On Tue, Jul 19, 2022 at 12:30:17PM -0700, Yosry Ahmed wrote: > Is there a reason why these resources cannot be moved across cgroups > dynamically? The only scenario I imagine is if you already have tmpfs > mounted and files charged to different cgroups, but once you attribute > tmpfs to one cgroup.charge_for.tmpfs (or sticky,..), I assume that we > can dynamically move the resources, right? > > In fact, is there a reason why we can't move the tmpfs charges in that > scenario as well? When we move processes we loop their pages tables > and move pages and their stats, is there a reason why we wouldn't be > able to do this with tmpfs mounts or bpf maps as well? Nothing is impossible but nothing is free as well. Moving charges around traditionally caused a lot of headaches in the past and never became reliable. There are inherent trade-offs here. You can make things more dynamic usually by making hot paths more expensive or doing some synchronization dancing which tends to be pretty hairy. People generally don't wanna make hot paths slower, so we tend to end up with something twisted which unfortunately turns out to be a headache in the long term. In general, I'd rather keep resource associations as static as possible. It's okay if we do something neat inside the kernel but if we create userspace expectation that resources can be moved around dynamically, we'll be stuck with that for a long time likely forfeiting future simplification / optimization opportunities. So, that's gonna be a fairly strong nack from my end. Thanks. -- tejun
