On Wed, May 11, 2022 at 06:34:39PM +0200, Michal Koutny wrote: > On Tue, May 10, 2022 at 08:06:24PM -0700, Roman Gushchin <roman.gushchin@xxxxxxxxx> wrote: > > My primary goal was to apply the memory pressure on memory cgroups with a lot > > of (dying) children cgroups. On a multi-cpu machine a memory cgroup structure > > is way larger than a page, so a cgroup which looks small can be really large > > if we calculate the amount of memory taken by all children memcg internals. > > > > Applying this pressure to another cgroup (e.g. the one which contains systemd) > > doesn't help to reclaim any pages which are pinning the dying cgroups. > > Just a note -- this another usecase of cgroups created from within the > subtree (e.g. a container). I agree that cgroup-manager/systemd case is > also valid (as dying memcgs may accumulate after a restart). > > memcgs with their retained state with footprint are special. > > > For other controllers (maybe blkcg aside, idk) it shouldn't matter, because > > there is no such problem there. > > > > For consistency reasons I'd suggest to charge all *large* allocations > > (e.g. percpu) to the parent cgroup. Small allocations can be ignored. > > Strictly speaking, this would mean that any controller would have on > implicit dependency on the memory controller (such as io controller > has). > In the extreme case even controller-less hierarchy would have such a > requirement (for precise kernfs_node accounting). > Such a dependency is not enforceable on v1 (with various topologies of > different hierarchies). > > Although, I initially favored the consistency with memory controller too, > I think it's simpler to charge to the creator's memcg to achieve > consistency across v1 and v2 :-) Ok, v1/v2 consistency is a valid point. As I said, I'm fine with both options, it shouldn't matter that much for anything except the memory controller: cgroup internal objects are not that large and the total memory footprint is usually small unless we have a lot of (dying) sub-cgroups. From my experience no other controllers should be affected (blkcg was affected due to a cgwb reference, but should be fine now), so it's not an issue at all. Thanks!
