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Date:   Thu, 21 Apr 2022 19:59:00 -0700
From:   Roman Gushchin <roman.gushchin@linux.dev>
To:     Waiman Long <longman@redhat.com>
Cc:     Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>,
        Shakeel Butt <shakeelb@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org, cgroups@vger.kernel.org,
        linux-mm@kvack.org, Muchun Song <songmuchun@bytedance.com>,
        "Matthew Wilcox (Oracle)" <willy@infradead.org>,
        Yang Shi <shy828301@gmail.com>,
        Vlastimil Babka <vbabka@suse.cz>
Subject: Re: [PATCH] mm/memcg: Free percpu stats memory of dying memcg's
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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?

Some time ago Muchun posted patches based on the reusing of the obj_cgroup API.

I'm not strictly against this approach, but in my opinion it's not the best.
I suggested to use lru vectors as an intermediate objects. In theory, it might
allow to avoid bumping reference counters for all charged pages at all: live
cgroups will be protected by being live, dying cgroups will only need
a temporarily protection while lru vectors and associated pages are reparenting.

There are pros and cons:
+ cgroup reference counting becomes simpler and more debuggable
+ potential perf wins from fewer operations with live cgroups css refcounters
= I hope to see code simplifications (but not guaranteed)
- deleting cgroups becomes more expensive, but the cost can be spread to
  asynchronous workers

Idk if Muchun tried to implement it. If not, I might try myself.

Thanks!