On Mon, Aug 29, 2016 at 3:20 PM, Tejun Heo <tj@xxxxxxxxxx> wrote: >> > These base-system operations are special regardless of cgroup and we >> > already have sometimes crude ways to affect their behaviors where >> > necessary through sysctl knobs, priorities on specific kernel threads >> > and so on. cgroup doesn't change the situation all that much. What >> > gets left in the root cgroup usually are the base-system operations >> > which are outside the scope of cgroup resource control in the first >> > place and cgroup resource graph can treat the root as an opaque anchor >> > point. >> >> This seems to explain why the controllers need to be able to handle >> things being charged to the root cgroup (or to an unidentifiable >> cgroup, anyway). That isn't quite the same thing as allowing, from an >> ABI point of view, the root cgroup to contain processes and cgroups >> but not allowing other cgroups to do the same thing. Consider: > > The points are 1. we need the root to be a special container anyway But you don't need to let userspace see that. > 2. allowing it to be special and contain system-wide consumptions > doesn't make the resource graph inconsistent once all non-system-wide > consumptions are put in non-root cgroups, and 3. this is the most > natural way to handle the situation both from implementation and > interface standpoints as it makes non-cgroup configuration a natural > degenerate case of cgroup configuration. > >> suppose that systemd (or some competing cgroup manager) is designed to >> run in the root cgroup namespace. It presumably expects *itself* to >> be in the root cgroup. Now try to run it using cgroups v2 in a >> non-root namespace. I don't see how it can possibly work if it the >> hierarchy constraints don't permit it to create sub-cgroups while it's >> still in the root. In fact, this seems impossible to fix even with >> user code changes. The manager would need to simultaneously create a >> new child cgroup to contain itself and assign itself to that child >> cgroup, because the intermediate state is illegal. > > Please re-read the constraint. It doesn't prevent any organizational > operations before resource control is enabled. > >> I really, really think that cgroup v2 should supply the same >> *interface* inside and outside of a non-root namespace. If this is > > It *does*. That's what I tried to explain, that it's exactly > isomorhpic once you discount the system-wide consumptions. > I don't think I agree. Suppose I wrote an init program or a cgroup manager. I can expect that init program to be started in the root cgroup. The program can be lazy and write +io to /cgroup/cgroup.subtree_control and then create some new cgroup /cgroup/a and it will work (I just tried it). Now I run that program in a namespace. It will not work because it'll get -EBUSY when it tries to write to cgroup.subtree_control. (I just tried this, too, only using cd instead of a namespace.) So it's *not* isomorphic. It *also* won't work (I think) if subtree control is enabled on the root, but I don't think this is a problem in practice because subtree control won't be enabled on the namespace root by a sensible cgroup manager. --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
