Re: [PATCH 4/5] memcg: lockless update of file_mapped

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Mon, 23 Aug 2010 17:50:15 +0900
Daisuke Nishimura <nishimura@mxp.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@jp.fujitsu.com> wrote:
> 
> > No changes from v4.
> > ==
> > From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.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@jp.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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