Chris Li <chrisl@xxxxxxxxxx> writes: > Hi T.J., > > On Tue, Apr 11, 2023 at 04:36:37PM -0700, T.J. Mercier 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. > > I am also interested in this topic. T.J. and I have some offline > discussion about this. We have some proposals to solve this > problem. > > I will share the write up here for the up coming LSF/MM discussion. Unfortunately I won't be attending LSF/MM in person this year but I am interested in this topic as well from the point of view of limiting pinned pages with cgroups. I am hoping to revive this patch series soon: https://lore.kernel.org/linux-mm/cover.c238416f0e82377b449846dbb2459ae9d7030c8e.1675669136.git-series.apopple@xxxxxxxxxx/ The main problem with this series was getting agreement on whether to add pinning as a separate cgroup (which is what the series currently does) or whether it should be part of a per-page memcg limit. The issue with per-page memcg limits is what to do for shared mappings. The below suggestion sounds promising because the pins for shared pages could be charged to the smemcg. However I'm not sure how it would solve the problem of a process in cgroup A being able to raise the pin count of cgroup B when pinning a smemcg page which was one of the reason I introduced a new cgroup controller. > Shared Memory Cgroup Controllers > > = Introduction > > The current memory cgroup controller does not support shared memory > objects. For the memory that is shared between different processes, it > is not obvious which process should get charged. Google has some > internal tmpfs “memcg=” mount option to charge tmpfs data to a > specific memcg that’s often different from where charging processes > run. However it faces some difficulties when the charged memcg exits > and the charged memcg becomes a zombie memcg. > Other approaches include “re-parenting” the memcg charge to the parent > memcg. Which has its own problem. If the charge is huge, iteration of > the reparenting can be costly. > > = Proposed Solution > > The proposed solution is to add a new type of memory controller for > shared memory usage. E.g. tmpfs, hugetlb, file system mmap and > dma_buf. This shared memory cgroup controller object will have the > same life cycle of the underlying shared memory. > > Processes can not be added to the shared memory cgroup. Instead the > shared memory cgroup can be added to the memcg using a “smemcg” API > file, similar to adding a process into the “tasks” API file. > When a smemcg is added to the memcg, the amount of memory that has > been shared in the memcg process will be accounted for as the part of > the memcg “memory.current”.The memory.current of the memcg is make up > of two parts, 1) the processes anonymous memory and 2) the memory > shared from smemcg. > > When the memcg “memory.current” is raised to the limit. The kernel > will active try to reclaim for the memcg to make “smemcg memory + > process anonymous memory” within the limit. That means a process in one cgroup could force reclaim of smemcg memory in use by a process in another cgroup right? I guess that's no different to the current situation though. > Further memory allocation > within those memcg processes will fail if the limit can not be > followed. If many reclaim attempts fail to bring the memcg > “memory.current” within the limit, the process in this memcg will get > OOM killed. How would this work if say a charge for cgroup A to a smemcg in both cgroup A and B would cause cgroup B to go over its memory limit and not enough memory could be reclaimed from cgroup B? OOM killing a process in cgroup B due to a charge from cgroup A doesn't sound like a good idea. > = Benefits > > The benefits of this solution include: > * No zombie memcg. The life cycle of the smemcg match the share memory file system or dma_buf. If we added pinning it could get a bit messier, as it would have to hang around until the driver unpinned the pages. But I don't think that's a problem. > * No reparenting. The shared memory only charge once to the smemcg > object. A memcg can include a smemcg to as part of the memcg memory > usage. When process exit and memcg get deleted, the charge remain to > the smemcg object. > * Much cleaner mental model of the smemcg, each share memory page is charge to one smemcg only once. > * Notice the same smemcg can add to more than one memcg. It can better > describe the shared memory relation. > > Chris > > > >> Suggested attendees: >> Yosry Ahmed <yosryahmed@xxxxxxxxxx> >> Yu Zhao <yuzhao@xxxxxxxxxx> >> T.J. Mercier <tjmercier@xxxxxxxxxx> >> Tejun Heo <tj@xxxxxxxxxx> >> Shakeel Butt <shakeelb@xxxxxxxxxx> >> Muchun Song <muchun.song@xxxxxxxxx> >> Johannes Weiner <hannes@xxxxxxxxxxx> >> Roman Gushchin <roman.gushchin@xxxxxxxxx> >> Alistair Popple <apopple@xxxxxxxxxx> >> Jason Gunthorpe <jgg@xxxxxxxxxx> >> Kalesh Singh <kaleshsingh@xxxxxxxxxx> >>
