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Date: Wed, 12 Jan 2011 13:36:20 +0530
From: Bharata B Rao <bharata@linux.vnet.ibm.com>
To: Mike Galbraith <efault@gmx.de>
Cc: Yong Zhang <yong.zhang0@gmail.com>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>, Ingo Molnar <mingo@elte.hu>,
        linux-kernel@vger.kernel.org
Subject: Re: [patch] Re: autogroup: sched_setscheduler() fails
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Reply-To: bharata@linux.vnet.ibm.com
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On Wed, Jan 12, 2011 at 08:24:41AM +0100, Mike Galbraith wrote:
> On Wed, 2011-01-12 at 12:05 +0530, Bharata B Rao wrote:
> > On Wed, Jan 12, 2011 at 01:40:32PM +0800, Yong Zhang wrote:
> 
> > > IMHO, task->cgroups will not change when autogroup take effect.
> > 
> > Yes and I believe this is cause for some of the weird semantics I see
> > with autogroup and cgroups. I am not sure if this has already been
> > discussed ealier, may be I need to go back and check the archives, but
> > consider this:
> > 
> > I have cpu cgroup mounted at /cgroup and see that all the tasks in the
> > system are listed in /cgroup/tasks file.
> > 
> > Now I start a task like this:
> > 
> > # ./while1 &
> > [1] 2761
> > 
> > and see that this task belongs to root cgroup.
> 
> Yes, cgroups sees cgroup associations, not autogroup associations.
> 
> > # grep 2761 /cgroup/tasks
> > 2671
> > 
> > But we know that this task really belongs to an autogroup and is not
> > sitting directly on root_task_group.
> 
> Yup, cgroups doesn't want it, so autogroup (if enabled) claims it.
> 
> > # cat /proc/2761/autogroup 
> > /autogroup-49 nice 0
> > 
> > So we have a task in an autogroup (which is a sub group of root_task_group)
> > but is being shown as part of root_task_group. Is this by design ?
> 
> Yes, it's supposed to be transparent to cgroups.

If cgroups doesn't want that task and if its transparent to cgroups,
why is it shown as part of /cgroups/tasks ? The task really doesn't
belong in there. Its not on the runqueue of root_task_group.

So basically I feel that a task consumed by autogroup shouldn't ideally be
shown in root cgroup's tasks file.

Regards,
Bharata.