linux-kernel - Re: [PATCH] sched: reset sched_entity depth on changing parent

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Date:	Mon, 27 Oct 2014 14:45:08 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Kirill Tkhai <tkhai@...dex.ru>
Cc:	Tejun Heo <tj@...nel.org>, Burke Libbey <burke.libbey@...pify.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"mingo@...nel.org" <mingo@...nel.org>
Subject: Re: [PATCH] sched: reset sched_entity depth on changing parent

On Mon, Oct 27, 2014 at 04:36:02PM +0300, Kirill Tkhai wrote:
> 
> 
> 27.10.2014, 16:28, "Peter Zijlstra" <peterz@...radead.org>:
> > On Mon, Oct 27, 2014 at 08:40:02AM -0400, Tejun Heo wrote:
> >>  On Mon, Oct 27, 2014 at 01:07:34PM +0100, Peter Zijlstra wrote:
> >>>  On Fri, Oct 24, 2014 at 09:18:42PM +0400, Kirill Tkhai wrote:
> >>>>  I was seeking a places where task_group of a task may change. I can't understand
> >>>>  how changing of parent's cgroup during fork() applies to a child.
> >>>  I didn't know we could change cgroup on fork(), I though the idea was
> >>>  you always inherited your parents cgroup.
> >>>
> >>>  How can this be?
> >>  Hmmm? -ENEEDMORECONTEXT but the inheriting happens at one point during
> >>  fork in cgroup_post_fork().  The child inherits whatever the parent
> >>  cgroup is at that point.
> >
> > So Kirill is saying that there is a race between fork and attach such
> > that a child can end up in a different cgroup than the parent and we
> > need to use the cgroup_subsys::fork call to fix that up.
> 
> I mean cgroup is the same, but sched_task_group is other (sched_task_group
> is equal to parent's on the moment of dup_task_struct()).

But that still means the parent changed cgroup during fork right? It
started out in a different cgroup than it ended up with, and we need
that .fork callback to fixup state.

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