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

[Glauber Costa]

On 09/26/2011 07:52 AM, KAMEZAWA Hiroyuki wrote:
> 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 am working on something here that may allow it.
But I think it is independent of the rest, and I can repost the series 
fixing the problems raised here without it, + EXPERIMENTAL.

Btw, using EXPERIMENTAL here is a very good idea. I think that we should
turn EXPERIMENTAL on even if I fix for that exists, for a least a couple
of months until we see how this thing really evolves.

What do you think?

> > 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.

I personally think disallowing task movement once accounted is 
reasonable. At least for starters.

I think I can add at least that to the next proposal. Famous last words 
is, it should not be that hard...

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