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

[Luca Abeni]

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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