Re: [Xenomai] usage of rtdm_task_sleep_abs

[Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>]

On Thu, Jul 09, 2015 at 05:43:03PM +0200, Johann Obermayr wrote:
> Am 09.07.2015 um 16:19 schrieb Gilles Chanteperdrix:
> >On Thu, Jul 09, 2015 at 02:15:55PM +0200, Johann Obermayr wrote:
> >>Hello,
> >>
> >>we have follow situation
> >>
> >>u64 lrtdrv_time_of_irq_ns ;
> >>void worker_task()
> >>{
> >>while(1)
> >>     {
> >>     rtdm_task_sleep_abs(lrtdrv_time_of_irq_ns + 950000ull,
> >>RTDM_TIMERMODE_ABSOLUTE);
> >>     do_something();
> >>     }
> >>}
> >>
> >>_kernel_rtdm_irq_handler()
> >>{
> >>     lrtdrv_time_of_irq_ns = rtdm_clock_read_monotonic();
> >>}
> >>
> >>the  _kernel_irq_callback() is called every 1ms.
> >>we will , that the worker_task begin 50us before next irq
> >>
> >>But sometime, the worker task start ~50us after irq. Why ?
> >Best way to know, enable the I-pipe tracer, setup sufficient back
> >trace points, and when the wrong wake up happens, trigger an I-pipe
> >trace.
> >
> i will enable i-pipe tracer.
> found out, that
> 
> rtdm_task_sleep_abs(xxx, RTDM_TIMERMODE_ABSOLUTE);
> u64 calc = rtdm_clock_read_monotonic();
> 
> sometime the diff between xxx & calc is biger than 61000ns.

This is called "kernel task scheduling latency". It can be measured by
the latency -t 1 test.

> 
> forget to say.
> worker_task run on Core1
> irq_handler run on Core0

Are you sure that the tsc is synchronized on both cores (you did not
give us much information, in particular we do not know on what
hardware you have this issue).

-- 
					    Gilles.
https://click-hack.org