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

[Kirill Tkhai <tkhai@yandex.ru>]

27.10.2014, 16:28, "Peter Zijlstra" <peterz@infradead.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().