Date:	Wed, 07 Jan 2015 13:45:10 +0100
From:	Luca Abeni <luca.abeni@...tn.it>
To:	tkhai@...dex.ru
CC:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Juri Lelli <juri.lelli@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Another SCHED_DEADLINE bug (with bisection and possible fix)

On 01/07/2015 01:29 PM, Kirill Tkhai wrote:
[...]
>>>> Based on your comments, I suspect my patch can be further
>>>> simplified by moving the call to init_dl_task_timer() in
>>>> __sched_fork().
>>>
>>> It seems this way has problems. The first one is that task may become
>>> throttled again, and we will start dl_timer again.
>> Well, in my understanding if I change the parameters of a
>> SCHED_DEADLINE task when it is throttled, it stays throttled... So, the
>> task might not become throttled again before the dl timer fires.
>> So, I hoped this problem does not exist. But I might be wrong.
>
> You keep zeroing of dl_se->dl_throttled
Right... Now that you point this out, I realize that dl_se->dl_throttled = 0
should be inside the if().

> and further enqueue_task() places it on the dl_rq.
I was under the impression that no further enqueue_task() will happen (since
the task is throttled, it is not on runqueue, so __sched_setscheduler() will
not dequeue/enqueue it).
But I am probably missing something else :)

>>> The second is that
>>> it's better to minimize number of combination of situations we have.
>>> Let's keep only one combination: timer is set <-> task is throttled.
>> Yes, this was my goal too... So, if I change the parameters of a task
>> when it is throttled, I leave dl_throttled set to 1 and I leave the
>> timer active.
>
> As I see,
>
> dl_se->dl_throttled = 0;
>
> is still in __setparam_dl() after your patch, so you do not leave
> it set to 1.
You are right, my fault.

[...]
>>> I think that when people change task's parameters, they want the
>>> kernel reacts on this immediately. For example, you want to kill
>>> throttled deadline task. You change parameters, but nothing happens.
>>> I think all developers had this use case when they were debugging
>>> deadline class.
>> I see... Different people have different requirements :)
>> My goal was to do something like adaptive scheduling (or scheduling
>> tasks with mode changes), so I did not want that changing the
>> scheduling parameters of a task affected the scheduling of the other
>> tasks... But if a task exits the throttled state when I change its
>> parameters, it might consume much more than the reserved CPU time.
>> Also, I suspect this kind of approach can be exploited by malicious
>> users: if I create a task with runtime 30ms and period 100ms, and I
>> change its scheduling parameters (to runtime=29ms and back) frequently
>> enough, I can consume much more than 30% of the CPU time...
>>
>> Anyway, I am fine with every patch that fixes the bug :)
>
> Deadline class requires root privileges. So, I do not see a problem
> here. Please, see __sched_setscheduler().
I know... But the final goal is to allow non-root users to use SCHED_DEADLINE,
so I was thinking about future problems.


> If in the future we allow non-privileged users to increase deadline,
> we will reflect that in __setparam_dl() too.
Ok.



			Thanks,
				Luca

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