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

[Yafang Shao <laoar.shao@gmail.com>]

On Tue, Aug 20, 2019 at 2:31 PM Michal Hocko <mhocko@suse.com> wrote:
>
> [hmm the email got stuck on my send queue - sending again]
>
> On Mon 19-08-19 16:15:08, Yafang Shao wrote:
> > On Mon, Aug 19, 2019 at 3:31 PM Michal Hocko <mhocko@suse.com> wrote:
> > >
> > > On Sun 18-08-19 00:24:54, 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.
> > >
> > > Could you be more specific about the configuration that leads to this
> > > situation?
> >
> > When I did memory pressure test to verify memory.min I found that issue.
> > This issue can be produced as bellow,
> >     memcg setting,
> >         memory.max: 1G
> >         memory.min: 512M
> >         some processes are running is this memcg, with both serveral
> > hundreds MB  file mapping and serveral hundreds MB anon mapping.
> >     system setting,
> >          swap: off.
> >          some memory pressure test are running on the system.
> >
> > When the memory usage of this memcg is bellow the memory.min, the
> > global reclaimers stop reclaiming pages in this memcg, and when
> > there's no available memory, the OOM killer will be invoked.
> > Unfortunately the OOM killer can chose the process running in the
> > protected memcg.
>
> Well, the memcg protection was designed to prevent from regular
> memory reclaim.  It was not aimed at acting as a group wide oom
> protection. The global oom killer (but memcg as well) simply cares only
> about oom_score_adj when selecting a victim.
>

OOM is a kind of memory reclaim, isn't it ?

> Adding yet another oom protection is likely to complicate the oom
> selection logic and make it more surprising. E.g. why should workload
> fitting inside the min limit be so special? Do you have any real world
> example?
>

The problem here is we want to use it ini the real world, but the
issuses we found  prevent us from using it in the real world.


> > In order to produce it easy, you can incease the memroy.min and set
> > -1000 to the oom_socre_adj of the processes outside of the protected
> > memcg.
>
> This sounds like a very dubious configuration to me. There is no other
> option than chosing from the protected group.
>

This is only an easy example to produce it.

> > Is this setting proper ?
> >
> > Thanks
> > Yafang
>
> --
> Michal Hocko
> SUSE Labs