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

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Message-Id: <799391414416962@web4j.yandex.ru>
Date:	Mon, 27 Oct 2014 16:36:02 +0300
From:	Kirill Tkhai <tkhai@...dex.ru>
To:	Peter Zijlstra <peterz@...radead.org>, Tejun Heo <tj@...nel.org>
Cc:	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



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()).


> I was always under the impression that fork was an 'atomic' operation
> from the point of cgroups, an attach (or move) would happen either
> before the fork or after, not during. But I appear to be mistaken in
> that assumption, going by the comments around cgroup_post_fork().
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