linux-kernel - Re: [PATCHSET cgroup/for-3.15] cgroup: implement unified hierarchy

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Message-ID: <20140502151250.GB10204@htj.dyndns.org>
Date:	Fri, 2 May 2014 11:12:50 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Raghavendra KT <raghavendra.kt@...ux.vnet.ibm.com>
Cc:	Vivek Goyal <vgoyal@...hat.com>, lizefan@...wei.com,
	containers@...ts.linux-foundation.org, cgroups@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCHSET cgroup/for-3.15] cgroup: implement unified hierarchy

Hello,

On Wed, Apr 30, 2014 at 04:27:51PM +0530, Raghavendra KT wrote:
> For some controllers like cpuset, when we have exclusive_cpu is set,
> what about having a knob in the cpuset controller
> to move the task to non-root (when option is set).

I'm doubtful.

> Because all the way along, though we have freedom to make the cpusets
> exclusive and move tasks (say VMs) into them,
> making sure they do not interfere with each other, we can not prevent
> the other tasks spawned in a system eating into cpus of
> exclusive cpuset since they go to root automatically.

I believe the right thing to do would be starting / confining other
tasks in the appropriate non-root cgroups.  cgroup already provides
mechanisms to achieve that.  The rest is upto userland.

> Do you think having a knob, to make sure new tasks spawned go to say a
> default  directory under root makes sense?
> 
>  I understand that we could easily have a userspace script which could
> achieve intended goal, but kernel solution
>  would really make the exclusive cpusets have exclusive access to cpus
> it should have.

This would just be a more reliable implementation of an ad-hoc
mechanism when it can already be properly achieved by managing cgroups
of all processes in the system.

> (I also have a POC implemented for pre-unified hierarchy tree and
> understand some bit of complications involved in that and
> understand that we should not have complex policies in kernel for e.g.
> filtering tasks based on patterns etc..).

As I wrote above, I think this is something to be solved from
userland.  I'm very positively against adding this sort of hacky
ad-hoc policies which serves one or a few use cases well while
introducing possible long-term maintenance issues.

Thanks.

-- 
tejun
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