Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1756914AbaJXRS4 (ORCPT <rfc822;w@1wt.eu>);
	Fri, 24 Oct 2014 13:18:56 -0400
Received: from forward20.mail.yandex.net ([95.108.253.145]:52541 "EHLO
	forward20.mail.yandex.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1756653AbaJXRSz (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Fri, 24 Oct 2014 13:18:55 -0400
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>
In-Reply-To: <20141024155805.GF21513@worktop.programming.kicks-ass.net>
References: <20141024150746.GB25260@burke.local> <20141024155805.GF21513@worktop.programming.kicks-ass.net>
Subject: Re: [PATCH] sched: reset sched_entity depth on changing parent
MIME-Version: 1.0
Message-Id: <164441414171122@web11g.yandex.ru>
X-Mailer: Yamail [ http://yandex.ru ] 5.0
Date: Fri, 24 Oct 2014 21:18:42 +0400
Content-Transfer-Encoding: 8bit
Content-Type: text/plain; charset=koi8-r
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org



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?
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/