[Devel] Re: [PATCH v2 09/11] memcg: propagate kmem limiting information to children

[Greg Thelen]

On Thu, Aug 23 2012, Glauber Costa wrote:

> On 08/23/2012 03:23 AM, Greg Thelen wrote:
>> On Wed, Aug 22 2012, Glauber Costa wrote:
>> 
>>>>>>
>>>>>> I am fine with either, I just need a clear sign from you guys so I don't
>>>>>> keep deimplementing and reimplementing this forever.
>>>>>
>>>>> I would be for make it simple now and go with additional features later
>>>>> when there is a demand for them. Maybe we will have runtimg switch for
>>>>> user memory accounting as well one day.
>>>>>
>>>>> But let's see what others think?
>>>>
>>>> In my use case memcg will either be disable or (enabled and kmem
>>>> limiting enabled).
>>>>
>>>> I'm not sure I follow the discussion about history.  Are we saying that
>>>> once a kmem limit is set then kmem will be accounted/charged to memcg.
>>>> Is this discussion about the static branches/etc that are autotuned the
>>>> first time is enabled?  
>>>
>>> No, the question is about when you unlimit a former kmem-limited memcg.
>>>
>>>> The first time its set there parts of the system
>>>> will be adjusted in such a way that may impose a performance overhead
>>>> (static branches, etc).  Thereafter the performance cannot be regained
>>>> without a reboot.  This makes sense to me.  Are we saying that
>>>> kmem.limit_in_bytes will have three states?
>>>
>>> It is not about performance, about interface.
>>>
>>> Michal says that once a particular memcg was kmem-limited, it will keep
>>> accounting pages, even if you make it unlimited. The limits won't be
>>> enforced, for sure - there is no limit, but pages will still be accounted.
>>>
>>> This simplifies the code galore, but I worry about the interface: A
>>> person looking at the current status of the files only, without
>>> knowledge of past history, can't tell if allocations will be tracked or not.
>> 
>> In the current patch set we've conflating enabling kmem accounting with
>> the kmem limit value (RESOURCE_MAX=disabled, all_other_values=enabled).
>> 
>> I see no problem with simpling the kernel code with the requirement that
>> once a particular memcg enables kmem accounting that it cannot be
>> disabled for that memcg.
>> 
>> The only question is the user space interface.  Two options spring to
>> mind:
>> a) Close to current code.  Once kmem.limit_in_bytes is set to
>>    non-RESOURCE_MAX, then kmem accounting is enabled and cannot be
>>    disabled.  Therefore the limit cannot be set to RESOURCE_MAX
>>    thereafter.  The largest value would be something like
>>    RESOURCE_MAX-PAGE_SIZE.  An admin wondering if kmem is enabled only
>>    has to cat kmem.limit_in_bytes - if it's less than RESOURCE_MAX, then
>>    kmem is enabled.
>> 
>
> If we need to choose between them, I like this better than your (b).
> At least it is all clear, and "fix" the history problem, since it is
> possible to look up the status of the files and figure it out.
>
>> b) Or, if we could introduce a separate sticky kmem.enabled file.  Once
>>    set it could not be unset.  Kmem accounting would only be enabled if
>>    kmem.enabled=1.
>> 
>> I think (b) is clearer.
>> 
> Depends on your definition of clearer. We had a knob for
> kmem_independent in the beginning if you remember, and it was removed.
> The main reason being knobs complicate minds, and we happen to have a
> very natural signal for this. I believe the same reasoning applies here.

Sounds good to me, so let's go with (a).