Re: [PATCH v6 3/4] cgroup: add xattr support

[Lennart Poettering <lpoetter-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

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