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

[Kirill Tkhai]

24.10.2014, 19:58, "Peter Zijlstra" <peterz@xxxxxxxxxxxxx>:
> 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@xxxxxxxxxx>
>>  CC: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>>  Signed-off-by: Burke Libbey <burke.libbey@xxxxxxxxxxx>
>>  ---
>>
>>  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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