linux-kernel - Re: [RFC 0/5] forced comounts for cgroups.

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Message-ID: <50471C0C.7050600@parallels.com>
Date:	Wed, 5 Sep 2012 13:31:56 +0400
From:	Glauber Costa <glommer@...allels.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	Tejun Heo <tj@...nel.org>, <linux-kernel@...r.kernel.org>,
	<cgroups@...r.kernel.org>, <linux-mm@...ck.org>,
	<davej@...hat.com>, <ben@...adent.org.uk>, <pjt@...gle.com>,
	<lennart@...ttering.net>, <kay.sievers@...y.org>
Subject: Re: [RFC 0/5] forced comounts for cgroups.

On 09/05/2012 01:26 PM, Peter Zijlstra wrote:
> On Wed, 2012-09-05 at 13:12 +0400, Glauber Costa wrote:
>> On 09/05/2012 01:11 PM, Tejun Heo wrote:
>>> Hello, Peter.
>>>
>>> On Wed, Sep 05, 2012 at 11:06:33AM +0200, Peter Zijlstra wrote:
>>>> *confused* I always thought that was exactly what you meant with unified
>>>> hierarchy.
>>>
>>> No, I never counted out differing granularity.
>>>
>>
>> Can you elaborate on which interface do you envision to make it work?
>> They will clearly be mounted in the same hierarchy, or as said
>> alternatively, comounted.
>>
>> If you can turn them on/off on a per-subtree basis, which interface
>> exactly do you propose for that?
> 
> I wouldn't, screw that. That would result in the exact same problem
> we're trying to fix. I want a single hierarchy walk, that's expensive
> enough.
> 
>> Would a pair of cgroup core files like available_controllers and
>> current_controllers are a lot of drivers do, suffice?
> 
> No.. its not a 'feature' I care to support for 'my' controllers.
> 
> I simply don't want to have to do two (or more) hierarchy walks for
> accounting on every schedule event, all that pointer chasing is stupidly
> expensive.
> 

You wouldn't have to do more than one hierarchy walks for that. What
Tejun seems to want, is the ability to not have a particular controller
at some point in the tree. But if they exist, they are always together.


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