Re: [Xenomai-help] Question about getting system time

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

Steve Deiters wrote:
>> Periodically setting the time is risky if timed jobs depend 
>> on Xenomai's real-time clock - it may jump in all directions...
>>
>>> Any other suggestions for providing timestamps to real time 
>> tasks in 
>>> this case?
>> Do you just need precise timestamps from with real-time 
>> tasks, or do you have to synchronize timer events of the 
>> Xenomai core on an external clock?
>>
>> For the former case (precisely our scenario), we laid the 
>> ground to extend Xenomai 2.5 with RT-safe syscalls to obtain 
>> Linux's view on gettimeofday. It "just" needs some polishing 
>> to post this for upstream.
>> Wolfgang (CC'ed) is working on this.
> 
> I'm just looking to get timestamps in the real time task.  At least in
> my case being able to call gettimeofday from the real time thread would
> be exactly what I need.
> 
> By the way, calling gettimeofday currently within a real time thread
> seems to occasionally freeze up my whole system.  I was going to make
> another post for this after I got a chance to verify some details.  Has
> anyone noticed anything similar to this?
> 

Yes, that's expected: If gettimeofday runs syscall-less, it tries to
read the time offset from a page which the Linux kernel updates
regularly. When user-space detects an ongoing update, it spins until
that has completed. But as Xenomai runs the task with higher priority
than the Linux update handler, you just ran into a live-lock.

Switching on the Xenomai watchdog will confirm this: It will shoot that
task, and the system will recover.

Jan

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