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Date: Wed, 18 Jan 2012 10:25:09 +0100
From: Johannes Weiner <hannes@cmpxchg.org>
To: Sha <handai.szj@gmail.com>
Cc: Ying Han <yinghan@google.com>, Andrew Morton <akpm@linux-foundation.org>,
        Michal Hocko <mhocko@suse.cz>,
        KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
        Balbir Singh <bsingharora@gmail.com>, cgroups@vger.kernel.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [patch 2/2] mm: memcg: hierarchical soft limit reclaim
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On Wed, Jan 18, 2012 at 03:17:25PM +0800, Sha wrote:
> > > I don't think it solve the root of the problem, example:
> > > root
> > > -> A (hard limit 20G, soft limit 12G, usage 20G)
> > >   -> A1 ( soft limit 2G,   usage 1G)
> > >   -> A2 ( soft limit 10G, usage 19G)
> > >          ->B1 (soft limit 5G, usage 4G)
> > >          ->B2 (soft limit 5G, usage 15G)
> > >
> > > Now A is hitting its hard limit and start hierarchical reclaim under A.
> > > If we choose B1 to go through mem_cgroup_over_soft_limit, it will
> > > return true because its parent A2 has a large usage and will lead to
> > > priority=0 reclaiming. But in fact it should be B2 to be punished.
> 
> > Because A2 is over its soft limit, the whole hierarchy below it should
> > be preferred over A1, so both B1 and B2 should be soft limit reclaimed
> > to be consistent with behaviour at the root level.
> 
> Well it is just the behavior that I'm expecting actually. But with my
> humble comprehension, I can't catch the soft-limit-based hierarchical
> reclaiming under the target cgroup (A2) in the current implementation
> or after the patch. Both the current mem_cgroup_soft_reclaim or
> shrink_zone select victim sub-cgroup by mem_cgroup_iter, but it
> doesn't take soft limit into consideration, do I left anything ?

No, currently soft limits are ignored if pressure originates from
below root_mem_cgroup.

But iff soft limits are applied right now, they are applied
hierarchically, see mem_cgroup_soft_limit_reclaim().

In my opinion, the fact that soft limits are ignored when pressure is
triggered sub-root_mem_cgroup is an artifact of the per-zone tree, so
I allowed soft limits to be taken into account below root_mem_cgroup.

But IMO, this is something different from how soft limit reclaim is
applied once triggered: currently, soft limit reclaim applies to a
whole hierarchy, including all children.  And this I left unchanged.