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

[Ying Han <yinghan@google.com>]

On Fri, Dec 7, 2012 at 9:27 AM, Michal Hocko <mhocko@suse.cz> wrote:
> On Fri 07-12-12 09:12:25, Ying Han wrote:
>> On Fri, Dec 7, 2012 at 12:58 AM, Michal Hocko <mhocko@suse.cz> wrote:
>> > On Thu 06-12-12 19:43:52, Ying Han wrote:
>> > [...]
>> >> Forgot to mention, I was testing 3.7-rc6 with the two cgroup changes :
>> >
>> > Could you give a try to -mm tree as well. There are some changes for
>> > memcgs removal in that tree which are not in Linus's tree.
>>
>> I will give a try, which patchset you have in mind so i can double check?
>
> Have a look at ba5e0e6be1c76fd37508b2825372b28a90a5b729 in my tree.

Tried the tag: mmotm-2012-12-05-16-59 which includes the commit above.
The test runs better. Thank you for the pointer.

Looking into the patch itself, it includes 9 patchset where 6 from
cgroup and 3 from memcg.

    Michal Hocko (3):
          memcg: make mem_cgroup_reparent_charges non failing
          hugetlb: do not fail in hugetlb_cgroup_pre_destroy
          Merge remote-tracking branch
'tj-cgroups/cgroup-rmdir-updates' into mmotm

    Tejun Heo (6):
          cgroup: kill cgroup_subsys->__DEPRECATED_clear_css_refs
          cgroup: kill CSS_REMOVED
          cgroup: use cgroup_lock_live_group(parent) in cgroup_create()
          cgroup: deactivate CSS's and mark cgroup dead before
invoking ->pre_destroy()
          cgroup: remove CGRP_WAIT_ON_RMDIR, cgroup_exclude_rmdir()
and cgroup_release_and_wakeup_rmdir()
          cgroup: make ->pre_destroy() return void

Any suggestion of the minimal patchset I need to apply for testing
this patchset? (hopefully not all of them)

--Ying

>
>> I didn't find the place where the css pins the memcg, which either
>> something i missed or not in place in my tree.
>
> Yeah, it is carefully hidden ;).
> css pins cgroup (last css_put will call dput on the cgroup dentry - via
> css_dput_fn) and the last reference to memcg is dropped from ->destroy
> callback (mem_cgroup_destroy) called from cgroup_diput.
>
>> I twisted the patch a bit to make it closer to your v2 version,
>> but instead keep the mem_cgroup_put() as well as using
>> css_tryget(). Again, my test is happy with it:
>
> This is really strange, there must be something weird with ref counting
> going on.
> Anyway, thanks for your testing! I will try to enahance my testing some
> more next week.
>
>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>> index f2eeee6..acec05a 100644
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -1004,6 +1004,7 @@ struct mem_cgroup *mem_cgroup_iter(struct
>> mem_cgroup *root,
>>                         if (prev && reclaim->generation != iter->generation) {
>>                                 if (last_visited) {
>>                                         css_put(&last_visited->css);
>> +                                       mem_cgroup_put(last_visited);
>>                                         iter->last_visited = NULL;
>>                                 }
>>                                 spin_unlock(&iter->iter_lock);
>> @@ -1041,15 +1042,22 @@ struct mem_cgroup *mem_cgroup_iter(struct
>> mem_cgroup *root,
>>                 if (reclaim) {
>>                         struct mem_cgroup *curr = memcg;
>>
>> -                       if (last_visited)
>> +                       if (last_visited) {
>>                                 css_put(&last_visited->css);
>> +                               mem_cgroup_put(last_visited);
>> +                       }
>>
>>                         if (css && !memcg)
>>                                 curr = container_of(css, struct
>> mem_cgroup, css);
>>
>>                         /* make sure that the cached memcg is not removed */
>> -                       if (curr)
>> -                               css_get(&curr->css);
>> +                       if (curr) {
>> +                               mem_cgroup_get(curr);
>> +                               if (!css_tryget(&curr->css)) {
>> +                                       mem_cgroup_put(curr);
>> +                                       curr = NULL;
>> +                               }
>> +                       }
>>                         iter->last_visited = curr;
>>
>>                         if (!css)
>>
>>
>> --Ying
>>
>> > --
>> > Michal Hocko
>> > SUSE Labs
>> --
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>
> --
> Michal Hocko
> SUSE Labs

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