Re: [PATCH] mm: memcontrol: fix occasional OOMs due to proportional memory.low reclaim

[Johannes Weiner <hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org>]

Hi Michal,

On Mon, Aug 23, 2021 at 06:09:29PM +0200, Michal Koutný wrote:
> Hello
> 
> (and sorry for a belated reply).

It's never too late, thanks for taking a look.

> On Tue, Aug 17, 2021 at 02:05:06PM -0400, Johannes Weiner <hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org> wrote:
> > @@ -2576,6 +2578,15 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
> > [...]
> > +			/* memory.low scaling, make sure we retry before OOM */
> > +			if (!sc->memcg_low_reclaim && low > min) {
> > +				protection = low;
> > +				sc->memcg_low_skipped = 1;
> 
> IIUC, this won't result in memory.events:low increment although the
> effect is similar (breaching (partial) memory.low protection) and signal
> to the user is comparable (overcommited memory.low).

Good observation. I think you're right, we should probably count such
partial breaches as LOW events as well.

Note that this isn't new behavior. My patch merely moved this part
from mem_cgroup_protection():

-       if (in_low_reclaim)
-               return READ_ONCE(memcg->memory.emin);

Even before, if we retried due to just one (possibly insignificant)
cgroup below low, we'd ignore proportional reclaim and partially
breach ALL protected cgroups, while only counting a low event for the
one group that is usage < low.

> Admittedly, this patch's behavior adheres to the current documentation
> (Documentation/admin-guide/cgroup-v2.rst):
> 
> > The number of times the cgroup is reclaimed due to high memory
> > pressure even though its usage is under the low boundary,
> 
> however, that definition might not be what the useful indicator would
> be now.
> Is it worth including these partial breaches into memory.events:low?

I think it is. How about:

"The number of times the cgroup's memory.low-protected memory was
reclaimed in order to avoid OOM during high memory pressure."

And adding a MEMCG_LOW event to partial breaches. BTW, the comment
block above this code is also out-of-date, because it says we're
honoring memory.low on the retries, but that's not the case.

I'll prepare a follow-up patch for these 3 things as well as the more
verbose comment that Michal Hocko asked for on the retry logic.