From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1754138AbbBFBbG (ORCPT <rfc822;w@1wt.eu>);
	Thu, 5 Feb 2015 20:31:06 -0500
Received: from szxga01-in.huawei.com ([119.145.14.64]:15320 "EHLO
	szxga01-in.huawei.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1753747AbbBFBbD (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Thu, 5 Feb 2015 20:31:03 -0500
Message-ID: <54D4194B.2040808@huawei.com>
Date: Fri, 6 Feb 2015 09:30:51 +0800
From: Zefan Li <lizefan@huawei.com>
User-Agent: Mozilla/5.0 (Windows NT 6.1; rv:17.0) Gecko/20130801 Thunderbird/17.0.8
MIME-Version: 1.0
To: Peter Zijlstra <peterz@infradead.org>
CC: Ingo Molnar <mingo@kernel.org>, Mike Galbraith <umgwanakikbuti@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Stefan Bader <stefan.bader@canonical.com>
Subject: Re: [PATCH] sched, autogroup: Fix failure when writing to cpu.rt_runtime_us
References: <54D32AD4.1060003@huawei.com> <20150205142527.GI5029@twins.programming.kicks-ass.net>
In-Reply-To: <20150205142527.GI5029@twins.programming.kicks-ass.net>
X-Enigmail-Version: 1.6
Content-Type: text/plain; charset="ISO-8859-1"
Content-Transfer-Encoding: 7bit
X-Originating-IP: [10.177.18.230]
X-CFilter-Loop: Reflected
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On 2015/2/5 22:25, Peter Zijlstra wrote:
> On Thu, Feb 05, 2015 at 04:33:24PM +0800, Zefan Li wrote:
>> This is how to reproduce the bug:
>>
>> int main() {
>>     struct sched_param param = {.sched_priority=1};
>>
>>     if (fork() > 0)
>>         exit(0);
>>
>>     setsid();
>>
>>     if (sched_setscheduler(0, SCHED_RR, &param) < 0){
>>         perror("failed to sched_setscheduler()");
>>         return -1;
>>     }
>>

btw, if you move setscheduler() abvoe fork(), changing rt_runtime
will succeed.

>>     while(1)
>>         ;
>> }
>>
>>   # ./test
>>   # mount -t cgroup -o cpu xxx /cgroup
>>   # cat /cgroup/cpu.rt_runtime_us
>>   950000
>>   # echo 940000 > /cgroup/cpu.rt_runtime_us
>>   Device or Resource busy
> 
> That's -EBUSY, but you're changing an -EPERM condition. Neither your
> patch nor explanation of the matter make sense.
> 

I'm not changing the -EPERM condition. I'm adding an else condition which
resets p->sched_task_group.

After running the test program, we have:

  root
   |
  autogroup   <-- the RT test program is now here

root.rt_runtime = 950000
autogroup.rt_runtime = 0

Now if you try to change the rt_runtime of the root cgroup, it returns
-EBUSY.

That comes from:

static int tg_rt_schedulable(struct task_group *tg, void *data)
(
	...
        if (rt_bandwidth_enabled() && !runtime && tg_has_rt_tasks(tg))
                return -EBUSY;
	...
}

static inline int tg_has_rt_tasks(struct task_group *tg)
{
        struct task_struct *g, *p;

        for_each_process_thread(g, p) {
                if (rt_task(p) && task_group(p) == tg)
                        return 1;
        }

        return 0;
}

here tg == autogroup, p == the test program, and task_group(p)
returns autogroup, which is wrong.

See this commit:

commit f44937718ce3b8360f72f6c68c9481712517a867
Author: Mike Galbraith <efault@gmx.de>
Date:   Thu Jan 13 04:54:50 2011 +0100

    sched, autogroup: Fix CONFIG_RT_GROUP_SCHED sched_setscheduler() failure
...
+       /*
+        * Autogroup RT tasks are redirected to the root task group
	...
+        * the policy change to proceed.  Thereafter, task_group()
+        * returns &root_task_group, ...
+        */