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

[Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

(2013/03/20 18:30), Glauber Costa wrote:
> 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.
>
I agree.

Thanks,
-Kame

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