On Thu, Oct 20, 2016 at 02:59:45PM -0400, Tejun Heo wrote: The big question I have around cgroup v2 is state of support for all controllers that libvirt uses (cpu, cpuacct, cpuset, memory, devices, freezer, blkio). IIUC, not all of these have been ported to cgroup v2 setup and the cpu port in particular was rejected by Linux maintainers. Libvirt has a general policy that we won't support features that only exist in out of tree patches (applies to kernel and any other software we build against or use). IIRC from earlier discussions, the model for dealing with processes in cgroup v2 was quite different. In libvirt we rely on the ability to assign different threads within a process to different cgroups, because we need to control CPU schedular parameters on different threads in QEMU. eg we have vCPU threads, I/O threads and general emulator threads each of which get different policies. When I spoke with Lennart about cgroup v2, way back in Jan, he indicated that while systemd can technically work with a system where some controllers are mounted as v1, while others are mounted as v2, this would not be an officially supported solution. Thus systemd in Fedora was not likely to switch to v2 until all required controllers could use v2. I'm not sure if this still corresponds to Lennarts current views, so CC'ing him to confirm/deny. I think from Libvirt POV it would greatly simplify life if we could likewise restrict ourselves to dealing with hosts which are exclusively v1 or exclusively v2, and not a mixture. ie we can completely isolate our codebases for v1 vs v2 management, making it easier to reason about and test their correctness, reducing QA testing burden. I recall that systemd policy for v2 was inteded to be that no app should write to cgroup sysfs except for systemd, unless there was a sub-tree created with Delegate=yes set on the scope. So this clearly means when using v2 we'll have to use the systemd DBus APIs for managing cgroups v2 on such hosts. While it is certainly nice that the vast majority of OS distros have switched over to using systemd for init, there's still enough users out there that I think we'll need to continue to have libvirt support for using sysfs for v2 on non-systemd hosts. Any way in summary, we'd like to see v2 support of course, since that is clearly the future. The big question is what we do about situation wrt not all controllers being supported in v2 - the lack of complete conversion is what has stopped me from doing any work in this area upto now. Regards, Daniel -- |: http://berrange.com -o- http://www.flickr.com/photos/dberrange/ :| |: http://libvirt.org -o- http://virt-manager.org :| |: http://entangle-photo.org -o- http://search.cpan.org/~danberr/ :|

Hello, Daniel. On Fri, Oct 21, 2016 at 11:19:02AM +0100, Daniel P. Berrange wrote: I see and that's understandable. However, I think supporting resource control through systemd can be a good way of navigating the situation. The back and forward compatibility issues are handled by systemd allowing libvirt users to make use of what's available on the system without burdening libvirt with complications. How thread granularity will be handled in cgroup v2 is still contentious but I believe that we'll eventually have something. I have always been curious about the QEMU thread control tho. What prevents it from using the usual nice level adjustments? Does it actually require hierarchical resource distribution? The hybrid mode implemented in systemd uses cgroup v2 for process management (the "name=systemd" hierarchy) but keeps using v1 hierarchies for all resource control. For "Delegate=" users, I don't think it'd matter all that much. Such users either see all v1 hierarchies for all resource controllers as before or the v2 hierarchy. I think that's gonna be the case. People *may* try to mix v1 and v2 hierarchies for resource control manually but supporting the mixture in any major software project would require a lot of complications which are difficult to justify. Hmmm... maybe I'm mistaken but it's also kinda broken without "Delegate=" on v1 too and we got bitten by that already. An internal software assumed that it can branch down from the cgroups that the target process is in at the time of startup and ended up building sub-hierarchies at different positions in different hierarchies. Later somebody launched a systemd service which requested some resource accounting and systemd ended up relocating processes from those sub-hierarchies. On systemd systems, I don't think it makes sense to try to do sub-hierarchy management directly without telling systemd about it. The flip side is the same too. With "Delegate=" set, cgroup v2 doesn't pose any more problems than v1 does. Definitely. What I'm suggesting now is, if available, to use systemd to set up resource control up to delegation point. This also would make control ownership arbitration between systemd and libvirt easier to solve. Beyond arbitration point, libvirt can keep doing whatever it has been doing. If there are v1 hierarchies, it can keep doing the subhierarchy management. If v2, it can ignore it for now until cgroup v2 and libvirt support for it are ready. IMHO, this would give a substantial part of resource containment that people want without libvirt having to deal with the headaches of transitional period. Thanks. -- tejun