linux-kernel - Re: [patch] Re: autogroup: sched

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Date:	Wed, 12 Jan 2011 08:24:41 +0100
From:	Mike Galbraith <efault@....de>
To:	bharata@...ux.vnet.ibm.com
Cc:	Yong Zhang <yong.zhang0@...il.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org
Subject: Re: [patch] Re: autogroup: sched_setscheduler() fails

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.

> Now say I create a sub cgroup and move this task to it.
> 
> # mkdir /cgroup/1
> # echo 2671 > /cgroup/1/tasks
> 
> So now the task moved to a real cgroup.
> 
> # cat /cgroup/1/tasks 
> 2761
> 
> But the /proc/2761/autogroup isn't updated. It still shows
> /autogroup-49 nice 0

The proc interface shows task->autogroup associations, which always
exist.  This is autogroups' view of the world, just as cgroup/tasks is
cgroups' view of it's world.

> May be this was all discussed earlier as I noted in the beginning, but I find
> this semantics a bit unusual.

Makes perfect sense to me.

	-Mike

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