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

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Message-ID: <20110824175431.GA26417@somewhere.redhat.com>
Date:	Wed, 24 Aug 2011 19:54:35 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Paul Menage <paul@...lmenage.org>
Cc:	Kay Sievers <kay.sievers@...y.org>, Li Zefan <lizf@...fujitsu.com>,
	Tim Hockin <thockin@...kin.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Aditya Kali <adityakali@...gle.com>,
	Oleg Nesterov <oleg@...hat.com>
Subject: Re: [RFD] Task counter: cgroup core feature or cgroup subsystem?
 (was Re: [PATCH 0/8 v3] cgroups: Task counter subsystem)

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@...il.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.
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