Re: [PATCH V8 2/2] mm: memcg detect no memcgs above softlimit under zone reclaim

[Michal Hocko <mhocko@suse.cz>]

On Fri 03-08-12 09:28:22, Ying Han wrote:
> On Fri, Aug 3, 2012 at 7:02 AM, Michal Hocko <mhocko@suse.cz> wrote:
> > On Thu 02-08-12 14:24:24, Ying Han wrote:
[...]
> >> diff --git a/mm/vmscan.c b/mm/vmscan.c
> >> index 88487b3..8622022 100644
> >> --- a/mm/vmscan.c
> >> +++ b/mm/vmscan.c
[...]
> >> @@ -1879,10 +1883,15 @@ static void shrink_zone(struct zone *zone, struct scan_control *sc)
> >>                * we have to reclaim under softlimit instead of burning more
> >>                * cpu cycles.
> >>                */
> >> -             if (!global_reclaim(sc) || sc->priority < DEF_PRIORITY ||
> >> -                             mem_cgroup_over_soft_limit(memcg))
> >> +             if (ignore_softlimit || !global_reclaim(sc) ||
> >> +                             sc->priority < DEF_PRIORITY ||
> >> +                             mem_cgroup_over_soft_limit(memcg)) {
> >>                       shrink_lruvec(lruvec, sc);
> >>
> >> +                     if (!mem_cgroup_is_root(memcg))
> >> +                             over_softlimit = true;
> >> +             }
> >> +
> >
> > I think this is still not sufficient because you do not want to hammer
> > root in the ignore_softlimit case.
> 
> Are you worried about over-reclaiming from root cgroup while the rest
> of the cgroup are under softimit? 

yes

> Hmm.. That only affect the DEF_PRIORITY level, and not sure how bad it
> is.

Even if it was for DEF_PRIORITY it would mean that the root group is
second class citizen which is definitely not good.

> On the other hand, I wonder if it is necessary bad since the pages
> under root cgroup are mainly re-parented pages which only get chance
> to be reclaimed under global pressure.

Hmm, I do not think this is true in general and you shouldn't rely on
it.

> 
> --Ying
> 
> > --
> > Michal Hocko
> > SUSE Labs

-- 
Michal Hocko
SUSE Labs

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