linux-kernel - Re: [RFD] cgroup: about multiple hierarchies

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Date:	Thu, 1 Mar 2012 16:38:37 -0600
From:	C Anthony Risinger <anthony@...x.me>
To:	Mike Galbraith <efault@....de>
Cc:	Vivek Goyal <vgoyal@...hat.com>,
	Kay Sievers <kay.sievers@...y.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	containers@...ts.linux-foundation.org,
	Michal Schmidt <mschmidt@...hat.com>,
	linux-kernel@...r.kernel.org,
	Christoph Hellwig <hch@...radead.org>,
	Lennart Poettering <lennart@...ttering.net>,
	Tejun Heo <tj@...nel.org>, cgroups@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFD] cgroup: about multiple hierarchies

On Thu, Mar 1, 2012 at 4:04 PM, Mike Galbraith <efault@....de> wrote:
> On Thu, 2012-03-01 at 16:02 -0500, Vivek Goyal wrote:
>> On Thu, Mar 01, 2012 at 09:26:43PM +0100, Mike Galbraith wrote:
>
>> > Q: you say systemd requires CONFIG_CGROUPS=y.  Why is that?  It's taking
>> > over sysvinits job afaiui, what does that have to do with cgroups?
>>
>> I think they were using it to track all the children forked by a service
>> and cleanup all of them if need be. So they just need it for logical
>> grouping functionality and don't require any controllers as such.
>
> Hm.  Controllers are perhaps not required, but cpu controller was
> configured and used without consent.  I didn't receive an offer.

AFAIK it does in fact only require `name` cgroup for it's own
monitoring purposes.  i believe the systemd folks also tried (and are
trying? see TODO) to get PR_SET_ANCHOR merged upstream:

https://lkml.org/lkml/2010/2/2/165

... which is a sort of recursive/persistent parenting flag; without
that or cgroups, there is no way to reliably supervise processes under
Linux.

the other problem is there is no way for a process to enumerate the
available cgroups -- IIRC a list had to be hard-coded in systemd
sources -- and mounting the cgroupfs without specifying a specific
subsystem simply mounts everything in one whack.

you should be able to tell systemd to ignore that specific controller,
or tell it to use existing mounts.  i for one have been using it
*exclusively* on my personal machines/home servers [archlinux] and am
very pleased ... it's very flexible and gives you an unprecedented
level of control and introspection into the system (man systemd.*) ...
i can create new services in about 3-5 lines.

... obviously this thread is not about systemd, but since it makes
such extensive use of cgroup facilities it only proves to highlight
it's deficiencies.  i think the notes and practices systemd has
established should be viewed as a good reference for what's clumsy, at
the very least, and should not be attributed to systemd, but to
cgroups.

-- 

C Anthony
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