Messages in this thread | | | Subject | Re: [PATCH] sched_rt.c: resch needed in rt_rq_enqueue() for the root rt_rq | From | Peter Zijlstra <> | Date | Sat, 04 Oct 2008 13:17:45 +0200 |
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On Fri, 2008-10-03 at 17:40 +0200, Dario Faggioli wrote: > Hi all, > > While working on the new version of the code for SCHED_SPORADIC I > noticed something strange in the present throttling mechanism. More > specifically in the throttling timer handler in sched_rt.c > (do_sched_rt_period_timer()) and in rt_rq_enqueue(). > > The problem is that, when unthrottling a runqueue, rt_rq_enqueue() only > asks for rescheduling if the runqueue has a sched_entity associated to > it (i.e., rt_rq->rt_se != NULL). > Now, if the runqueue is the root rq (which has a rt_se = NULL) > rescheduling does not take place, and it is delayed to some undefined > instant in the future. > > This imply some random bandwidth usage by the RT tasks under throttling. > For instance, setting rt_runtime_us/rt_period_us = 950ms/1000ms an RT > task will get less than 95%. In our tests we got something varying > between 70% to 95%. > Using smaller time values, e.g., 95ms/100ms, things are even worse, and > I can see values also going down to 20-25%!! > > The tests we performed are simply running 'yes' as a SCHED_FIFO task, > and checking the CPU usage with top, but we can investigate thoroughly > if you think it is needed. > > Things go much better, for us, with the attached patch... Don't know if > it is the best approach, but it solved the issue for us.
Its consistent with John Blackwood's change to the !group case (f3ade837), and looks good.
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Ingo, please pickup (might be 27.1 material as well).
Thanks Dario!
> > --- > Signed-off-by: Dario Faggioli <raistlin@linux.it> > Signed-off-by: Michael Trimarchi <trimarchimichael@yahoo.it> > --- > diff --git a/kernel/sched_rt.c b/kernel/sched_rt.c > index 1113157..37f0721 100644 > --- a/kernel/sched_rt.c > +++ b/kernel/sched_rt.c > @@ -102,12 +102,12 @@ static void dequeue_rt_entity(struct sched_rt_entity *rt_se); > > static void sched_rt_rq_enqueue(struct rt_rq *rt_rq) > { > + struct task_struct *curr = rq_of_rt_rq(rt_rq)->curr; > struct sched_rt_entity *rt_se = rt_rq->rt_se; > > - if (rt_se && !on_rt_rq(rt_se) && rt_rq->rt_nr_running) { > - struct task_struct *curr = rq_of_rt_rq(rt_rq)->curr; > - > - enqueue_rt_entity(rt_se); > + if (rt_rq->rt_nr_running) { > + if (rt_se && !on_rt_rq(rt_se)) > + enqueue_rt_entity(rt_se); > if (rt_rq->highest_prio < curr->prio) > resched_task(curr); > } >
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