <gthelen@...gle.com>
Cc: linux-mm@...ck.org,
"nishimura\@mxp.nes.nec.co.jp" <nishimura@....nes.nec.co.jp>,
"balbir\@linux.vnet.ibm.com" <balbir@...ux.vnet.ibm.com>,
m-ikeda@...jp.nec.com,
"akpm\@linux-foundation.org" <akpm@...ux-foundation.org>,
"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
kamezawa.hiroyuki@...il.com
Subject: Re: [PATCH] memcg: use ID in page_cgroup
On Tue, 24 Aug 2010 00:47:50 -0700
Greg Thelen <gthelen@...gle.com> wrote:
> KAMEZAWA Hiroyuki <kamezawa.hiroyu@...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@...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@...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).
>
No. (see 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()?
>
No.
- lock_page_cgroup() is for keeping page_cgroup's status stable.
- rcu_read_lock() is for delaying to discard mem_cgroup object.
rcu_read_lock() is just for delaying to discard object, not for avoiding
racy updates. All _updates_ requires proper lock or speculative logic as
atomic_inc_not_zero() etc... Basically, RCU is for avoiding use-after-free.
Thanks,
-Kame
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