Re: [RFC] cgroup gets release after long time

From: Pavel Hrdina <phrdina@redhat.com>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: Roman Gushchin <guro@fb.com>, Jiri Olsa <jolsa@redhat.com>,
	Tejun Heo <tj@kernel.org>, Li Zefan <lizefan@huawei.com>,
	Daniel Mack <daniel@zonque.org>,
	"cgroups@vger.kernel.org" <cgroups@vger.kernel.org>,
	"bpf@vger.kernel.org" <bpf@vger.kernel.org>,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	"David S. Miller" <davem@davemloft.net>
Subject: Re: [RFC] cgroup gets release after long time
Date: Fri, 17 May 2019 12:12:51 +0200	[thread overview]
Message-ID: <20190517101222.GF1981@antique-laptop> (raw)
In-Reply-To: <CAADnVQ+c4HW+1jrurHDX0M4-yn13fmU=TYhF+8wPrxNZZRcjTw@mail.gmail.com>

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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'?

Pavel

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