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Message-Id: <20100824084916.078d6a82.kamezawa.hiroyu@jp.fujitsu.com>
Date: Tue, 24 Aug 2010 08:49:16 +0900
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To: Daisuke Nishimura <nishimura@....nes.nec.co.jp>
Cc: linux-mm@...ck.org,
"balbir@...ux.vnet.ibm.com" <balbir@...ux.vnet.ibm.com>,
gthelen@...gle.com, m-ikeda@...jp.nec.com,
"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
kamezawa.hiroyuki@...il.com
Subject: Re: [PATCH 4/5] memcg: lockless update of file_mapped
On Mon, 23 Aug 2010 17:50:15 +0900
Daisuke Nishimura <nishimura@....nes.nec.co.jp> wrote:
> This patch looks good to me, but I have one question.
>
> Why do we need to acquire sc.lock inside mem_cgroup_(start|end)_move() ?
> These functions doesn't access mc.*.
>
just reusing a lock to update status. If you don't like, I'll add a new lock.
Thanks,
-Kame
> Thanks,
> Daisuke Nishimura.
>
> On Fri, 20 Aug 2010 19:02:56 +0900
> KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> wrote:
>
> > No changes from v4.
> > ==
> > From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> >
> > At accounting file events per memory cgroup, we need to find memory cgroup
> > via page_cgroup->mem_cgroup. Now, we use lock_page_cgroup().
> >
> > But, considering the context which page-cgroup for files are accessed,
> > we can use alternative light-weight mutual execusion in the most case.
> > At handling file-caches, the only race we have to take care of is "moving"
> > account, IOW, overwriting page_cgroup->mem_cgroup. Because file status
> > update is done while the page-cache is in stable state, we don't have to
> > take care of race with charge/uncharge.
> >
> > Unlike charge/uncharge, "move" happens not so frequently. It happens only when
> > rmdir() and task-moving (with a special settings.)
> > This patch adds a race-checker for file-cache-status accounting v.s. account
> > moving. The new per-cpu-per-memcg counter MEM_CGROUP_ON_MOVE is added.
> > The routine for account move
> > 1. Increment it before start moving
> > 2. Call synchronize_rcu()
> > 3. Decrement it after the end of moving.
> > By this, file-status-counting routine can check it needs to call
> > lock_page_cgroup(). In most case, I doesn't need to call it.
> >
> >
> > Changelog: 20100804
> > - added a comment for possible optimization hint.
> > Changelog: 20100730
> > - some cleanup.
> > Changelog: 20100729
> > - replaced __this_cpu_xxx() with this_cpu_xxx
> > (because we don't call spinlock)
> > - added VM_BUG_ON().
> >
> > Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> > ---
> > mm/memcontrol.c | 79 +++++++++++++++++++++++++++++++++++++++++++++++---------
> > 1 file changed, 67 insertions(+), 12 deletions(-)
> >
> > Index: mmotm-0811/mm/memcontrol.c
> > ===================================================================
> > --- mmotm-0811.orig/mm/memcontrol.c
> > +++ mmotm-0811/mm/memcontrol.c
> > @@ -90,6 +90,7 @@ enum mem_cgroup_stat_index {
> > MEM_CGROUP_STAT_PGPGOUT_COUNT, /* # of pages paged out */
> > MEM_CGROUP_STAT_SWAPOUT, /* # of pages, swapped out */
> > MEM_CGROUP_EVENTS, /* incremented at every pagein/pageout */
> > + MEM_CGROUP_ON_MOVE, /* A check for locking move account/status */
> >
> > MEM_CGROUP_STAT_NSTATS,
> > };
> > @@ -1086,7 +1087,50 @@ static unsigned int get_swappiness(struc
> > return swappiness;
> > }
> >
> > -/* A routine for testing mem is not under move_account */
> > +static void mem_cgroup_start_move(struct mem_cgroup *mem)
> > +{
> > + int cpu;
> > + /* for fast checking in mem_cgroup_update_file_stat() etc..*/
> > + spin_lock(&mc.lock);
> > + /* TODO: Can we optimize this by for_each_online_cpu() ? */
> > + for_each_possible_cpu(cpu)
> > + per_cpu(mem->stat->count[MEM_CGROUP_ON_MOVE], cpu) += 1;
> > + spin_unlock(&mc.lock);
> > +
> > + synchronize_rcu();
> > +}
> > +
> > +static void mem_cgroup_end_move(struct mem_cgroup *mem)
> > +{
> > + int cpu;
> > +
> > + if (!mem)
> > + return;
> > + /* for fast checking in mem_cgroup_update_file_stat() etc..*/
> > + spin_lock(&mc.lock);
> > + for_each_possible_cpu(cpu)
> > + per_cpu(mem->stat->count[MEM_CGROUP_ON_MOVE], cpu) -= 1;
> > + spin_unlock(&mc.lock);
> > +}
> > +
> > +/*
> > + * mem_cgroup_is_moved -- checking a cgroup is mc.from target or not.
