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From: Luca Abeni <luca.abeni@unitn.it>
To: tkhai@yandex.ru
Cc: Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>, Juri Lelli <juri.lelli@gmail.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: Another SCHED_DEADLINE bug (with bisection and possible fix)
Date: Wed, 07 Jan 2015 13:45:10 +0100	[thread overview]
Message-ID: <54AD2A56.70307@unitn.it> (raw)
In-Reply-To: <1420633741.12772.10.camel@yandex.ru>

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


  reply	other threads:[~2015-01-07 12:45 UTC|newest]

Thread overview: 26+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2014-12-29 23:27 luca abeni
2015-01-04 22:51 ` Kirill Tkhai
2015-01-05 15:21   ` Luca Abeni
2015-01-05 23:07     ` Kirill Tkhai
2015-01-07  7:01       ` Luca Abeni
2015-01-07 12:29         ` Kirill Tkhai
2015-01-07 12:45           ` Luca Abeni [this message]
2015-01-07 13:04             ` Kirill Tkhai
2015-01-07 13:14               ` Luca Abeni
2015-01-09 11:15               ` Luca Abeni
2015-01-09 11:46                 ` Kirill Tkhai
2015-01-13  8:10           ` Juri Lelli
2015-01-13  9:26             ` Kirill Tkhai
2015-01-13 14:04               ` Peter Zijlstra
2015-01-14 12:43                 ` Kirill Tkhai
2015-01-15 11:23                   ` Luca Abeni
2015-01-15 12:23                     ` Peter Zijlstra
2015-01-15 13:35                       ` Luca Abeni
2015-01-28 14:08                         ` Peter Zijlstra
2015-01-28 14:40                           ` Luca Abeni
2015-01-30 10:25                           ` Luca Abeni
2015-01-30 10:35                           ` Juri Lelli
2015-01-31  9:56                             ` Peter Zijlstra
2015-02-02 11:31                               ` Juri Lelli
2015-02-02 13:57                               ` Luca Abeni
2015-02-04 14:34                           ` [tip:sched/core] sched/deadline: Fix deadline parameter modification handling tip-bot for Peter Zijlstra

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