Re: [PATCH 4/5] memcg: lockless update of file stat with move-account safe method

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

On Tue, 31 Aug 2010 12:51:18 +0900
Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> wrote:

> On Wed, 25 Aug 2010 17:10:50 +0900
> KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
> 
> > 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: 20100825
> >  - added a comment about mc.lock
> >  - fixed bad lock.
> > 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>
> 
> (snip)
> 
> > @@ -1505,29 +1551,36 @@ 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);
> It doesn't cause any problem, but I think it would be better to change this to
> "id_to_memcg(..., false)". It's just under rcu_read_lock(), not under page_cgroup
> lock anymore.
> 
ok, I'll apply your suggestion.

> Otherwise, it looks good to me.
> 
> Reviewed-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
> 

Thanks!
-Kame

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