On Wed, Apr 21, 2021 at 03:00:48PM +0800, Muchun Song wrote: > Because memory allocations pinning memcgs for a long time - it exists > at a larger scale and is causing recurring problems in the real world: > page cache doesn't get reclaimed for a long time, or is used by the > second, third, fourth, ... instance of the same job that was restarted > into a new cgroup every time. Unreclaimable dying cgroups pile up, > waste memory, and make page reclaim very inefficient. > > We can convert LRU pages and most other raw memcg pins to the objcg > direction to fix this problem, and then the page->memcg will always > point to an object cgroup pointer. > > Therefore, the infrastructure of objcg no longer only serves > CONFIG_MEMCG_KMEM. In this patch, we move the infrastructure of the > objcg out of the scope of the CONFIG_MEMCG_KMEM so that the LRU pages > can reuse it to charge pages. > > We know that the LRU pages are not accounted at the root level. But > the page->memcg_data points to the root_mem_cgroup. So the > page->memcg_data of the LRU pages always points to a valid pointer. > But the root_mem_cgroup dose not have an object cgroup. If we use > obj_cgroup APIs to charge the LRU pages, we should set the > page->memcg_data to a root object cgroup. So we also allocate an > object cgroup for the root_mem_cgroup. Overall the patch looks very good to me. There are few small things to enhance: 1) I'd rename it. Looking at the title I expect a trivial code move, however the patch is doing more than this: e.g. allocating an objcg for the root memcg. Something like "prepare objcg API for non-kmem usage". 2) How about obj_cgroup_release_kmem() instead of obj_cgroup_release_uncharge()? 3) The first paragraph of the commit log looks a bit vague: which allocations pinning memcgs? How about something like this? Pagecache pages are charged at the allocation time and holding a reference to the original memory cgroup until being reclaimed. Depending on the memory pressure, specific patterns of the page sharing between different cgroups and the cgroup creation and destruction rates, a large number of dying memory cgroups can be pinned by pagecache pages. It makes the page reclaim less efficient and wastes memory. Thanks!
