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

[Lennart Poettering]

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@xxxxxxxxxx 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
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