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Date:   Thu, 8 Dec 2022 09:14:10 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     =?utf-8?B?56iL5Z6y5rab?= Chengkaitao Cheng 
        <chengkaitao@didiglobal.com>
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Subject: Re: [PATCH v2] mm: memcontrol: protect the memory in cgroup from
 being oom killed
Message-ID: <Y5Gc0jiDlWlRlMYH@dhcp22.suse.cz>
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 <CEFD5AB7-17FB-4CC0-B818-1988484B8E55@didiglobal.com>
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On Thu 08-12-22 07:59:27, 程垲涛 Chengkaitao Cheng wrote:
> At 2022-12-08 15:33:07, "Michal Hocko" <mhocko@suse.com> wrote:
> >On Thu 08-12-22 11:46:44, chengkaitao wrote:
> >> From: chengkaitao <pilgrimtao@gmail.com>
> >> 
> >> We created a new interface <memory.oom.protect> for memory, If there is
> >> the OOM killer under parent memory cgroup, and the memory usage of a
> >> child cgroup is within its effective oom.protect boundary, the cgroup's
> >> tasks won't be OOM killed unless there is no unprotected tasks in other
> >> children cgroups. It draws on the logic of <memory.min/low> in the
> >> inheritance relationship.
> >> 
> >> It has the following advantages,
> >> 1. We have the ability to protect more important processes, when there
> >> is a memcg's OOM killer. The oom.protect only takes effect local memcg,
> >> and does not affect the OOM killer of the host.
> >> 2. Historically, we can often use oom_score_adj to control a group of
> >> processes, It requires that all processes in the cgroup must have a
> >> common parent processes, we have to set the common parent process's
> >> oom_score_adj, before it forks all children processes. So that it is
> >> very difficult to apply it in other situations. Now oom.protect has no
> >> such restrictions, we can protect a cgroup of processes more easily. The
> >> cgroup can keep some memory, even if the OOM killer has to be called.
> >> 
> >> Signed-off-by: chengkaitao <pilgrimtao@gmail.com>
> >> ---
> >> v2: Modify the formula of the process request memcg protection quota.
> >
> >The new formula doesn't really address concerns expressed previously.
> >Please read my feedback carefully again and follow up with questions if
> >something is not clear.
> 
> The previous discussion was quite scattered. Can you help me summarize
> your concerns again?

The most important part is http://lkml.kernel.org/r/Y4jFnY7kMdB8ReSW@dhcp22.suse.cz
: Let me just emphasise that we are talking about fundamental disconnect.
: Rss based accounting has been used for the OOM killer selection because
: the memory gets unmapped and _potentially_ freed when the process goes
: away. Memcg changes are bound to the object life time and as said in
: many cases there is no direct relation with any process life time.

That is to the per-process discount based on rss or any per-process
memory metrics.

Another really important question is the actual configurability. The
hierarchical protection has to be enforced and that means that same as
memory reclaim protection it has to be enforced top-to-bottom in the
cgroup hierarchy. That makes the oom protection rather non-trivial to
configure without having a good picture of a larger part of the cgroup
hierarchy as it cannot be tuned based on a reclaim feedback.
-- 
Michal Hocko
SUSE Labs