Re: [PATCH v2] mm, memcg: skip killing processes under memcg protection at first scan

[Roman Gushchin <guro@fb.com>]

On Sun, Aug 18, 2019 at 09:18:06PM -0400, Yafang Shao wrote:
> In the current memory.min design, the system is going to do OOM instead
> of reclaiming the reclaimable pages protected by memory.min if the
> system is lack of free memory. While under this condition, the OOM
> killer may kill the processes in the memcg protected by memory.min.
> This behavior is very weird.
> In order to make it more reasonable, I make some changes in the OOM
> killer. In this patch, the OOM killer will do two-round scan. It will
> skip the processes under memcg protection at the first scan, and if it
> can't kill any processes it will rescan all the processes.
> 
> Regarding the overhead this change may takes, I don't think it will be a
> problem because this only happens under system  memory pressure and
> the OOM killer can't find any proper victims which are not under memcg
> protection.

Also, after the second thought, what your patch really does,
it basically guarantees that no processes out of memory cgroups
with memory.min set will be ever killed (unless there are any other
processes). In most cases (at least on our setups) it's basically
makes such processes immune to the OOM killer (similar to oom_score_adj
set to -1000).

This is by far a too strong side effect of setting memory.min,
so I don't think the idea is acceptable at all.

Thanks!