Re: [RFC 1/3] memcg: integrate soft reclaim tighter with zone shrinking code

[Michal Hocko <mhocko@suse.cz>]

On Tue 09-04-13 17:57:54, Glauber Costa wrote:
> On 04/09/2013 04:13 PM, Michal Hocko wrote:
[...]
> > +static void shrink_zone(struct zone *zone, struct scan_control *sc)
> > +{
> > +	bool do_soft_reclaim = mem_cgroup_should_soft_reclaim(sc);
> > +	unsigned long nr_scanned = sc->nr_scanned;
> > +	unsigned nr_shrunk;
> > +
> > +	nr_shrunk = __shrink_zone(zone, sc, do_soft_reclaim);
> > +
> > +	/*
> > +	 * No group is over the soft limit or those that are do not have
> > +	 * pages in the zone we are reclaiming so we have to reclaim everybody
> > +	 */
> > +	if (do_soft_reclaim && (!nr_shrunk || sc->nr_scanned == nr_scanned)) {
> > +		__shrink_zone(zone, sc, false);
> > +		return;
> > +	}
> >  }
> 
> If I read this correctly, you stop shrinking when you reach a group in
> which you manage to shrink some pages. Is it really what we want?

Well, this is what we do during standard reclaim __shrink_zone either
walks all children of the target_memcg or reclaim enough pages.

> We have no guarantee that we're now under the soft limit, so shouldn't
> we keep shrinking downwards until every parent of ours is within limits ?

I do not think we should reclaim until we are under soft limit because
our primary target is different - balance zones resp. get under hard
limit. Soft limit just helps us to point at victims (and newly also to
protect high class citizens).

So the second round is just a way to reclaim at least something if
the is nobody eligible for the soft game part. I can see some harder
conditions for the fallback (e.g. only fallback after certain priority
but let's keep this simple for now and do additional parts on top).

Thanks
-- 
Michal Hocko
SUSE Labs

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