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SubjectRe: [patch v2 3/6] memcg: rework mem_cgroup_iter to use cgroup iterators
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On Wed, Dec 12, 2012 at 1:06 AM, Michal Hocko <mhocko@suse.cz> wrote:
> 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.

Yes, and the patch doesn't change that.

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.

Agree that the last_visited is shared between all the memcgs accessing
the per-zone-per-iterator.

Also agree that we don't want to drop reference of it if last_visited
is cached after the loop.

But If i look at the callers of mem_cgroup_iter(), they all look like
the following:

memcg = mem_cgroup_iter(root, NULL, &reclaim);
do {

// do something

memcg = mem_cgroup_iter(root, memcg, &reclaim);
} while (memcg);

So we get out of the loop when memcg returns as NULL, where the
last_visited is cached as NULL as well thus no css_get(). That is what
I meant by "each reclaim thread closes the loop". If that is true, the
current implementation of mem_cgroup_iter_break() changes that.


>
>> 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).

Not sure whether that or just change the mem_cgroup_iter_break() by
dropping the refcnt of last_visited.

--Ying
>
>> 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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