linux-kernel - Re: [PATCH] mm: memcontrol: protect the memory in cgroup from being oom killed

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Date:   Thu, 1 Dec 2022 14:30:11 +0000
From:   程垲涛 Chengkaitao Cheng 
        <chengkaitao@...iglobal.com>
To:     Michal Hocko <mhocko@...e.com>
CC:     Tao pilgrim <pilgrimtao@...il.com>,
        "tj@...nel.org" <tj@...nel.org>,
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        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH] mm: memcontrol: protect the memory in cgroup from being
 oom killed

At 2022-12-01 21:08:26, "Michal Hocko" <mhocko@...e.com> wrote:
>On Thu 01-12-22 13:44:58, Michal Hocko wrote:
>> On Thu 01-12-22 10:52:35, 程垲涛 Chengkaitao Cheng wrote:
>> > At 2022-12-01 16:49:27, "Michal Hocko" <mhocko@...e.com> wrote:
>[...]
>> There is a misunderstanding, oom.protect does not replace the user's 
>> tailed policies, Its purpose is to make it easier and more efficient for 
>> users to customize policies, or try to avoid users completely abandoning 
>> the oom score to formulate new policies.
>
> Then you should focus on explaining on how this makes those policies and
> easier and moe efficient. I do not see it.

In fact, there are some relevant contents in the previous chat records. 
If oom.protect is applied, it will have the following benefits
1. Users only need to focus on the management of the local cgroup, not the 
impact on other users' cgroups.
2. Users and system do not need to spend extra time on complicated and 
repeated scanning and configuration. They just need to configure the 
oom.protect of specific cgroups, which is a one-time task

>> > >Why cannot you simply discount the protection from all processes
>> > >equally? I do not follow why the task_usage has to play any role in
>> > >that.
>> > 
>> > If all processes are protected equally, the oom protection of cgroup is 
>> > meaningless. For example, if there are more processes in the cgroup, 
>> > the cgroup can protect more mems, it is unfair to cgroups with fewer 
>> > processes. So we need to keep the total amount of memory that all 
>> > processes in the cgroup need to protect consistent with the value of 
>> > eoom.protect.
>> 
>> You are mixing two different concepts together I am afraid. The per
>> memcg protection should protect the cgroup (i.e. all processes in that
>> cgroup) while you want it to be also process aware. This results in a
>> very unclear runtime behavior when a process from a more protected memcg
>> is selected based on its individual memory usage.
>
The correct statement here should be that each memcg protection should 
protect the number of mems specified by the oom.protect. For example, 
a cgroup's usage is 6G, and it's oom.protect is 2G, when an oom killer occurs, 
In the worst case, we will only reduce the memory used by this cgroup to 2G 
through the om killer.

>Let me be more specific here. Although it is primarily processes which
>are the primary source of memcg charges the memory accounted for the oom
>badness purposes is not really comparable to the overal memcg charged
>memory. Kernel memory, non-mapped memory all that can generate rather
>interesting cornercases.

Sorry, I'm thoughtless enough about some special memory statistics. I will fix 
it in the next version
 
Thanks for your comment!
chengkaitao