Re: [libvirt] [systemd-devel] systemd-cgroups-agent not working in containers

On Wed, 26.11.14 22:29, Richard Weinberger (richard(a)nod.at) wrote: cgroup empty notification is seriously broken unfortunately in the kernel the way it is currently implemented. And we'll miss the callouts in a number of cases (for example, if somebody has any dir in a cgroup still we get no events for it. It's also not available at all inside of containers, since the callouts take place on the main pid namespace, and nowhere else). Our current strategy for still being able to clean everything up is this: a) for service units we keep track of main and control PID (control PID is the PID of any script or so we invoke to shutdown a service, via ExecStop= or so, or for reload via ExecReload, and so on) and if they are gone we consider the service dead, and kill all other processes of a service forcibly, not waiting for them between SIGTERM and SIGKILL, simply because we can't. b) For scope units (which login sessions are exposed as) things are more difficult. While for service units the relevant processes are children of PID 1 and we hence get SIGCHLD signals for this is usually not the case for scope units, the processes might be child processes of arbitrary processes, we hence cannot reliably get notifications for. For dealing with this we have two strategies: [1] the registrar of the scope must explicitly stop the scope when appropriate. [2] the registrar of the scope must explicitly "abandon" the scope when appropriate. In the case of logind both stopping and abandoning are available, depending on the KillUserProcesses= setting of logind.conf. logind triggers the stopping/abandoning as soon as either: I) the PAM session end hook is invoked for the specific session II) or the session fifo is closed. Each session logind keeps track of has one of these. The FIFO is simply created in the PAM open session hook, and normally closed in the session end hook. Should the session die abnormally though (without going through the PAM end hook) logind sees this as POLLHUP on the the other end of the FIFO and can act on it. (Note that the FIFO is passed with O_CLOEXEC to the PAM session to ensure that it only is kept around in the parent process between PAM open and end hooks, but not passed to the child processes, which then go an and invoke login/bash or whatever else that is the user session. When a scope is "stopped" this has the effect of killing all the scopes processes, immedietely. When it is "abandoned" however we iterate through all remaining processes of the scope, add them to a wacthlist and wait for a SIGCHLD for them, checking on each one we get if the scope is now empty. If it isn't empty then we collect the PIDs again at that time. The rationale for this is: the abandoning should normally happen when the main process of the scope dies. At this time the other processes of the scope (which are its children usually) would get reparented to PID 1 (because UNIX) which allows us to get SIGCHLD for them again. Complex? Awful? Disgusting? Yes, absolutely. But as far as I can see it should actually be good enough to all cases I ran into. The proper fix in the long run is to get better notifications for cgroups from the kernel. Great thing is, they are now available, but only in the new "unified" cgroup hierarchy, which we haven't ported things to yet. With that in place we finally can watch cgroups comprehensively and safely without all this madness. Yay! Now, if the tracking logic described above doesn't work for you, it would be good if you would first try with pristine upstream systemd. In the past we had problems with PAM clients that didn't implement the PAM session logic correctly and didn't invoke the PAM session close hooks, didn't keep the parent process around to do so, or suchlike. What kind of PAM session do you into this problem with? Well, I as a developer just build it from the git tree, after installing all deps, with ./autogen.sh c && make -j6 && sudo make install Lennart -- Lennart Poettering, Red Hat