Re: [PATCH v3 2/7] socket: initial cgroup code.

[KAMEZAWA Hiroyuki]

On Sat, 24 Sep 2011 11:45:04 -0300
Glauber Costa <glommer@xxxxxxxxxxxxx> wrote:

> On 09/22/2011 12:09 PM, Balbir Singh wrote:
> > On Thu, Sep 22, 2011 at 11:30 AM, Greg Thelen<gthelen@xxxxxxxxxx>  wrote:
> >> On Wed, Sep 21, 2011 at 11:59 AM, Glauber Costa<glommer@xxxxxxxxxxxxx>  wrote:
> >>> Right now I am working under the assumption that tasks are long lived inside
> >>> the cgroup. Migration potentially introduces some nasty locking problems in
> >>> the mem_schedule path.
> >>>
> >>> Also, unless I am missing something, the memcg already has the policy of
> >>> not carrying charges around, probably because of this very same complexity.
> >>>
> >>> True that at least it won't EBUSY you... But I think this is at least a way
> >>> to guarantee that the cgroup under our nose won't disappear in the middle of
> >>> our allocations.
> >>
> >> Here's the memcg user page behavior using the same pattern:
> >>
> >> 1. user page P is allocate by task T in memcg M1
> >> 2. T is moved to memcg M2.  The P charge is left behind still charged
> >> to M1 if memory.move_charge_at_immigrate=0; or the charge is moved to
> >> M2 if memory.move_charge_at_immigrate=1.
> >> 3. rmdir M1 will try to reclaim P (if P was left in M1).  If unable to
> >> reclaim, then P is recharged to parent(M1).
> >>
> >
> > We also have some magic in page_referenced() to remove pages
> > referenced from different containers. What we do is try not to
> > penalize a cgroup if another cgroup is referencing this page and the
> > page under consideration is being reclaimed from the cgroup that
> > touched it.
> >
> > Balbir Singh
> Do you guys see it as a showstopper for this series to be merged, or can 
> we just TODO it ?
> 

In my experience, 'I can't rmdir cgroup.' is always an important/difficult
problem. The users cannot know where the accouting is leaking other than
kmem.usage_in_bytes or memory.usage_in_bytes. and can't fix the issue.

please add EXPERIMENTAL to Kconfig until this is fixed.

> I can push a proposal for it, but it would be done in a separate patch 
> anyway. Also, we may be in better conditions to fix this when the slab 
> part is merged - since it will likely have the same problems...
> 

Yes. considering sockets which can be shared between tasks(cgroups)
you'll finally need
  - owner task of socket 
  - account moving callback

Or disallow task moving once accounted.


Thanks,
-Kame








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