Re: [PATCHSET cgroup/for-3.16] cgroup: iterate cgroup_subsys_states directly

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

On2014/5/14 21:07, Tejun Heo wrote:
> Hello, Li.
> 
> On Wed, May 14, 2014 at 12:21:25PM +0800, Li Zefan wrote:
>>> There are now use cases where controllers need to iterate through
>>> csses regardless of their online state as long as they have positive
>>
>> What use cases are we talking about here?
> 
> memcg wants to be able to iterate all csses whose refcnts haven't
> reached zero yet so that it can treat offline csses the same way as
> online ones in terms of memory reclaim.  They don't contain new tasks
> so new charges won't be created but offlining won't try to transfer
> all charges to the parent but just leave the offline child attached
> until all charges are eventually reclaimed from the pressure from the
> parent.
> 
> I'm not too familiar with the details but this makes sense in generic
> sense too.  Offline marks an object starting its draining phase and
> release marks the actual destruction point.  For controllers with
> persistent states like memcg, it's a lot more natural to deal offlined
> csses as "active but draining following the usual hierarchical
> operation" rather than trying to explicitly update the states from
> offline to move them to the parent especially as the effort there
> essentially is a waste as most of those moved charges aren't gonna be
> used in the parent and will be released eventually.
> 
> Guaranteeing iteration of offline but not-released csses allow
> controllers to treat the draining stage between offline and release
> more or less identically to online state which in turn can make
> ->css_offline() significantly simpler and lighter.
> 

yeah, fair enough.