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Re: [libvirt] [systemd-devel] systemd-cgroups-agent not working in containers
- From: Lennart Poettering <mzerqung 0pointer de>
- To: Richard Weinberger <richard nod at>
- Cc: "libvir-list redhat com" <libvir-list redhat com>, Linux Containers <containers lists linux-foundation org>, "systemd-devel lists freedesktop org" <systemd-devel lists freedesktop org>
- Subject: Re: [libvirt] [systemd-devel] systemd-cgroups-agent not working in containers
- Date: Sun, 30 Nov 2014 23:30:25 +0100
On Wed, 26.11.14 22:29, Richard Weinberger (richard nod at) wrote:
> I run a Linux container setup with openSUSE 13.1/2 as guest distro.
> After some time containers slow down.
> An investigation showed that the containers slow down because a lot of stale
> user sessions slow down almost all systemd tools, mostly systemctl.
> loginctl reports many thousand sessions.
> All in state "closing".
> The vast majority of these sessions are from crond an ssh logins.
> It turned out that sessions are never closed and stay around.
> The control group of a said session contains zero tasks.
> So I started to explore why systemd keeps it.
> After another few hours of debugging I realized that systemd never
> issues the release signal from cgroups.
> Also calling the release agent by hand did not help. i.e.
> /usr/lib/systemd/systemd-cgroups-agent /user.slice/user-0.slice/session-c324.scope
> Therefore systemd never recognizes that a server/session has no more tasks
> and will close it.
> First I thought it is an issue in libvirt combined with user namespaces.
> But I can trigger this also without user namespaces and also with systemd-nspawn.
> Tested with systemd 208 and 210 from openSUSE, their packages have all known bugfixes.
> Any idea where to look further?
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
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:
 the registrar of the scope must explicitly stop the scope when
 the registrar of the scope must explicitly "abandon" the scope
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
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
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?
> How do you run the most current systemd on your distro?
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 Poettering, Red Hat
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