Re: [PATCH 3/3] memcg: simplify mem_cgroup_reclaim_iter

[Tejun Heo <tj@kernel.org>]

Hello, Michal.

On Wed, Jun 05, 2013 at 09:30:23AM +0200, Michal Hocko wrote:
> > I don't really get that.  As long as the amount is bound and the
> > overhead negligible / acceptable, why does it matter how long the
> > pinning persists? 
> 
> Because the amount is not bound either. Just create a hierarchy and
> trigger the hard limit and if you are careful enough you can always keep
> some of the children in the cached pointer (with css reference, if you
> will) and then release the hierarchy. You can do that repeatedly and
> leak considerable amount of memory.

It's still bound, no?  Each live memcg can only keep limited number of
cgroups cached, right?

> > We aren't talking about something gigantic or can
> 
> mem_cgroup is 888B now (depending on configuration). So I wouldn't call
> it negligible.

Do you think that the number can actually grow harmful?  Would you be
kind enough to share some calculations with me?

> > In the off chance that this is a real problem, which I strongly doubt,
> > as I wrote to Johannes, we can implement extremely dumb cleanup
> > routine rather than this weak reference beast.
> 
> That was my first version (https://lkml.org/lkml/2013/1/3/298) and
> Johannes didn't like. To be honest I do not care _much_ which way we go
> but we definitely cannot pin those objects for ever.

I'll get to the barrier thread but really complex barrier dancing like
that is only justifiable in extremely hot paths a lot of people pay
attention to.  It doesn't belong inside memcg proper.  If the cached
amount is an actual concern, let's please implement a simple clean up
thing.  All we need is a single delayed_work which scans the tree
periodically.

Johannes, what do you think?

Thanks.

-- 
tejun

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