Re: [PATCH v2 2/5] memcg: provide root figures from system totals

[Glauber Costa <glommer@parallels.com>]

On 03/20/2013 12:18 PM, Michal Hocko wrote:
> On Wed 20-03-13 12:08:17, Glauber Costa wrote:
>> On 03/20/2013 12:03 PM, Michal Hocko wrote:
>>> On Wed 20-03-13 11:03:17, Glauber Costa wrote:
>>>> On 03/19/2013 04:55 PM, Michal Hocko wrote:
>>>>> On Tue 19-03-13 13:46:50, Michal Hocko wrote:
>>>>>> On Tue 05-03-13 17:10:55, Glauber Costa wrote:
>>>>>>> For the root memcg, there is no need to rely on the res_counters if hierarchy
>>>>>>> is enabled The sum of all mem cgroups plus the tasks in root itself, is
>>>>>>> necessarily the amount of memory used for the whole system. Since those figures
>>>>>>> are already kept somewhere anyway, we can just return them here, without too
>>>>>>> much hassle.
>>>>>>>
>>>>>>> Limit and soft limit can't be set for the root cgroup, so they are left at
>>>>>>> RESOURCE_MAX. Failcnt is left at 0, because its actual meaning is how many
>>>>>>> times we failed allocations due to the limit being hit. We will fail
>>>>>>> allocations in the root cgroup, but the limit will never the reason.
>>>>>>
>>>>>> I do not like this very much to be honest. It just adds more hackery...
>>>>>> Why cannot we simply not account if nr_cgroups == 1 and move relevant
>>>>>> global counters to the root at the moment when a first group is
>>>>>> created?
>>>>>
>>>>> OK, it seems that the very next patch does what I was looking for. So
>>>>> why all the churn in this patch?
>>>>> Why do you want to make root even more special?
>>>>
>>>> Because I am operating under the assumption that we want to handle that
>>>> transparently and keep things working. If you tell me: "Hey, reading
>>>> memory.usage_in_bytes from root should return 0!", then I can get rid of
>>>> that.
>>>
>>> If you simply switch to accounting for root then you do not have to care
>>> about this, don't you?
>>>
>> Of course not, but the whole point here is *not* accounting root.
> 
> I thought the objective was to not account root if there are no
> children. 

It is the goal, yes. As I said: I want the root-only case to keep
providing userspace with meaningful statistics, therefore the bypass.
But since the machinery is in place, it is trivial to keep bypassing for
use_hierarchy = 1 at the root level. If you believe it would be simpler,
I could refrain from doing it.

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