Re: [PATCH v2] vmpressure: make sure memcg stays alive until all users are signaled

[Li Zefan <lizefan-hv44wF8Li93QT0dZR+AlfA@public.gmane.org>]

On 2013/7/12 18:37, Michal Hocko wrote:
> On Fri 12-07-13 17:54:27, Li Zefan wrote:
>> On 2013/7/12 17:29, Michal Hocko wrote:
>>> On Fri 12-07-13 17:20:09, Li Zefan wrote:
>>> [...]
>>>> But if I read the code correctly, even no one registers a vmpressure event,
>>>> vmpressure() is always running and queue the work item.
>>>
>>> True but checking there is somebody is rather impractical. First we
>>> would have to take a events_lock to check this and then drop it after
>>> scheduling the work. Which doesn't guarantee that the registered event
>>> wouldn't go away.
>>> And even trickier, we would have to do the same for all parents up the
>>> hierarchy.
>>>
>>
>> The thing is, we can forget about eventfd. eventfd is checked in
>> vmpressure_work_fn(), while vmpressure() is always called no matter what.
> 
> But vmpressure is called only for an existing memcg. This means that
> it cannot be called past css_offline so it must happen _before_ cgroup
> eventfd cleanup code.
> 
> Or am I missing something?
> 

Yeah.

The vmpressure work item is queued if we sense some memory pressure, no matter
if there is any eventfd ever registered. This is the point.