On Sat, Aug 14, 2021 at 01:25:08PM +0800, Muchun Song wrote: > 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. > > 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. > > Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx> I like the "move objcg stuff to memcg/css level" part. I'm less convinced about making byte-sized charging kmem-specific (both naming and #ifdef CONFIG_MEMCG_KMEM). Do we really win a lot? I understand why we might wanna compile out some checks from the hot allocation path, but few bytes in struct objcg will not make a big difference, as well as few lines of code in cgroup creation/removal paths. Also it might be useful for byte-sized accounting outside kmem, e.g. zswap. So, I'd remove this dependency and rename to something like obj_cgroup_release_bytes(). In the long run we might wanna to eliminate CONFIG_MEMCG_KMEM completely, so let's at least not add new dependencies. Thanks!
