Re: [patch] Re: autogroup: sched_setscheduler() fails
From: Mike Galbraith
Date: Wed Jan 12 2011 - 02:24:58 EST
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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