Re: [PATCH v2] mm, memcg: avoid oom if cgroup is not populated

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

On Wed, Nov 27, 2019 at 7:11 PM David Hildenbrand <david@redhat.com> wrote:
>
> On 27.11.19 02:28, Yafang Shao wrote:
>
> Let me give this patch description an overhaul:
>

Well done!
Thanks for your work.

> > 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.
>
> "When there are no more processes in a memcg (e.g., due to OOM
> group), we can still have file pages in the page cache."
>
> > 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.
>
> "If these pages are protected by memory.min, they can't be reclaimed.
> Especially if there won't be another process in this memcg and the memcg
> is kept online, we do want to drop these pages from the page cache."
>
> > 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.
>
> "By dropping these page caches we can avoid reclaimers (e.g., kswapd or
> direct) to scan and reclaim pages from all memcgs in the system -
> because the reclaimers will try to fairly reclaim pages from all memcgs
> in the system when 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 outputs, that should not happen.
> > The OOM output is not expected by the admin if he or she wants to drop
> > the cahes and knows there're no processes in this memcg.
>
> "By setting the hard limit of such a memcg to 0, we allow to drop the
> page cache of such memcgs. Unfortunately, this may invoke the OOM killer
> and generate a lot of output. The OOM output is not expected by an admin
> who wants to drop these caches and knows that there are no processes in
> this memcg anymore."
>
> >
> > If memcg is not populated, we should not invoke the OOM killer because
> > there's nothing to kill. Next time when you start a new process and if the
> > max is still bellow usage, the OOM killer will be invoked and your new
> > process is killed, so we can cosider it as lazy OOM, that is we have been
> > always doing in the kernel.
>
> "Therefore, if a memcg is not populated, we should not invoke the OOM
> killer - there is nothing to kill. The next time a new process is
> started in the memcg and the "max" is still below usage, the OOM killer
> will be invoked and the new process will be killed."
>
> 1. I don't think the "lazy OOM" part is relevant.
>

That doesn't imporatant.

> 2. Where is the part that modifies the limits? or did you drop that? is
> it part of another patch?
>

No. it is not part of another patch.
Modifying the limits is really a workaround that Michal[1] has told me
to fix my problem,
while actually it doesn't work, that is why I submit this patch.

1. https://lore.kernel.org/linux-mm/20191126073129.GA20912@dhcp22.suse.cz/


> 3. I think I agree with Michal that modifying the limits smells more
> like a configuration thingy to be handled by an admin (especially, adapt
> min/max properly). But again, not sure where that change is located :)
>

I agree with you all, but that is Michal told me to do. See above and
the disccussion in this thread.

> 4. This patch on its own (if there are no processes, there is nothing to
> kill) does not sound too wrong to me. Instead of an endless loop
> (besides signals) where we can't make any progress, we exit right away.
>

Thanks for you feedback.

> (I am not yet too familiar with memgc, Michal is clearly the expert :) )
>

I agree with you that Michal is an expert, but clearly that Michal is
not an expert on this issue.

> >
> > Fixes: b6e6edcf ("mm: memcontrol: reclaim and OOM kill when shrinking memory.max below usage")
> > Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
> > Cc: Johannes Weiner <hannes@cmpxchg.org>
> > Cc: Michal Hocko <mhocko@suse.com>
> > ---
> >  mm/memcontrol.c | 15 +++++++++++++--
> >  1 file changed, 13 insertions(+), 2 deletions(-)
> >
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 1c4c08b..e936f1b 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -6139,9 +6139,20 @@ static ssize_t memory_max_write(struct kernfs_open_file *of,
> >                       continue;
> >               }
> >
> > -             memcg_memory_event(memcg, MEMCG_OOM);
> > -             if (!mem_cgroup_out_of_memory(memcg, GFP_KERNEL, 0))
> > +             /* If there's no procesess, we don't need to invoke the OOM
> > +              * killer. Then next time when you try to start a process
> > +              * in this memcg, the max may still bellow usage, and then
> > +              * this OOM killer will be invoked. This can be considered
> > +              * as lazy OOM, that is we have been always doing in the
> > +              * kernel. Pls. Michal, that is really consistency.
> > +              */
> > +             if (cgroup_is_populated(memcg->css.cgroup)) {
> > +                     memcg_memory_event(memcg, MEMCG_OOM);
> > +                     if (!mem_cgroup_out_of_memory(memcg, GFP_KERNEL, 0))
> > +                             break;
> > +             } else  {
> >                       break;
> > +             }
> >       }
> >
> >       memcg_wb_domain_size_changed(memcg);
> >
>
>
> --
> Thanks,
>
> David / dhildenb
>


Thanks
Yafang