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From: Kirill Tkhai <tkhai@yandex.ru>
To: Peter Zijlstra <peterz@infradead.org>,
        Burke Libbey <burke.libbey@shopify.com>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "mingo@kernel.org" <mingo@kernel.org>
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References: <20141024150746.GB25260@burke.local> <20141024155805.GF21513@worktop.programming.kicks-ass.net> <164441414171122@web11g.yandex.ru>
Subject: Re: [PATCH] sched: reset sched_entity depth on changing parent
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I've dived into this and found, we are really need this.
I'll send a patch with description soon.

24.10.2014, 22:19, "Kirill Tkhai" <tkhai@yandex.ru>:
> 24.10.2014, 19:58, "Peter Zijlstra" <peterz@infradead.org>:
>> ?On Fri, Oct 24, 2014 at 11:07:46AM -0400, Burke Libbey wrote:
>>> ??From 2014-02-15: https://lkml.org/lkml/2014/2/15/217
>>>
>>> ??This issue was reported and patched, but it still occurs in some situations on
>>> ??newer kernel versions.
>>>
>>> ??[2249353.328452] BUG: unable to handle kernel NULL pointer dereference at 0000000000000150
>>> ??[2249353.336528] IP: [<ffffffff810b1cf7>] check_preempt_wakeup+0xe7/0x210
>>>
>>> ??se.parent gets out of sync with se.depth, causing a panic when the algorithm in
>>> ??find_matching_se assumes they are correct. This patch forces se.depth to be
>>> ??updated every time se.parent is, so they can no longer become desync'd.
>>>
>>> ??CC: Ingo Molnar <mingo@kernel.org>
>>> ??CC: Peter Zijlstra <peterz@infradead.org>
>>> ??Signed-off-by: Burke Libbey <burke.libbey@shopify.com>
>>> ??---
>>>
>>> ??I haven't been able to isolate the problem. Though I'm pretty confident this
>>> ??fixes the issue I've been having, I have not been able to prove it.
>> ?So this isn't correct, switching rq should not change depth. I suspect
>> ?you're just papering over the issue by frequently resetting the value,
>> ?which simply narrows the race window.
>
> Just a hypothesis.
>
> 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.
>
> Child's cgroup is the same as parent's after dup_task_struct(). The only function
> changing task_group is sched_move_task(), but we do not call it between
> dup_task_struct() and wake_up_new_task(). Shouldn't we do something like this?
>
> (compile tested only)
> ---
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index cc18694..0ccbbdb 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -7833,6 +7833,11 @@ static void cpu_cgroup_css_offline(struct cgroup_subsys_state *css)
> ?????????sched_offline_group(tg);
> ?}
>
> +static void cpu_cgroup_fork(struct task_struct *task)
> +{
> + sched_move_task(task);
> +}
> +
> ?static int cpu_cgroup_can_attach(struct cgroup_subsys_state *css,
> ??????????????????????????????????struct cgroup_taskset *tset)
> ?{
> @@ -8205,6 +8210,7 @@ struct cgroup_subsys cpu_cgrp_subsys = {
> ?????????.css_free = cpu_cgroup_css_free,
> ?????????.css_online = cpu_cgroup_css_online,
> ?????????.css_offline = cpu_cgroup_css_offline,
> + .fork = cpu_cgroup_fork,
> ?????????.can_attach = cpu_cgroup_can_attach,
> ?????????.attach = cpu_cgroup_attach,
> ?????????.exit = cpu_cgroup_exit,
>
> Or we just should set tsk->sched_task_group?
> --
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