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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
