rt_task_set_priority does not increase priority of other task

Philippe Gerum rpm at xenomai.org
Thu Sep 17 19:49:22 CEST 2020

Jan Kiszka <jan.kiszka at siemens.com> writes:

> On 17.09.20 14:01, Harco Kuppens wrote:
>> On 17/09/2020 13:51, Jan Kiszka wrote:
>>> On 16.09.20 20:12, Harco Kuppens via Xenomai wrote:
>>>> Hi,
>>>> I found a problem with rt_task_set_priority function which does not
>>>> increase priority of another task.
>>>> However it works fine if you increase the priority of another task.
>>>> Below is an en example program and its output, and we run this program
>>>> on xenomai 3.08.
>>>> The problem appears if we run the program on our xenomai image for the
>>>> raspberry pi 3,
>>>> and is also appears in our virtual box image.
>>>> Both images can be found at :
>>>>    * http://www.cs.ru.nl/lab/xenomai/raspberrypi.html
>>>>    * http://www.cs.ru.nl/lab/xenomai/virtualbox.html
>>>> The easiest way is to run the virtualbox image.
>>>> The final question I have: is there an wrong usage of xenomai API in the
>>>> example program,
>>>> or is this a bug in xenomai?
>>> Something is inconsistent here. Did you also check via
>>> /proc/xenomai/sched/threads if that view is consistent with the result
>>> of inquire?
>> yes, and they also said the priority was not increased.
>> You can repeat the experiment in the virtualbox image.
>> Note: we use virtualbox so that students can do some exercise at
>> home. The exerecises on hardware they must do on raspberry pi 3 in
>> the lab.
>> Normally a rt os on virtualbox would make no sense.
> I'm doing most of Xenomai development in KVM, including kernel
> debugging - no need to explain ;).
>>> I vaguely recall issues of the latter but I also do not
>>> recall any fix to 3.1, not to speak of anything that was not backported.
>>> BTW, tried 3.1 as well?
>> no, because I don't have it  installed. Could someone who has it
>> running try this example on it, and check whether this   problem
>> also occurs there?
> There is something unexpected with master and also with
> --enable-lazy-setsched. It's important to note that without that 
> feature, include for 3.0 which lacked that, the setprio call will
> switch the caller into secondary mode. Still, that alone does not
> explain the result to me yet. Thanks in advanced to Philippe to
> picking this up!


In absence of --enable-lazy-setsched, we know that t2 switches to
secondary mode as a result of calling rt_task_set_priority(), for the
purpose of eagerly propagating the priority update first to the main
(kernel) scheduler, _before_ telling Cobalt about it.

t1 then t0 - which are still controlled by the Cobalt scheduler - may
preempt t2, which runs code somewhere between
__STD(pthread_setschedparam()) and
XENOMAI_SYSCALL(sc_cobalt_thread_setschedparam_ex) in
pthread_setschedparam_ex(), as they wake up from rt_timer_spin().

The printf() output may be confusing, because as we see the "change prio
task X to Y" message, we still cannot assume the operation was completed
just yet. As mentioned earlier, t2 is crawling on the root stage at this
point (low priority stage of the pipeline).

As t1 then t0 grab the CPU which t2 just yielded, they manage to run
their respective loop entirely before t2 has a chance to leave the
(low-priority) secondary mode, displaying the old priority value
Cobalt-wise, which is still pending update.

In short, building with --enable-lazy-setsched may mitigate the issue in
most cases, but there is no way to strictly synchronize the main and
Cobalt schedulers when it comes to updating thread priorities only using
the plain rt_task_set_priority()/pthread_setschedparam_ex() calls. There
will always be a delay between the two updates, you only get to chose
whether you want the main (linux) scheduler to be updated first at the
expense of a mode switch, or Cobalt should be told first about the
change (sparing a transition to secondary mode in the process), and the
main kernel would be notified next.

In the latter case, there is another gotcha involving glibc's caching of
a pthread priority value: with --enable-lazy-setsched, that cached value
won't be updated with the new value passed to rt_task_set_priority(), so
__STD(pthread_getschedparam()) may return the old priority. Some
comments in pthread_getschedparam_ex() give details.

>> Anyway it was pretty difficult to get xenomai 3.08 with gpio support
>> to work on the raspberry pi 3.
> Not sure ATM where we stand with that platform and the GPIO
> enabling. Greg, Philippe?

The common bcm2835-gpio chip is supported by the real-time GPIO
framework. The RPI-specific GPIO expander is not though.


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