linux-kernel - Re: [RFC 2/5] memcg: rework mem_cgroup

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Date:	Thu, 15 Nov 2012 06:47:32 -0800
From:	Tejun Heo <htejun@...il.com>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Ying Han <yinghan@...gle.com>,
	Glauber Costa <glommer@...allels.com>
Subject: Re: [RFC 2/5] memcg: rework mem_cgroup_iter to use cgroup iterators

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.  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.

Thanks.

-- 
tejun
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