Re: [RFC] cgroup gets release after long time

[Roman Gushchin <guro@fb.com>]

On Mon, May 20, 2019 at 10:41:26AM +0200, Pavel Hrdina wrote:
> On Sat, May 18, 2019 at 12:56:12AM +0000, Roman Gushchin wrote:
> > On Fri, May 17, 2019 at 12:12:51PM +0200, Pavel Hrdina wrote:
> > > On Thu, May 16, 2019 at 10:25:50AM -0700, Alexei Starovoitov wrote:
> > > > On Thu, May 16, 2019 at 10:15 AM Roman Gushchin <guro@fb.com> wrote:
> > > > >
> > > > > On Thu, May 16, 2019 at 05:31:44PM +0200, Pavel Hrdina wrote:
> > > > > > On Thu, May 16, 2019 at 03:22:33PM +0000, Roman Gushchin wrote:
> > > > > > > On Thu, May 16, 2019 at 12:39:15PM +0200, Jiri Olsa wrote:
> > > > > > > > hi,
> > > > > > > > Pavel reported an issue with bpf programs (attached to cgroup)
> > > > > > > > not being released at the time when the cgroup is removed and
> > > > > > > > are still visible in 'bpftool prog' list afterwards.
> > > > > > >
> > > > > > > Hi Jiri!
> > > > > > >
> > > > > > > Can you, please, try the patch from
> > > > > > > https://github.com/rgushchin/linux/commit/f77afa1952d81a1afa6c4872d342bf6721e148e2 ?
> > > > > > >
> > > > > > > It should solve the problem, and I'm about to post it upstream.
> > > > > >
> > > > > > Perfect, I'll give it a try with full libvirt setup as well.
> > > > > >
> > > > > > Can we have this somehow detectable from user-space so libvirt can
> > > > > > decide when to use BPF or not?  I would like to avoid using BPF with
> > > > > > libvirt if this issue is not fixed and we cannot simply workaround it
> > > > > > as systemd automatically removes cgroups for us.
> > > > >
> > > > > Hm, I don't think there is a good way to detect it from userspace.
> > > > > At least I have no good ideas. Alexei? Daniel?
> > > > >
> > > > > If you're interested in a particular stable version, we can probably
> > > > > treat it as a "fix", and backport.
> > > > 
> > > > right.
> > > > also user space workaround is trivial.
> > > > Just detach before rmdir.
> > > 
> > > Well yes, it's trivial but not if you are using machined from systemd.
> > > Once libvirt kills QEMU process systemd automatically removes the
> > > cgroup so we don't have any chance to remove the BPF program.
> > > 
> > > Would it be too ugly to put something into
> > > '/sys/kernel/cgroup/features'?
> > 
> > I thought about it, but it seems that /sys/kernel/cgroup/features is also
> > relatively new. So if we're not going to backport it (I mean auto-detaching),
> > than we can simple look at the kernel version, right?
> 
> If you think only about upstream then the version check is in most cases
> good enough, but usually that's not the case and patches are backported
> to downstream distributions as well.
> 
> Yes, that file was introduced in kernel 4.15 so there are some
> limitations where the fix would be introspectable.
> 
> > If we're going to backport it, the question is which stable version we're
> > looking at.
> > 
> > In general, I don't see any reasons why cgroup/features can't be used.
> 
> Perfect, in that case I would prefer if we could export it in
> cgroup/features as it will be easier for user-space to figure out
> whether it's safe to relay on proper cleanup behavior or not and
> it will make downstream distributions life easier.

Hello, Pavel!

Tejun noticed that cgroup features are supposed to match cgroupfs mount options,
so it can't be used here. And this >= 4.15 limitation is also a significant
constraint.

Thanks!