Re: [Xenomai-help] How to use Xenomai libraries with "normal" (non Xenomai) linux processes ?

[Jan Kiszka <jan.kiszka@domain.hid>]

Gilles Chanteperdrix wrote:
> roderik.wildenburg@domain.hid wrote:
>>>> 4.) Is there a way, to force back a process to secondary mode
>>>> (after the Xenomai-systemcall has been executed) ?
>>> Yes, but no, you do not want to do that. Xenomai automatically
>>> switches the process when needed.
>>>
>> But yes, I want, I want, I want  ;-)) Think about the following
>> scenario: 1)"linuxwithxenolib" calls my xenomai-library. 2)Due to a
>> xenomai systemcall in my library "linuxwithxenolib" switches to
>> primary mode 3)"linuxwithxenolib" leaves my library function 
>> 4)"linuxwithxenolib" is still in primary mode and acts as a realtime
>> task till it calls a linux-systemcall. This isn´t the way
>> "linuxwithxenolib" should act. It should be scheduled by linux as
>> often and as long as possible. 5)So, if I could force secondary mode
>> at the end of my library function everything would be fine.
>>
>> So how can I force secondary mode ?
> 
> The point is that if you force it to secondary mode, and the next system
> call is in fact a xenomai syscall, xenomai will switch the thread to
> primary mode again. So, you have two useless mode switches which you
> could have avoided if you had not forcibly switched to secondary mode.
> Note that even if a xenomai thread is running in secondary mode and has
> a priority higher than another xenomai thread running in primary mode,
> the one running in secondary mode will run. So, you should not use
> primary mode and secondary mode to decide which thread should run, you
> should use the priorities.

The only imaginable scenario for having to force some thread back to
secondary mode is when that thread is running significant CPU load
_without_ issuing a single regular syscall. If such a thread enters
primary mode and is then left there, Linux will not be able to run
anymore. In that case one may insert a tiny nanosleep etc. to enforce
the switch. But that's still a fairly theoretic scenario.

Jan

-- 
Siemens AG, Corporate Technology, CT SE 2
Corporate Competence Center Embedded Linux