linux-kernel - Re: [mm] [PATCH 2/4] Memory cgroup resource counters for hierarchy

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Message-ID: <490D931B.8000200@linux.vnet.ibm.com>
Date:	Sun, 02 Nov 2008 17:16:35 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
CC:	linux-mm@...ck.org, YAMAMOTO Takashi <yamamoto@...inux.co.jp>,
	Paul Menage <menage@...gle.com>, lizf@...fujitsu.com,
	linux-kernel@...r.kernel.org,
	Nick Piggin <nickpiggin@...oo.com.au>,
	David Rientjes <rientjes@...gle.com>,
	Pavel Emelianov <xemul@...nvz.org>,
	Dhaval Giani <dhaval@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [mm] [PATCH 2/4] Memory cgroup resource counters for hierarchy

KAMEZAWA Hiroyuki wrote:
> On Sun, 02 Nov 2008 11:19:38 +0530
> Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:
> 
>> KAMEZAWA Hiroyuki wrote:
>>> On Sun, 02 Nov 2008 00:18:37 +0530
>>> Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:
>>>
>>>> Add support for building hierarchies in resource counters. Cgroups allows us
>>>> to build a deep hierarchy, but we currently don't link the resource counters
>>>> belonging to the memory controller control groups, which are linked in
>>>> cgroup hiearchy. This patch provides the infrastructure for resource counters
>>>> that have the same hiearchy as their cgroup counter parts.
>>>>
>>>> These set of patches are based on the resource counter hiearchy patches posted
>>>> by Pavel Emelianov.
>>>>
>>>> NOTE: Building hiearchies is expensive, deeper hierarchies imply charging
>>>> the all the way up to the root. It is known that hiearchies are expensive,
>>>> so the user needs to be careful and aware of the trade-offs before creating
>>>> very deep ones.
>>>>
>>> ...isn't it better to add "root_lock" to res_counter rather than taking
>>> all levels of lock one by one ?
>>>
>>>  spin_lock(&res_counter->hierarchy_root->lock);
>>>  do all charge/uncharge to hierarchy
>>>  spin_unlock(&res_counter->hierarchy_root->lock);
>>>
>>> Hmm ?
>>>
>> Good thought process, but that affects and adds code complexity for the case
>> when hierarchy is enabled/disabled. It is also inefficient, since all charges
>> will now contend on root lock, in the current process, it is step by step, the
>> contention only occurs on common parts of the hierarchy (root being the best case).
>>
> 
> Above code's contention level is not different from "only root no children" case.
> Just inside-lock is heavier.

Yes, correct! I think the approach in the patches is better.

-- 
	Balbir
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