On Tue, Apr 11, 2023 at 4:36 PM T.J. Mercier <tjmercier@google.com> wrote:
>
> When a memcg is removed by userspace it gets offlined by the kernel.
> Offline memcgs are hidden from user space, but they still live in the
> kernel until their reference count drops to 0. New allocations cannot
> be charged to offline memcgs, but existing allocations charged to
> offline memcgs remain charged, and hold a reference to the memcg.
>
> As such, an offline memcg can remain in the kernel indefinitely,
> becoming a zombie memcg. The accumulation of a large number of zombie
> memcgs lead to increased system overhead (mainly percpu data in struct
> mem_cgroup). It also causes some kernel operations that scale with the
> number of memcgs to become less efficient (e.g. reclaim).
>
> There are currently out-of-tree solutions which attempt to
> periodically clean up zombie memcgs by reclaiming from them. However
> that is not effective for non-reclaimable memory, which it would be
> better to reparent or recharge to an online cgroup. There are also
> proposed changes that would benefit from recharging for shared
> resources like pinned pages, or DMA buffer pages.
>
> Suggested attendees:
> Yosry Ahmed <yosryahmed@google.com>
> Yu Zhao <yuzhao@google.com>
> T.J. Mercier <tjmercier@google.com>
> Tejun Heo <tj@kernel.org>
> Shakeel Butt <shakeelb@google.com>
> Muchun Song <muchun.song@linux.dev>
> Johannes Weiner <hannes@cmpxchg.org>
> Roman Gushchin <roman.gushchin@linux.dev>
> Alistair Popple <apopple@nvidia.com>
> Jason Gunthorpe <jgg@nvidia.com>
> Kalesh Singh <kaleshsingh@google.com>

For the record, here are the slides that were presented for this
discussion (attached).