Re: [RFC][PATCH 5/7][memcg] memcg lockless update of file mapped

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

On Wed, 28 Jul 2010 00:09:21 -0700
Greg Thelen <gthelen@google.com> wrote:

> KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> writes:
> 
> > 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.
> >
> > Note: update_file_mapped is safe against charge/uncharge even if it's
> > not under address_space->tree_lock or lock_page(). Because it's under
> > page_table_lock(), anyone can't unmap it...then, anyone can't uncharge().
> >
> >
> >
> > Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> > ---
> >  mm/memcontrol.c |   64 ++++++++++++++++++++++++++++++++++++++++++++++++++++----
> >  1 file changed, 60 insertions(+), 4 deletions(-)
> >
> > Index: mmotm-0719/mm/memcontrol.c
> > ===================================================================
> > --- mmotm-0719.orig/mm/memcontrol.c
> > +++ mmotm-0719/mm/memcontrol.c
> > @@ -89,6 +89,7 @@ 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,
> >  };
> > @@ -1071,7 +1072,48 @@ 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);
> > +	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)
> > +{
> 
> Could we add an assertion to confirm locking contract is upheld:
> 	VM_BUG_ON(!rcu_read_lock_held());
> 

Hmm. there is an only one caller...I'll add one or I don't make
this as a funciton.


> > +	return this_cpu_read(mem->stat->count[MEM_CGROUP_ON_MOVE]) > 0;
> > +}
> >  
> >  static bool mem_cgroup_under_move(struct mem_cgroup *mem)
> >  {
> > @@ -1470,13 +1512,21 @@ 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_mem(pc->mem_cgroup);
> > +	if (!mem)
> > +		goto done;
> > +	need_lock = mem_cgroup_is_moved(mem);
> > +	if (need_lock) {
> > +		/* need to serialize with move_account */
> > +		lock_page_cgroup(pc);
> > +		mem = id_to_mem(pc->mem_cgroup);
> > +	}
> >  	if (!mem || !PageCgroupUsed(pc))
> >  		goto done;
> 
> Could we add a preemption() check here to ensure that the
> __this_cpu_xxx() is safe to use?
> 
Hmm, ok.


Thanks,
-Kame
> 	/*
> 	 * Preemption is already disabled. We can use __this_cpu_xxx
> 	 */
> +        VM_BUG_ON(preemptible());
> 
> > @@ -1492,7 +1542,9 @@ void mem_cgroup_update_file_mapped(struc
> >  	}
> >  
> >  done:
> > -	unlock_page_cgroup(pc);
> > +	if (need_lock)
> > +		unlock_page_cgroup(pc);
> > +	rcu_read_unlock();
> >  }
> >  
> >  /*
> > @@ -3024,6 +3076,7 @@ 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;
> > @@ -3037,6 +3090,7 @@ 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)
> > @@ -4503,6 +4557,7 @@ 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);
> > @@ -4533,6 +4588,7 @@ 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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