Re: [patch v2 4/6] memcg: simplify mem_cgroup_iter

[Michal Hocko]

On Mon 10-12-12 20:35:20, Ying Han wrote:
> On Mon, Nov 26, 2012 at 10:47 AM, Michal Hocko <mhocko@xxxxxxx> wrote:
> > Current implementation of mem_cgroup_iter has to consider both css and
> > memcg to find out whether no group has been found (css==NULL - aka the
> > loop is completed) and that no memcg is associated with the found node
> > (!memcg - aka css_tryget failed because the group is no longer alive).
> > This leads to awkward tweaks like tests for css && !memcg to skip the
> > current node.
> >
> > It will be much easier if we got rid off css variable altogether and
> > only rely on memcg. In order to do that the iteration part has to skip
> > dead nodes. This sounds natural to me and as a nice side effect we will
> > get a simple invariant that memcg is always alive when non-NULL and all
> > nodes have been visited otherwise.
> >
> > We could get rid of the surrounding while loop but keep it in for now to
> > make review easier. It will go away in the following patch.
> >
> > Signed-off-by: Michal Hocko <mhocko@xxxxxxx>
> > ---
> >  mm/memcontrol.c |   56 +++++++++++++++++++++++++++----------------------------
> >  1 file changed, 27 insertions(+), 29 deletions(-)
> >
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 6bcc97b..d1bc0e8 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -1086,7 +1086,6 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
> >         rcu_read_lock();
> >         while (!memcg) {
> >                 struct mem_cgroup_reclaim_iter *uninitialized_var(iter);
> > -               struct cgroup_subsys_state *css = NULL;
> >
> >                 if (reclaim) {
> >                         int nid = zone_to_nid(reclaim->zone);
> > @@ -1112,53 +1111,52 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
> >                  * explicit visit.
> >                  */
> >                 if (!last_visited) {
		    ^^^^^^^^
		    here

> > -                       css = &root->css;
> > +                       memcg = root;
> >                 } else {
> >                         struct cgroup *prev_cgroup, *next_cgroup;
> >
> >                         prev_cgroup = (last_visited == root) ? NULL
> >                                 : last_visited->css.cgroup;
> > -                       next_cgroup = cgroup_next_descendant_pre(prev_cgroup,
> > -                                       root->css.cgroup);
> > -                       if (next_cgroup)
> > -                               css = cgroup_subsys_state(next_cgroup,
> > -                                               mem_cgroup_subsys_id);
> > -               }
> > +skip_node:
> > +                       next_cgroup = cgroup_next_descendant_pre(
> > +                                       prev_cgroup, root->css.cgroup);
> >
> > -               /*
> > -                * Even if we found a group we have to make sure it is alive.
> > -                * css && !memcg means that the groups should be skipped and
> > -                * we should continue the tree walk.
> > -                * last_visited css is safe to use because it is protected by
> > -                * css_get and the tree walk is rcu safe.
> > -                */
> > -               if (css == &root->css || (css && css_tryget(css)))
> > -                       memcg = mem_cgroup_from_css(css);
> > +                       /*
> > +                        * Even if we found a group we have to make sure it is
> > +                        * alive. css && !memcg means that the groups should be
> > +                        * skipped and we should continue the tree walk.
> > +                        * last_visited css is safe to use because it is
> > +                        * protected by css_get and the tree walk is rcu safe.
> > +                        */
> > +                       if (next_cgroup) {
> > +                               struct mem_cgroup *mem = mem_cgroup_from_cont(
> > +                                               next_cgroup);
> > +                               if (css_tryget(&mem->css))
> > +                                       memcg = mem;
> 
> I see a functional change after this, where we now hold a refcnt of
> css if memcg is root. It is not the case before this change.

I know it is a bit obscure but this is not the case.
cgroup_next_descendant_pre never visits its root. That's why we have
that if (!last_visited) test above. We have to handle it separately.

Makes sense?

> 
> --Ying
[...]
-- 
Michal Hocko
SUSE Labs
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