From mboxrd@z Thu Jan 1 00:00:00 1970 From: Lennart Poettering Subject: Re: [PATCH v6 3/4] cgroup: add xattr support Date: Tue, 21 Aug 2012 23:43:44 +0200 Message-ID: <50340110.50607@redhat.com> References: <20120816174453.154143248@napanee.usersys.redhat.com> <20120816174454.087507415@napanee.usersys.redhat.com> <20120816200006.GG24861@google.com> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20120816200006.GG24861-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> Sender: cgroups-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-ID: Content-Type: text/plain; charset="us-ascii"; format="flowed" To: Tejun Heo Cc: aris-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Li Zefan , Hugh Dickins , Hillf Danton Heya, (sorry for the late reply) On 16.08.2012 22:00, Tejun Heo wrote: > On Thu, Aug 16, 2012 at 01:44:56PM -0400, aris-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org wrote: >>> Attaching meta information to services, in an easily discoverable >>> way. For example, in systemd we create one cgroup for each service, and >>> could then store data like the main pid of the specific service as an >>> xattr on the cgroup itself. That way we'd have almost all service state >>> in the cgroupfs, which would make it possible to terminate systemd and >>> later restart it without losing any state information. But there's more: >>> for example, some very peculiar services cannot be terminated on >>> shutdown (i.e. fakeraid DM stuff) and it would be really nice if the >>> services in question could just mark that on their cgroup, by setting an >>> xattr. On the more desktopy side of things there are other >>> possibilities: for example there are plans defining what an application >>> is along the lines of a cgroup (i.e. an app being a collection of >>> processes). With xattrs one could then attach an icon or human readable >>> program name on the cgroup. >>> >>> The key idea is that this would allow attaching runtime meta information >>> to cgroups and everything they model (services, apps, vms), that doesn't >>> need any complex userspace infrastructure, has good access control >>> (i.e. because the file system enforces that anyway, and there's the >>> "trusted." xattr namespace), notifications (inotify), and can easily be >>> shared among applications. > > I'm not against this but unsure whether using kmem is enough for the > suggested use case. Lennart, would this suit systemd? How much > metadata are we talking about? Just small things, like values, PIDs, i.e. a few 100 bytes or so per cgroup should be more than sufficient for our needs. Lennart