Date: Thu, 15 Jan 2009 13:54:05 +0900
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
To: balbir@linux.vnet.ibm.com
Cc: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>,
       LKML <linux-kernel@vger.kernel.org>, linux-mm <linux-mm@kvack.org>,
       Andrew Morton <akpm@linux-foundation.org>,
       Pavel Emelyanov <xemul@openvz.org>, Li Zefan <lizf@cn.fujitsu.com>,
       Paul Menage <menage@google.com>
Subject: Re: [RFC][PATCH 5/4] memcg: don't call res_counter_uncharge when
 obsolete
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On Thu, 15 Jan 2009 10:15:57 +0530
Balbir Singh <balbir@linux.vnet.ibm.com> wrote:

> * KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> [2009-01-15 13:41:14]:
> 
> > On Thu, 15 Jan 2009 09:47:50 +0530
> > Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
> > 
> > > * KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> [2009-01-15 12:24:16]:
> > 
> > > > But I don't like -EBUSY ;)
> > > > 
> > > > When rmdir() returns -EBUSY even if there are no (visible) children and tasks,
> > > > our customer will take kdump and send it to me "please explain this kernel bug"
> > > > 
> > > > I'm sure it will happen ;)
> > > >
> > > 
> > > OK, but memory.stat can show why the group is busy and with
> > > move_to_parent() such issues should not occur right? I'll relook at
> > > the code. Thanks for your input.
> > > 
> > 
> > There was a design choice at swap_cgroup.
> > 
> > At rmdir, there may be used swap entry in memcg. (mem->memsw.usage can be > 0)
> >   1. update all records in swap cgroup
> >   2. just ignore account from swap, we can treat then at swap-in.
> > 
> > I implemented "2" by refcnt.
> > 
> > To do "1", we have to scan all used swap_cgroup but I don't want to scan all
> > swap_cgroup entry at rmdir. It's heavy job.
> > (*) To reduce memory usage by swap_cgroup, swap_cgroup just have a pointer to memcg
> > (**) I implemented swap_cgroup as statically allocated array because I don't want
> >     any dynamic memory allocation at swap-out and want to avoid unnecessary memory
> >     usage.
> 
> Fair enough, but I don't like that we don't have any checks for
> 
> If parent still has children, parent should not go away. The
> problem that Daisuke-San is seeing.
> 
The parent has "no children" in cgroup layer. It just has children in
"memory subsys" layer.

It's better to add mem_cgroup_get()/put agaisnt the parent at create()/destroy().
I'm now preparing a patch.

-Kame


> -- 
> 	Balbir
> 

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