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    SubjectRe: [patch 2/2] mm: memcg: hierarchical soft limit reclaim
    On Fri 13-01-12 16:50:01, Johannes Weiner wrote:
    > On Fri, Jan 13, 2012 at 01:04:06PM +0100, Michal Hocko wrote:
    > > On Tue 10-01-12 16:02:52, Johannes Weiner wrote:
    [...]
    > > > +bool mem_cgroup_over_softlimit(struct mem_cgroup *root,
    > > > + struct mem_cgroup *memcg)
    > > > +{
    > > > + if (mem_cgroup_disabled())
    > > > + return false;
    > > > +
    > > > + if (!root)
    > > > + root = root_mem_cgroup;
    > > > +
    > > > + for (; memcg; memcg = parent_mem_cgroup(memcg)) {
    > > > + /* root_mem_cgroup does not have a soft limit */
    > > > + if (memcg == root_mem_cgroup)
    > > > + break;
    > > > + if (res_counter_soft_limit_excess(&memcg->res))
    > > > + return true;
    > > > + if (memcg == root)
    > > > + break;
    > > > + }
    > > > + return false;
    > > > +}
    > >
    > > Well, this might be little bit tricky. We do not check whether memcg and
    > > root are in a hierarchy (in terms of use_hierarchy) relation.
    > >
    > > If we are under global reclaim then we iterate over all memcgs and so
    > > there is no guarantee that there is a hierarchical relation between the
    > > given memcg and its parent. While, on the other hand, if we are doing
    > > memcg reclaim then we have this guarantee.
    > >
    > > Why should we punish a group (subtree) which is perfectly under its soft
    > > limit just because some other subtree contributes to the common parent's
    > > usage and makes it over its limit?
    > > Should we check memcg->use_hierarchy here?
    >
    > We do, actually. parent_mem_cgroup() checks the res_counter parent,
    > which is only set when ->use_hierarchy is also set.

    Of course I am blind.. We do not setup res_counter parent for
    !use_hierarchy case. Sorry for noise...
    Now it makes much better sense. I was wondering how !use_hierarchy could
    ever work, this should be a signal that I am overlooking something
    terribly.

    [...]
    > > > @@ -2121,8 +2121,16 @@ static void shrink_zone(int priority, struct zone *zone,
    > > > .mem_cgroup = memcg,
    > > > .zone = zone,
    > > > };
    > > > + int epriority = priority;
    > > > + /*
    > > > + * Put more pressure on hierarchies that exceed their
    > > > + * soft limit, to push them back harder than their
    > > > + * well-behaving siblings.
    > > > + */
    > > > + if (mem_cgroup_over_softlimit(root, memcg))
    > > > + epriority = 0;
    > >
    > > This sounds too aggressive to me. Shouldn't we just double the pressure
    > > or something like that?
    >
    > That's the historical value. When I tried priority - 1, it was not
    > aggressive enough.

    Probably because we want to reclaim too much. Maybe we should do
    reduce nr_to_reclaim (ugly) or reclaim only overlimit groups until certain
    priority level as Ying suggested in her patchset.
    --
    Michal Hocko
    SUSE Labs
    SUSE LINUX s.r.o.
    Lihovarska 1060/12
    190 00 Praha 9
    Czech Republic


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