Re: [PATCH] mm, memcg: avoid oom if cgroup is not populated

[Michal Hocko <mhocko@kernel.org>]

On Tue 26-11-19 22:25:27, Yafang Shao wrote:
> On Tue, Nov 26, 2019 at 9:16 PM Michal Hocko <mhocko@kernel.org> wrote:
> >
> > On Tue 26-11-19 08:02:49, Yafang Shao wrote:
> > > There's one case that the processes in a memcg are all exit (due to OOM
> > > group or some other reasons), but the file page caches are still exist.
> > > These file page caches may be protected by memory.min so can't be
> > > reclaimed. If we can't success to restart the processes in this memcg or
> > > don't want to make this memcg offline, then we want to drop the file page
> > > caches.
> > > The advantage of droping this file caches is it can avoid the reclaimer
> > > (either kswapd or direct) scanning and reclaiming pages from all memcgs
> > > exist in this system, because currently the reclaimer will fairly reclaim
> > > pages from all memcgs if the system is under memory pressure.
> > > The possible method to drop these file page caches is setting the
> > > hard limit of this memcg to 0. Unfortunately this may invoke the OOM killer
> > > and generates lots of misleading outputs, that should not happen.
> >
> > I disagree that the output is misleading. Quite contrary, it provides a
> > useful lead on the unreclaimable memory.
> >
> 
> We can show the unreclaimable memory independently, rather than print
> the full oom output.
> OOM killer is used to kill process, why do we invoke it when there's
> no process ?
> What's the advantage of doing it ?

Consistency.

> > > One misleading output is "Out of memory and no killable processes...",
> > > while really there is no tasks rather than no killable tasks.
> >
> > Again, this is nothing misleading. No task is a trivial subset of no
> > killable task. I do not see why we should treat one differently than the
> > other.
> >
> 
> No killable tasks means  there's task and the OOM killer may be invoked.
> While no tasks means the OOM killer is useless.

I disagree.

> > > Furthermore,
> > > the OOM output is not expected by the admin if he or she only wants to drop
> > > the cahes and knows there're no processes running in this memcg.
> >
> > But this is not what hard limit reduced to 0 really does. No matter
> > whether there is some task or not. It simply reclaims _all_ the memory
> > as explained in other email.
> >
> 
> Are there any way to reclaim page cache only ?
> No.

Correct. And in absence of a solid usecase then I do not see a reason to
add this. We have a global knob to achieve this and it has turned out to
be abused and just used incorrectly most of the time.

> I know it will relcaim all the memory.
> If you really think this expression is a prolem,  but does it
> improtant that we should distingush between  caches (both page caches
> and kmem) and _all_ memory, especially when there's no processes ?

I do not think we should distinguish different memory types and treat
them differently when applying hard limit.
-- 
Michal Hocko
SUSE Labs