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    SubjectRe: Another SCHED_DEADLINE bug (with bisection and possible fix)
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    On Ср, 2015-01-07 at 13:45 +0100, Luca Abeni wrote:
    > 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 :)

    We have two concept of "on runqueue". The first one is rq->on_rq. It means
    that a task is "queued". The second is on_dl_rq(dl_se).

    When task is not "queued", it's always not on dl_rq.

    When task is "queued" it may be in two states:
    1)on_dl_rq() -- this means the task is not throttled;
    2)!on_dl_rq() -- is task as throttled.

    So when we are discussing about a throttled task, the task is "queued". If
    you clear dl_throttled, __sched_setscheduler() places it back it the both
    meaning: on_rq and on_dl_rq, and the task becomes available for picking
    in __schedule().

    >
    > >>> 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.

    I think everything may change many times before we implement that. It's better
    to keep the code in the consistent state.

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

    Does my patch help you? It helps me, but anyway I need your confirmation.

    Thanks,
    Kirill.



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