Re: [PATCH] sched, autogroup: Fix failure when writing to cpu.rt_runtime_us

[Zefan Li <lizefan@huawei.com>]

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, ...
+        */