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    SubjectRe: [patch 4/8] memcg: rework soft limit reclaim
    On Wed 15-06-11 15:57:59, Ying Han wrote:
    > On Fri, Jun 10, 2011 at 12:36 AM, Michal Hocko <mhocko@suse.cz> wrote:
    > > On Thu 09-06-11 17:00:26, Michal Hocko wrote:
    > >> On Thu 02-06-11 22:25:29, Ying Han wrote:
    > >> > On Thu, Jun 2, 2011 at 2:55 PM, Ying Han <yinghan@google.com> wrote:
    > >> > > On Tue, May 31, 2011 at 11:25 PM, Johannes Weiner <hannes@cmpxchg.org> wrote:
    > >> > >> Currently, soft limit reclaim is entered from kswapd, where it selects
    > >> [...]
    > >> > >> diff --git a/mm/vmscan.c b/mm/vmscan.c
    > >> > >> index c7d4b44..0163840 100644
    > >> > >> --- a/mm/vmscan.c
    > >> > >> +++ b/mm/vmscan.c
    > >> > >> @@ -1988,9 +1988,13 @@ static void shrink_zone(int priority, struct zone *zone,
    > >> > >>                unsigned long reclaimed = sc->nr_reclaimed;
    > >> > >>                unsigned long scanned = sc->nr_scanned;
    > >> > >>                unsigned long nr_reclaimed;
    > >> > >> +               int epriority = priority;
    > >> > >> +
    > >> > >> +               if (mem_cgroup_soft_limit_exceeded(root, mem))
    > >> > >> +                       epriority -= 1;
    > >> > >
    > >> > > Here we grant the ability to shrink from all the memcgs, but only
    > >> > > higher the priority for those exceed the soft_limit. That is a design
    > >> > > change
    > >> > > for the "soft_limit" which giving a hint to which memcgs to reclaim
    > >> > > from first under global memory pressure.
    > >> >
    > >> >
    > >> > Basically, we shouldn't reclaim from a memcg under its soft_limit
    > >> > unless we have trouble reclaim pages from others.
    > >>
    > >> Agreed.
    > >>
    > >> > Something like the following makes better sense:
    > >> >
    > >> > diff --git a/mm/vmscan.c b/mm/vmscan.c
    > >> > index bdc2fd3..b82ba8c 100644
    > >> > --- a/mm/vmscan.c
    > >> > +++ b/mm/vmscan.c
    > >> > @@ -1989,6 +1989,8 @@ restart:
    > >> >         throttle_vm_writeout(sc->gfp_mask);
    > >> >  }
    > >> >
    > >> > +#define MEMCG_SOFTLIMIT_RECLAIM_PRIORITY       2
    > >> > +
    > >> >  static void shrink_zone(int priority, struct zone *zone,
    > >> >                                 struct scan_control *sc)
    > >> >  {
    > >> > @@ -2001,13 +2003,13 @@ static void shrink_zone(int priority, struct zone *zone,
    > >> >                 unsigned long reclaimed = sc->nr_reclaimed;
    > >> >                 unsigned long scanned = sc->nr_scanned;
    > >> >                 unsigned long nr_reclaimed;
    > >> > -               int epriority = priority;
    > >> >
    > >> > -               if (mem_cgroup_soft_limit_exceeded(root, mem))
    > >> > -                       epriority -= 1;
    > >> > +               if (!mem_cgroup_soft_limit_exceeded(root, mem) &&
    > >> > +                               priority > MEMCG_SOFTLIMIT_RECLAIM_PRIORITY)
    > >> > +                       continue;
    > >>
    > >> yes, this makes sense but I am not sure about the right(tm) value of the
    > >> MEMCG_SOFTLIMIT_RECLAIM_PRIORITY. 2 sounds too low.
    > >
    > > There is also another problem. I have just realized that this code path
    > > is shared with the cgroup direct reclaim. We shouldn't care about soft
    > > limit in such a situation. It would be just a wasting of cycles. So we
    > > have to:
    > >
    > > if (current_is_kswapd() &&
    > >        !mem_cgroup_soft_limit_exceeded(root, mem) &&
    > >        priority > MEMCG_SOFTLIMIT_RECLAIM_PRIORITY)
    > >        continue;
    >
    > Agreed.
    >
    > >
    > > Maybe the condition would have to be more complex for per-cgroup
    > > background reclaim, though.
    >
    > That would be the same logic for per-memcg direct reclaim. In general,
    > we don't consider soft_limit
    > unless the global memory pressure. So the condition could be something like:
    >
    > > if ( global_reclaim(sc) &&
    > >        !mem_cgroup_soft_limit_exceeded(root, mem) &&
    > >        priority > MEMCG_SOFTLIMIT_RECLAIM_PRIORITY)
    > >        continue;
    >
    > make sense?

    Yes seems to be more consistent.

    --
    Michal Hocko
    SUSE Labs
    SUSE LINUX s.r.o.
    Lihovarska 1060/12
    190 00 Praha 9
    Czech Republic
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