Re: [patch 2/2] mm: memcg: hierarchical soft limit reclaim

[Sha <handai.szj@gmail.com>]

On 01/18/2012 05:25 PM, Johannes Weiner wrote:
> 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().
Er... I'm even more confused: mem_cgroup_soft_limit_reclaim indeed
choses the biggest soft-limit excessor first, but in the succeeding reclaim
mem_cgroup_hierarchical_reclaim just selects a child cgroup  by css_id
which has nothing to do with soft limit (see mem_cgroup_select_victim).
IMHO, it's not a genuine hierarchical reclaim.
I check this from the latest memcg-devel git tree (branch since-3.1)...

> 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.

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