[PATCH 3/3] cgroup: implement cgroup.subtree_populated for the default hierarchy

[Tejun Heo]

Hey, Serge.

On Thu, Apr 10, 2014 at 05:08:55AM +0200, Serge E. Hallyn wrote:
> Quoting Tejun Heo (tj at kernel.org):
> > * It delivers events by forking and execing a userland binary
> >   specified as the release_agent.  This is a long deprecated method of
> >   notification delivery.  It's extremely heavy, slow and cumbersome to
> >   integrate with larger infrastructure.
> 
> (Not seriously worried about this, but it's a point worth considering)
> It does have one advantage though:  if the userspace agent goes bad,
> cgroups can still be removed on empty.
> 
> Do you plan on keeping release-on-empty around?  I assume only for a
> while?

The new mechanism is only for the unified hierarchy.  The old one will
be kept around for other hierarchies.

> Do you think there is any value in having a simpler "remove-when-empty"
> file?  Doesn't call out to userspace, just drops the cgroup when there
> are no more tasks or sub-cgroups?

I don't think so.  Implementing such simplistic mechanism in userland
is trivial and even independent failover mechanisms can be easily
implemented from userland as multiple entities can set up watches.  I
don't think there's much value in providing another mechanism from
kernel side.  The only reason why release_agent thing got as complex
as it is is because the mechanism is fundamentally flawed - clumsy
delivery, no multiple watches, single watch point - so people tried to
work around it by adding event filtering from kernel side, which is
quite backwards IMHO.  With proper event mechanism, everything should
be easily achievable from userland side.

> > * Events are filtered from the kernel side.  "notify_on_release" file
> >   is used to subscribe to or suppres release event and events are not
> >   generated if a cgroup becomes empty by moving the last task out of
> >   it; however, event is generated if it becomes empty because the last
> >   child cgroup is removed.  This is inconsistent, awkward and
> 
> Hm, maybe I'm misreading but this doesn't seem right.  If I move
> a task into x1 and kill the task, x1 goes away.  Likewise if I
> create x1/y1, and rmdir y1, x1 goes away.  I suspect I'm misunderstanding
> the case in which you say it doesn't happen?

The case where you move a task out of x1/y1 to another cgroup doesn't
generate an event.  One could say that that's unnecessary because the
mover knows that the cgroup is becoming empty; however, it excludes
any cases where there are more than one actors and the same can be
said for cases when the actor is removing a child.

> > This patch implements interface file "cgroup.subtree_populated" which
> > can be used to monitor whether the cgroup's subhierarchy has tasks in
> > it or not.  Its value is 1 if there is no task in the cgroup and its
> 
> I think you meant this backward?  It's 1 if there is *any task in
> the cgroup and its descendants, else 0?

Oops, yeap.  Will update.

Thanks!

-- 
tejun