Re: [patch v2 3/6] memcg: rework mem_cgroup_iter to use cgroup iterators

[Michal Hocko <mhocko@suse.cz>]

On Tue 11-12-12 14:36:10, Ying Han wrote:
> On Tue, Dec 11, 2012 at 7:54 AM, Michal Hocko <mhocko@suse.cz> wrote:
> > On Sun 09-12-12 11:39:50, Ying Han wrote:
> >> On Mon, Nov 26, 2012 at 10:47 AM, Michal Hocko <mhocko@suse.cz> wrote:
> > [...]
> >> >                 if (reclaim) {
> >> > -                       iter->position = id;
> >> > +                       struct mem_cgroup *curr = memcg;
> >> > +
> >> > +                       if (last_visited)
> >> > +                               css_put(&last_visited->css);
> >                             ^^^^^^^^^^^
> >                             here
> >> > +
> >> > +                       if (css && !memcg)
> >> > +                               curr = mem_cgroup_from_css(css);
> >> > +
> >> > +                       /* make sure that the cached memcg is not removed */
> >> > +                       if (curr)
> >> > +                               css_get(&curr->css);
> >> > +                       iter->last_visited = curr;
> >>
> >> Here we take extra refcnt for last_visited, and assume it is under
> >> target reclaim which then calls mem_cgroup_iter_break() and we leaked
> >> a refcnt of the target memcg css.
> >
> > I think you are not right here. The extra reference is kept for
> > iter->last_visited and it will be dropped the next time somebody sees
> > the same zone-priority iter. See above.
> >
> > Or have I missed your question?
> 
> Hmm, question remains.
> 
> My understanding of the mem_cgroup_iter() is that each call path
> should close the loop itself, in the sense that no *leaked* css refcnt
> after that loop finished. It is the case for all the caller today
> where the loop terminates at memcg == NULL, where all the refcnt have
> been dropped by then.

Now I am not sure I understand you. mem_cgroup_iter_break will always
drop the reference of the last returned memcg. So far so good. But if
the last memcg got cached in per-zone-priority last_visited then we
_have_ to keep a reference to it regardless we broke out of the loop.
The last_visited thingy is shared between all parallel reclaimers so we
cannot just drop a reference to it.

> One exception is mem_cgroup_iter_break(), where the loop terminates
> with *leaked* refcnt and that is what the iter_break() needs to clean
> up. We can not rely on the next caller of the loop since it might
> never happen.

Yes, this is true and I already have a half baked patch for that. I
haven't posted it yet but it basically checks all node-zone-prio
last_visited and removes itself from them on the way out in pre_destroy
callback (I just need to cleanup "find a new last_visited" part and will
post it).

> It makes sense to drop the refcnt of last_visited, the same reason as
> drop refcnt of prev. I don't see why it makes different.

Because then it might vanish when somebody else wants to access it. If
we just did mem_cgroup_get which would be enough to keep only memcg part
in memory then what can we do at the time we visit it? css_tryget would
tell us "no your buddy is gone", you do not have any links to the tree
so you would need to start from the beginning. That is what I have
implemented in the first version. Then I've realized that this could
make a bigger pressure on the groups created earlier which doesn't seem
to be right. With css pinning we are sure that there is a link to a next
node in the tree.

Thanks!
-- 
Michal Hocko
SUSE Labs

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