[Xenomai-core] Re: [Xenomai-help] rt_task_wait_period() and overruns

[Philippe Gerum <rpm@xenomai.org>]

Dmitry Adamushko wrote:
> 
> On 01/03/06, *Philippe Gerum* <rpm@xenomai.org <mailto:rpm@xenomai.org>> 
> wrote:
>  
> 
>      The other option I've described for
>     dealing with overruns in rt_task_wait_period would be as follows:
> 
>     - save the count of overruns
>     - clear the count of overruns  /* i.e. "purge" */
>     - return both the saved count and -ETIMEDOUT to the user.
> 
>     This way, rt_task_wait_period would return only once with an error
>     status, telling
>     the user about the exact count of pending overruns in the same time. 
> 
> 
> 
> I definitely agree with you here.
> 
> IMHO, there is no point in calling rt_task_wait_period() a few
> times in a row just to clean up the "poverrun" counter
> (if there were a few overruns) when the whole may be reported at once. 
> This former way just gives unnecessary overhead.
> 

My concern is that some recovery procedure might require to get the exact number 
of pending overruns to operate properly in order to catch up with the missing 
expiries, and there is no way to get this information out of the current API (!). 
Even calling rt_task_wait_period in loop and testing for -ETIMEDOUT is unusable, 
since well, we would obviously get blocked when the overrun count drops to zero, 
which is not what we want in order to be able to run the recovery procedure asap.

> 
> Actually, there is a kind of application that must not rely on
> the "poverrun" counter, the klatency/latency utility and alike.
> 
> They are run normally (at least at the very first time) in the untrusted 
> environment
> where SMI or something similar - that may prevent a CPU from handling normal
> interrupts for quite a long time - make occur happily.
> As the "poeverrun" counting is dependent on the timer interrupt,
> it becomes irrelevant.
> 
> Something like
> overruns = (real_time_of_wakeup - desired_time_of_wakeup) / period (*)
> should be rather used there (of course, the timing source must not be
> interrupt-dependent).

Ah! you know what, I'm pretty sure that one of your very first public posts on the 
RTAI/fusion mailing list at that time, was exactely about this issue :o) And yes, 
this is correct, we currently rely on some internal accounting that depends on the 
system's ability to get properly paced timer ticks, which is, mmh... silly. I'm 
the one to congratulate for that.

> 
> Other applications may likely rely on the "poverrun" counter
> (returned by rt_task_wait_period(&overrun)) as they normally
> run in the (more or less) studied and tweaked environment.
> 
> Ok, some "paranoid" programmers would think differently even then
> but there is always the (*) way :o)
> 
> 
> -- 
> Best regards,
> Dmitry Adamushko


-- 

Philippe.