Re: [PATCH RFC cgroup/for-3.7] cgroup: mark subsystems with brokenhierarchy support and whine if cgroups are nested for them

From: Tejun Heo
Date: Tue Sep 11 2012 - 13:15:59 EST

Hello, Vivek.

On Tue, Sep 11, 2012 at 10:51:06AM -0400, Vivek Goyal wrote:
> > * Move users away from using hierarchy on currently non-hierarchical
> > subsystems, so that implementing proper hierarchy support on those
> > doesn't surprise them.
> I know two current/potential users. systemd and libvirt. They are
> anyway going to create hierarchy irrespective of the fact whether
> controller supports it or not.

systemd mounts all controllers by default but it only creates
hierarchy for controllers other than cpu, so we're in the clear there.

> So even if we start screaming, nothing is going to change there, I
> suspect. Just that by default they expect every controller supports
> hiearchies.

As for libvirt, that's exactly the case where we want to be warning.
The problem is that they're currently creating a hierarchy and
expecting (or at least experiencing) flat behavior. We want them to
be either switch to explicit flat cgroups or at least know very well
that they're doing something unsupported and the behavior will change
beneath them.

> > * Keep track of which controllers are broken how and nudge the
> > subsystems to implement proper hierarchy support.
> I thought we can easily keep track of this in a simple .txt file and
> we really don't have to provide explicit warnings.
> I think for these controllers it is a known fact that they don't support
> hiearchy yet. I am skeptical that providing explicit warnings is going
> to help.

We should change blkio to support full hierarchy and soon and the
current users of the broken hierarchy should be warned loudly and
clearly; otherwise, this whole excercise is pointless, so I don't
think it makes sense to suppress warnings for known broken ones. In
fact, my concern there is that it's too quiet and polite and I'm
planning on promoting it to full-blown WARN in time.


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