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Date: Thu, 6 Sep 2012 16:09:20 +0400
From: Glauber Costa <glommer@parallels.com>
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To: Michal Hocko <mhocko@suse.cz>
CC: Tejun Heo <tj@kernel.org>, <linux-kernel@vger.kernel.org>,
        <cgroups@vger.kernel.org>, <linux-mm@kvack.org>,
        Dave Jones <davej@redhat.com>, Ben Hutchings <ben@decadent.org.uk>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>, Paul Turner <pjt@google.com>,
        Lennart Poettering <lennart@poettering.net>,
        Kay Sievers <kay.sievers@vrfy.org>,
        Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
        Johannes Weiner <hannes@cmpxchg.org>
Subject: Re: [PATCH v2] memcg: first step towards hierarchical controller
References: <20120904130905.GA15683@dhcp22.suse.cz> <504601B8.2050907@parallels.com> <20120904143552.GB15683@dhcp22.suse.cz> <50461241.5010300@parallels.com> <20120904145414.GC15683@dhcp22.suse.cz> <50461610.30305@parallels.com> <20120904162501.GE15683@dhcp22.suse.cz> <504709D4.2010800@parallels.com> <20120905144942.GH5388@dhcp22.suse.cz> <20120905201238.GE13737@google.com> <20120906120623.GE22426@dhcp22.suse.cz>
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On 09/06/2012 04:06 PM, Michal Hocko wrote:
> On Wed 05-09-12 13:12:38, Tejun Heo wrote:
>> Hello, Michal.
>>
>> On Wed, Sep 05, 2012 at 04:49:42PM +0200, Michal Hocko wrote:
>>> Can we settle on the following 3 steps?
>>> 1) warn about "flat" hierarchies (give it X releases) - I will push it
>>>    to as many Suse code streams as possible (hope other distributions
>>>    could do the same)
>>
>> I think I'm just gonna trigger WARN from cgroup core if anyone tries
>> to create hierarchy with a controller which doesn't support full
>> hierarchy.  WARN_ON_ONCE() at first and then WARN_ON() on each
>> creation later on.
> 
> How do you find out that a controller is not fully hierarchical? Memory
> controller can be both.
> 
>>> 2) flip the default on the root cgroup & warn when somebody tries to
>>>    change it to 0 (give it another X releases) that the knob will be
>>>    removed
>>> 3) remove the knob and the whole nonsese
>>> 4) revert 3 if somebody really objects
>>
>> If we can get to 3, I don't think 4 would be a problem.
> 
> Agreed.
> 
Just so I understand it:

Michal clearly objected before folding his patch with my Kconfig patch.
But is there still opposition to merge both?

By having it default-n, only people that are either sure that this is
safe for them, or have more clearly defined lifecycles could set it.