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

[Roman Gushchin <roman.gushchin@linux.dev>]

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.

Thanks!