linux-kernel - Re: [PATCH v2 04/11] kmem accounting basic infrastructure

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Message-ID: <502BA4AC.9040000@parallels.com>
Date:	Wed, 15 Aug 2012 17:31:24 +0400
From:	Glauber Costa <glommer@...allels.com>
To:	Michal Hocko <mhocko@...e.cz>
CC:	<linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
	<cgroups@...r.kernel.org>, <devel@...nvz.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	<kamezawa.hiroyu@...fujitsu.com>, Christoph Lameter <cl@...ux.com>,
	"David Rientjes" <rientjes@...gle.com>,
	Pekka Enberg <penberg@...nel.org>
Subject: Re: [PATCH v2 04/11] kmem accounting basic infrastructure

On 08/15/2012 05:26 PM, Michal Hocko wrote:
> On Wed 15-08-12 17:04:31, Glauber Costa wrote:
>> On 08/15/2012 05:02 PM, Michal Hocko wrote:
>>> On Wed 15-08-12 16:53:40, Glauber Costa wrote:
>>> [...]
>>>>>>> This doesn't check for the hierachy so kmem_accounted might not be in 
>>>>>>> sync with it's parents. mem_cgroup_create (below) needs to copy
>>>>>>> kmem_accounted down from the parent and the above needs to check if this
>>>>>>> is a similar dance like mem_cgroup_oom_control_write.
>>>>>>>
>>>>>>
>>>>>> I don't see why we have to.
>>>>>>
>>>>>> I believe in a A/B/C hierarchy, C should be perfectly able to set a
>>>>>> different limit than its parents. Note that this is not a boolean.
>>>>>
>>>>> Ohh, I wasn't clear enough. I am not against setting the _limit_ I just
>>>>> meant that the kmem_accounted should be consistent within the hierarchy.
>>>>>
>>>>
>>>> If a parent of yours is accounted, you get accounted as well. This is
>>>> not the state in this patch, but gets added later. Isn't this enough ?
>>>
>>> But if the parent is not accounted, you can set the children to be
>>> accounted, right? Or maybe this is changed later in the series? I didn't
>>> get to the end yet.
>>>
>>
>> Yes, you can. Do you see any problem with that?
> 
> Well, if a child contributes with the kmem charges upwards the hierachy
> then a parent can have kmem.usage > 0 with disabled accounting.
> I am not saying this is a no-go but it definitely is confusing and I do
> not see any good reason for it. I've considered it as an overlook rather
> than a deliberate design decision.
> 

No, it is not an overlook.
It is theoretically possible to skip accounting on non-limited parents,
but how expensive is that? This is, indeed, confusing.

Of course I can be biased, but the way I see it, once you have
hierarchy, you account everything your child accounts.

I really don't see what is the concern here.
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