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

[Glauber Costa <glommer@parallels.com>]

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. So if
we are entirely skipping root account, it, I personally believe we need
to replace it with something else so we can keep things working as much
as we can.

It doesn't need to be perfect, though: There is no way we can have
max_usage without something like a res_counter that locks memory
charges. I believe we can live without that. But as for the basic
statistics and numbers, I believe they should keep working.

>> The fact that I keep bypassing when hierarchy is present, it is
>> more of a reuse of the infrastructure since it's there anyway.
>>
>> Also, I would like the root memcg to be usable, albeit cheap, for
>> projects like memory pressure notifications.
>  
> root memcg without any childre, right?
> 
yes, of course.


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