Re: [PATCH] mm/memcg: Free percpu stats memory of dying memcg's

Waiman Long
2022-04-25T01:01:45Z

On 4/21/22 22:29, Muchun Song wrote:
> On Thu, Apr 21, 2022 at 02:46:00PM -0400, Waiman Long wrote:
>> On 4/21/22 13:59, Roman Gushchin wrote:
>>> On Thu, Apr 21, 2022 at 01:28:20PM -0400, Waiman Long wrote:
>>>> On 4/21/22 12:33, Roman Gushchin wrote:
>>>>> On Thu, Apr 21, 2022 at 10:58:45AM -0400, Waiman Long wrote:
>>>>>> For systems with large number of CPUs, the majority of the memory
>>>>>> consumed by the mem_cgroup structure is actually the percpu stats
>>>>>> memory. When a large number of memory cgroups are continuously created
>>>>>> and destroyed (like in a container host), it is possible that more
>>>>>> and more mem_cgroup structures remained in the dying state holding up
>>>>>> increasing amount of percpu memory.
>>>>>>
>>>>>> We can't free up the memory of the dying mem_cgroup structure due to
>>>>>> active references in some other places. However, the percpu stats memory
>>>>>> allocated to that mem_cgroup is a different story.
>>>>>>
>>>>>> This patch adds a new percpu_stats_disabled variable to keep track of
>>>>>> the state of the percpu stats memory. If the variable is set, percpu
>>>>>> stats update will be disabled for that particular memcg. All the stats
>>>>>> update will be forward to its parent instead. Reading of the its percpu
>>>>>> stats will return 0.
>>>>>>
>>>>>> The flushing and freeing of the percpu stats memory is a multi-step
>>>>>> process. The percpu_stats_disabled variable is set when the memcg is
>>>>>> being set to offline state. After a grace period with the help of RCU,
>>>>>> the percpu stats data are flushed and then freed.
>>>>>>
>>>>>> This will greatly reduce the amount of memory held up by dying memory
>>>>>> cgroups.
>>>>>>
>>>>>> By running a simple management tool for container 2000 times per test
>>>>>> run, below are the results of increases of percpu memory (as reported
>>>>>> in /proc/meminfo) and nr_dying_descendants in root's cgroup.stat.
>>>>> Hi Waiman!
>>>>>
>>>>> I've been proposing the same idea some time ago:
>>>>> https://lore.kernel.org/all/20190312223404.28665-7-guro@fb.com/T/ .
>>>>>
>>>>> However I dropped it with the thinking that with many other fixes
>>>>> preventing the accumulation of the dying cgroups it's not worth the added
>>>>> complexity and a potential cpu overhead.
>>>>>
>>>>> I think it ultimately comes to the number of dying cgroups. If it's low,
>>>>> memory savings are not worth the cpu overhead. If it's high, they are.
>>>>> I hope long-term to drive it down significantly (with lru-pages reparenting
>>>>> being the first major milestone), but it might take a while.
>>>>>
>>>>> I don't have a strong opinion either way, just want to dump my thoughts
>>>>> on this.
>>>> I have quite a number of customer cases complaining about increasing percpu
>>>> memory usages. The number of dying memcg's can go to tens of thousands. From
>>>> my own investigation, I believe that those dying memcg's are not freed
>>>> because they are pinned down by references in the page structure. I am aware
>>>> that we support the use of objcg in the page structure which will allow easy
>>>> reparenting, but most pages don't do that and it is not easy to do this
>>>> conversion and it may take quite a while to do that.
>>> The big question is whether there is a memory pressure on those systems.
>>> If yes, and the number of dying cgroups is growing, it's worth investigating.
>>> It might be due to the sharing of pagecache pages and this will be ultimately
>>> fixed with implementing of the pagecache reparenting. But it also might be due
>>> to other bugs, which are fixable, so it would be great to understand.
>>
>> Pagecache reparenting will probably fix the problem that I have seen. Is
>> someone working on this?
>>
> We also encountered dying cgroup issue on our servers for a long time.
> I have worked on this for a while and proposed a resolution [1] based
> on obj_cgroup APIs to charge the LRU pages.
>
> [1] https://lore.kernel.org/all/20220216115132.52602-1-songmuchun@bytedance.com/

Thanks for the pointer. I am interested in this patch series. Please cc
me if you need to generate a new revision.

Cheers,
Longman