> > + * used for avoiding race.
> > + * mem_cgroup_under_move -- checking a cgroup is mc.from or mc.to or
> > + * under hierarchy of them. used for waiting at
> > + * memory pressure.
> > + * Result of is_moved can be trusted until the end of rcu_read_unlock().
> > + * The caller must do
> > + * rcu_read_lock();
> > + * result = mem_cgroup_is_moved();
> > + * .....make use of result here....
> > + * rcu_read_unlock();
> > + */
> > +static bool mem_cgroup_is_moved(struct mem_cgroup *mem)
> > +{
> > + VM_BUG_ON(!rcu_read_lock_held());
> > + return this_cpu_read(mem->stat->count[MEM_CGROUP_ON_MOVE]) > 0;
> > +}
> >
> > static bool mem_cgroup_under_move(struct mem_cgroup *mem)
> > {
> > @@ -1502,29 +1546,36 @@ void mem_cgroup_update_file_mapped(struc
> > {
> > struct mem_cgroup *mem;
> > struct page_cgroup *pc;
> > + bool need_lock = false;
> >
> > pc = lookup_page_cgroup(page);
> > if (unlikely(!pc))
> > return;
> > -
> > - lock_page_cgroup(pc);
> > + rcu_read_lock();
> > mem = id_to_memcg(pc->mem_cgroup, true);
> > - if (!mem || !PageCgroupUsed(pc))
> > + if (likely(mem)) {
> > + if (mem_cgroup_is_moved(mem)) {
> > + /* need to serialize with move_account */
> > + lock_page_cgroup(pc);
> > + need_lock = true;
> > + mem = id_to_memcg(pc->mem_cgroup, true);
> > + if (unlikely(!mem))
> > + goto done;
> > + }
> > + }
> > + if (unlikely(!PageCgroupUsed(pc)))
> > goto done;
> > -
> > - /*
> > - * Preemption is already disabled. We can use __this_cpu_xxx
> > - */
> > if (val > 0) {
> > - __this_cpu_inc(mem->stat->count[MEM_CGROUP_STAT_FILE_MAPPED]);
> > + this_cpu_inc(mem->stat->count[MEM_CGROUP_STAT_FILE_MAPPED]);
> > SetPageCgroupFileMapped(pc);
> > } else {
> > - __this_cpu_dec(mem->stat->count[MEM_CGROUP_STAT_FILE_MAPPED]);
> > + this_cpu_dec(mem->stat->count[MEM_CGROUP_STAT_FILE_MAPPED]);
> > ClearPageCgroupFileMapped(pc);
> > }
> > -
> > done:
> > - unlock_page_cgroup(pc);
> > + if (need_lock)
> > + unlock_page_cgroup(pc);
> > + rcu_read_unlock();
> > }
> >
> > /*
> > @@ -3064,6 +3115,7 @@ move_account:
> > lru_add_drain_all();
> > drain_all_stock_sync();
> > ret = 0;
> > + mem_cgroup_start_move(mem);
> > for_each_node_state(node, N_HIGH_MEMORY) {
> > for (zid = 0; !ret && zid < MAX_NR_ZONES; zid++) {
> > enum lru_list l;
> > @@ -3077,6 +3129,7 @@ move_account:
> > if (ret)
> > break;
> > }
> > + mem_cgroup_end_move(mem);
> > memcg_oom_recover(mem);
> > /* it seems parent cgroup doesn't have enough mem */
> > if (ret == -ENOMEM)
> > @@ -4563,6 +4616,7 @@ static void mem_cgroup_clear_mc(void)
> > mc.to = NULL;
> > mc.moving_task = NULL;
> > spin_unlock(&mc.lock);
> > + mem_cgroup_end_move(from);
> > memcg_oom_recover(from);
> > memcg_oom_recover(to);
> > wake_up_all(&mc.waitq);
> > @@ -4593,6 +4647,7 @@ static int mem_cgroup_can_attach(struct
> > VM_BUG_ON(mc.moved_charge);
> > VM_BUG_ON(mc.moved_swap);
> > VM_BUG_ON(mc.moving_task);
> > + mem_cgroup_start_move(from);
> > spin_lock(&mc.lock);
> > mc.from = from;
> > mc.to = mem;
> >
>
--
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