linux-kernel - Re: [RFC][PATCH 5/4] memcg: don't call res_counter

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Message-Id: <20090115134114.dba6b83a.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Thu, 15 Jan 2009 13:41:14 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	balbir@...ux.vnet.ibm.com
Cc:	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Pavel Emelyanov <xemul@...nvz.org>,
	Li Zefan <lizf@...fujitsu.com>, Paul Menage <menage@...gle.com>
Subject: Re: [RFC][PATCH 5/4] memcg: don't call res_counter_uncharge when
 obsolete

On Thu, 15 Jan 2009 09:47:50 +0530
Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:

> * KAMEZAWA Hiroyuki <kamezawa.hiroyu@...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.

Thanks,
-Kame

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