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

[Glauber Costa <glommer@parallels.com>]

On 03/20/2013 12:58 PM, Michal Hocko wrote:
> On Wed 20-03-13 12:34:01, Glauber Costa wrote:
>> 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,
> 
> Sure, statistics need to stay at the place. I am not objecting on that.
> 
>> therefore the bypass.
> 
> I am just arguing about bypassing root even when there are children and
> use_hierarchy == 1 because it adds more code to maintain.
> 
>> 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.
> 
> I am all for "the simple the better" and add more optimizations on top.
> We have a real issue now and we should eliminate it. My original plan
> was to look at the bottlenecks and eliminate them one after another in
> smaller steps. But all the work I have on the plate is preempting me
> from looking into that...
> 
Been there, done that =)

I have no objections removing the special case for use_hierarchy == 1.

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