Re: [PATCH 1/4] memcg, mm: introduce lowlimit reclaim

[Johannes Weiner]

On Fri, May 02, 2014 at 11:36:28AM +0200, Michal Hocko wrote:
> On Wed 30-04-14 18:55:50, Johannes Weiner wrote:
> > On Mon, Apr 28, 2014 at 02:26:42PM +0200, Michal Hocko wrote:
> > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > > index 19d620b3d69c..40e517630138 100644
> > > --- a/mm/memcontrol.c
> > > +++ b/mm/memcontrol.c
> > > @@ -2808,6 +2808,29 @@ static struct mem_cgroup *mem_cgroup_lookup(unsigned short id)
> > >  	return mem_cgroup_from_id(id);
> > >  }
> > >  
> > > +/**
> > > + * mem_cgroup_reclaim_eligible - checks whether given memcg is eligible for the
> > > + * reclaim
> > > + * @memcg: target memcg for the reclaim
> > > + * @root: root of the reclaim hierarchy (null for the global reclaim)
> > > + *
> > > + * The given group is reclaimable if it is above its low limit and the same
> > > + * applies for all parents up the hierarchy until root (including).
> > > + */
> > > +bool mem_cgroup_reclaim_eligible(struct mem_cgroup *memcg,
> > > +		struct mem_cgroup *root)
> > 
> > Could you please rename this to something that is more descriptive in
> > the reclaim callsite?  How about mem_cgroup_within_low_limit()?
> 
> I have intentionally used somethig that is not low_limit specific. The
> generic reclaim code does't have to care about the reason why a memcg is
> not reclaimable. I agree that having follow_low_limit paramter explicit
> and mem_cgroup_reclaim_eligible not is messy. So something should be
> renamed. I would probably go with s@follow_low_limit@check_reclaim_eligible@
> but I do not have a strong preference.
> 
> > > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > > index c1cd99a5074b..0f428158254e 100644
> > > --- a/mm/vmscan.c
> > > +++ b/mm/vmscan.c
> [...]
> > > +static void shrink_zone(struct zone *zone, struct scan_control *sc)
> > > +{
> > > +	if (!__shrink_zone(zone, sc, true)) {
> > > +		/*
> > > +		 * First round of reclaim didn't find anything to reclaim
> > > +		 * because of low limit protection so try again and ignore
> > > +		 * the low limit this time.
> > > +		 */
> > > +		__shrink_zone(zone, sc, false);
> > > +	}

So I don't think this can work as it is, because we are not actually
changing priority levels yet.  It will give up on the guarantees of
bigger groups way before smaller groups are even seriously looked at.

> > I would actually prefer not having a second round here, and make the
> > low limit behave more like mlock memory.  If there is no reclaimable
> > memory, go OOM.
> 
> This was done in my previous attempt and I prefer OOM myself but it is
> also true that starting with a more relaxed limit and adding an
> option for hard guarantee later when we have a clear usecase is a better
> approach. Although I can see potential in go-oom-rather-than-reclaim
> configurations, usecases I am primarily interested in won't overcommit on
> low_limit.
> 
> That being said, I like the idea of having the hard guarantee but I also
> think it should be configurable. I can post those patches in this thread
> but I feel it is too early as nobody has explicitly asked for this yet.

As per above, this makes the semantics so much more fishy.  When
exactly do we stop honoring the guarantees in the process?

This is not even guarantees anymore, but rather another reclaim
prioritization scheme with best-effort semantics.  That went over
horribly with soft limits, and I don't want to repeat this.

Overcommitting on guarantees makes no sense, and you even agree you
are not interested in it.  We also agree that we can always add a knob
later on to change semantics when an actual usecase presents itself,
so why not start with the clear and simple semantics, and the simpler
implementation?
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