Re: Another SCHED_DEADLINE bug (with bisection and possible fix)

[Kirill Tkhai <tkhai@yandex.ru>]

Hi, Juri,

13.01.2015, 11:10, "Juri Lelli" <juri.lelli@arm.com>:
> Hi all,
>
> really sorry for the huge delay in replying to this! :(
>
> On 07/01/2015 12:29, Kirill Tkhai wrote:
>>  On Ср, 2015-01-07 at 08:01 +0100, Luca Abeni wrote:
>>>  Hi Kirill,
>>>
>>>  On Tue, 06 Jan 2015 02:07:21 +0300
>>>  Kirill Tkhai <tkhai@yandex.ru> wrote:
>>>>  On Пн, 2015-01-05 at 16:21 +0100, Luca Abeni wrote:
>>>  [...]
>>>>>  For reference, I attach the patch I am using locally (based on what
>>>>>  I suggested in my previous mail) and seems to work fine here.
>>>>>
>>>>>  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, and further enqueue_task()
>>  places it on the dl_rq. So, further update_curr_dl() may make it throttled
>>  again, and it will try to start dl_timer (which is already set).
>>>>  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.
>>>  [...]
>>>>>>  @@ -3250,16 +3251,19 @@ static void
>>>>>>    __setparam_dl(struct task_struct *p, const struct sched_attr
>>>>>>  *attr) {
>>>>>>            struct sched_dl_entity *dl_se = &p->dl;
>>>>>>  +        struct hrtimer *timer = &dl_se->dl_timer;
>>>>>>  +
>>>>>>  + if (!hrtimer_active(timer) ||
>>>>>>  hrtimer_try_to_cancel(timer) != -1) {
>>>>>  Just for the sake of curiosity, why trying to cancel the timer
>>>>>  ("|| hrtimer_try_to_cancel(timer)") here? If it is active, cannot
>>>>>  we leave it active (without touching dl_throttled, dl_new and
>>>>>  dl_yielded)?
>>>>>
>>>>>  I mean: if I try to change the parameters of a task when it is
>>>>>  throttled, I'd like it to stay throttled until the end of the
>>>>>  reservation period... Or am I missing something?
>>>>  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...
>
> Well, I'm inclined to agree to Luca's viewpoint. We should not change
> parameters of a throttled task or we may affect other tasks.

Could you explain your viewpoint more? How does this affects other tasks?

As I understand, in __setparam_dl() we are sure that there is enough
dl_bw. In __sched_setscheduler() we call it after dl_overflow() check.

>>>  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().
>>
>>  If in the future we allow non-privileged users to increase deadline,
>>  we will reflect that in __setparam_dl() too.
>
> I'd say it is better to implement the right behavior even for root, so
> that we will find it right when we'll grant access to non root users
> too. Also, even if root can do everything, we always try not to break
> guarantees that come with admission control (root or non root that is).