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Date:   Tue, 27 Sep 2022 21:07:02 +0800
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Subject: Re: [External] Re: [RFC] proc: Add a new isolated /proc/pid/mempolicy
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To:     Michal Hocko <mhocko@suse.com>
Cc:     Zhongkun He <hezhongkun.hzk@bytedance.com>, corbet@lwn.net,
        akpm@linux-foundation.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org
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From:   Abel Wu <wuyun.abel@bytedance.com>
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On 9/27/22 6:49 PM, Michal Hocko wrote:
> On Tue 27-09-22 11:20:54, Abel Wu wrote:
> [...]
>>>> Btw.in order to add per-thread-group mempolicy, is it possible to add
>>>> mempolicy in mm_struct?
>>>
>>> I dunno. This would make the mempolicy interface even more confusing.
>>> Per mm behavior makes a lot of sense but we already do have per-thread
>>> semantic so I would stick to it rather than introducing a new semantic.
>>>
>>> Why is this really important?
>>
>> We want soft control on memory footprint of background jobs by applying
>> NUMA preferences when necessary, so the impact on different NUMA nodes
>> can be managed to some extent. These NUMA preferences are given by the
>> control panel, and it might not be suitable to overwrite the tasks with
>> specific memory policies already (or vice versa).
> 
> Maybe the answer is somehow implicit but I do not really see any
> argument for the per thread-group semantic here. In other words why a
> new interface has to cover more than the local [sg]et_mempolicy?
> I can see convenience as one potential argument. Also if there is a
> requirement to change the policy in atomic way then this would require a
> single syscall.

Convenience is not our major concern. A well-tuned workload can have
specific memory policies for different tasks/vmas in one process, and
this can be achieved by set_mempolicy()/mbind() respectively. While
other workloads are not, they don't care where the memory residents,
so the impact they brought on the co-located workloads might vary in
different NUMA nodes.

The control panel, which has a full knowledge of workload profiling,
may want to interfere the behavior of the non-mempolicied processes
by giving them NUMA preferences, to better serve the co-located jobs.

So in this scenario, a process's memory policy can be assigned by two
objects dynamically:

  a) the process itself, through set_mempolicy()/mbind()
  b) the control panel, but API is not available right now

Considering the two policies should not fight each other, it sounds
reasonable to introduce a new syscall to assign memory policy to a
process through struct mm_struct.