linux-kernel - Question about SCHED_DEADLINE and sched

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Date:	Mon, 10 Aug 2015 22:47:35 +0200
From:	Michael Riesch <michael@...sch.at>
To:	linux-kernel@...r.kernel.org
CC:	j.lelli@...up.it
Subject: Question about SCHED_DEADLINE and sched_yield() usage

Hi all,

I connected two analog-to-digital converters to a BeagleBoneBlack (with
kernel version 3.14.33-ti-r51.2) and tried to use the deadline scheduler
to get samples at a constant rate. In my C++/Qt application the ADCs are
represented by a class which is derived from QThread. The run() method
is basically:

run()
{
   unsigned int flags;
   struct sched_attr attr;
   attr.sched_policy = SCHED_DEADLINE;
   attr.sched_runtime = 600 * 1000;
   attr.sched_deadline = 1250 * 1000;
   attr.sched_period = 1250 * 1000;
   sched_setattr(0, &attr, flags);

   while (active) {
      /* code that gets a sample from adc, takes around 500 ms */

      sched_yield();
   }
}

to get samples at a rate of 800 Hz. However, once sched_yield() is
called, the threads do not seem to be scheduled again (no samples are
acquired and when the application shuts down the threads remain as zombies).

As far as I understand, I have to call sched_yield() if the the
execution time of one loop iteration is either not constant or unknown
(both cases being very likely), because if I do not, a new loop
iteration could be started if the time budget is not empty.

Have I missed something? Any ideas or alternative approaches are welcome!

Thanks a lot in advance!
Best regards, Michael
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