Re: [RFD] Task counter: cgroup core feature or cgroup subsystem? (was Re: [PATCH 0/8 v3] cgroups: Task counter subsystem)

[Frederic Weisbecker <fweisbec@gmail.com>]

On Tue, Aug 23, 2011 at 09:07:59AM -0700, Paul Menage wrote:
> On Thu, Aug 18, 2011 at 7:33 AM, Frederic Weisbecker <fweisbec@gmail.com> wrote:
> >
> > So the problem with the task counter as a subsystem is that you could
> > mount it in your systemd cgroups hierarchy but then it's not anymore
> > available for those who want to use containers.
> 
> Another possible option is something that I prototyped a couple of
> years ago, but dropped due to lack of compelling need and demand - the
> ability to have subsystems that can be bound on multiple subsystems at
> once. See
> 
> http://lkml.indiana.edu/hypermail/linux/kernel/0907.0/00574.html
> http://lkml.indiana.edu/hypermail/linux/kernel/0907.0/00576.html
> http://lkml.indiana.edu/hypermail/linux/kernel/0907.0/00577.html
> 
> It's applicable to subsystems whose state isn't tied to any specific
> single resource in the kernel outside of cgroups (so e.g. the CPU
> scheduler couldn't be usefully multi-bindable, since the CPU cgroup
> state is tied to the machine's single CPU scheduler).
> 
> In the end I didn't work further on it, since it seemed that most
> things that needed to be available to multiple hierarchies could more
> simply be added to the core cgroups subsystem and automatically be
> available on all hierarchies. But the point about tracking overhead
> for fork/exit is certainly something that could make this worthwhile.

That sounds like a perfect fit. I like that much better because there
should be no noticeable overhead when the task counter subsys is
nowhere mounted, compared to a pure core feature.

So I'm going to continue to work on that task counter subsystem and
I will unearth your old patch afterward to make that work on several
mountpoints once we are sure this is needed for systemd.

It seems your patch doesn't handle the ->fork() and ->exit() calls.
We probably need a quick access to states of multi-subsystems from
the task, some lists available from task->cgroups, I don't know yet.