Deadline miss in periodic tasks

Gedare Bloom gedare at
Fri Sep 7 16:31:26 UTC 2012

On Fri, Sep 7, 2012 at 11:22 AM, Fered <a_Fered at> wrote:
>  Joel Sherrill wrote:
>> On 09/07/2012 02:14 AM, Fered wrote:
>> > Hi;
>> >
>> > In periodic tasks, if a task misses its deadline at middle of its
>> > execution, is the task stopped or it continues and just the
>> > rtems_rate_monotonic_period() function returns RTEMS_TIMEOUT?
>> If you are using the Deterministic Priority Scheduler (default)
>> or the Earliest Deadline First, this is the case.
>> If you are using the Constant Bandwidth Scheduler algorithm,
>> then it can detect when a thread is not going to make its
>> deadline and give you options to deal with that.
>> But in general, the task continues to execute after the period.
>> There is no magic way for RTEMS to make you go back to
>> the top of the loop without the user callbacks supported by the
>> CBS algorithm. When you get the RTEMS_TIMEOUT, you have
>> to know what that means to that task.
>> > Thanks.
>> > _______________________________________________
>> > rtems-users mailing list
>> > rtems-users at
>> >
>> --
>> Joel Sherrill, Ph.D.             Director of Research&   Development
>> joel.sherrill at        On-Line Applications Research
>> Ask me about RTEMS: a free RTOS  Huntsville AL 35805
>>      Support Available             (256) 722-9985
> Thank you Joel.
> If the task continues after deadline, then next periods will be shifted. Is it
> true?
I think so. IIRC the period timer is reset when the task polls its
periodic timer.

> How can I use EDF scheduler? I should assign priorities according to periods
> or there is some facilities in RTEMS?
> How can I use Constant Band Scheduler? I should implement it myself?
There are some examples in spedfsched and spcbssched under sptests,
and some documentation in the Scheduling section of the Manuals.

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