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

[Ying Han <yinghan@google.com>]

On Tue, Dec 11, 2012 at 8:01 AM, Michal Hocko <mhocko@suse.cz> wrote:
> On Mon 10-12-12 20:35:20, Ying Han wrote:
>> On Mon, Nov 26, 2012 at 10:47 AM, Michal Hocko <mhocko@suse.cz> 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@suse.cz>
>> > ---
>> >  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?

Ah, OK. The code is more readable after this patch then

--Ying

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

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