Date: Thu, 15 Nov 2012 16:12:55 +0100
From: Michal Hocko <mhocko@suse.cz>
To: Tejun Heo <htejun@gmail.com>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
        Johannes Weiner <hannes@cmpxchg.org>, Ying Han <yinghan@google.com>,
        Glauber Costa <glommer@parallels.com>
Subject: Re: [RFC 2/5] memcg: rework mem_cgroup_iter to use cgroup iterators
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On Thu 15-11-12 06:47:32, Tejun Heo wrote:
> Hello, Michal.
> 
> On Thu, Nov 15, 2012 at 10:51:03AM +0100, Michal Hocko wrote:
> > > I'm a bit confused.  Why would that make any difference?  Shouldn't it
> > > be just able to test the condition and continue?
> > 
> > Ohh, I misunderstood your proposal. So what you are suggesting is
> > to put all the logic we have in mem_cgroup_iter inside what you call
> > reclaim here + mem_cgroup_iter_break inside the loop, right?
> > 
> > I do not see how this would help us much. mem_cgroup_iter is not the
> > nicest piece of code but it handles quite a complex requirements that we
> > have currently (css reference count, multiple reclaimers racing). So I
> > would rather keep it this way. Further simplifications are welcome of
> > course.
> > 
> > Is there any reason why you are not happy about direct using of
> > cgroup_next_descendant_pre?
> 
> Because I'd like to consider the next functions as implementation
> detail, and having interations structred as loops tend to read better
> and less error-prone.  e.g. when you use next functions directly, it's
> way easier to circumvent locking requirements in a way which isn't
> very obvious. 

The whole point behind mem_cgroup_iter is to hide all the complexity
behind memcg iteration. Memcg code either use for_each_mem_cgroup_tree
for !reclaim case and mem_cgroup_iter otherwise.

> So, unless it messes up the code too much (and I can't see why it
> would), I'd much prefer if memcg used for_each_*() macros.

As I said this would mean that the current mem_cgroup_iter code would
have to be inverted which doesn't simplify the code much. I'd rather
hide all the grossy details inside the memcg iterator.
Or am I still missing your suggestion?
-- 
Michal Hocko
SUSE Labs
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