Re: [Xenomai-core] [RFC][PATCH 4/4] shorten overrun loops of periodic timers

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

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Gilles Chanteperdrix wrote:
> Jan Kiszka wrote:
>  > Gilles Chanteperdrix wrote:
>  > > Jan Kiszka wrote:
>  > >  > A simple patch, just like suggested by Gilles, to avoid looping over
>  > >  > periodic xntimer handlers in case of overruns.
>  > >  > 
>  > >  > It saves the current TSC on loop entry and uses this value later when
>  > >  > forwarding the timer. Is is the overhead of re-reading the TSC on all
>  > >  > archs negligible and should we rather go that way?
>  > > 
>  > >  >(...)
>  > >  > -				xntimerh_date(&timer->aplink) +=
>  > >  > -				    nkpod->htimer.interval;
>  > >  > +				while ((xntimerh_date(&timer->aplink) +=
>  > >  > +				        nkpod->htimer.interval) < now);
>  > > 
>  > > I think you are patching the wrong addition, the one you are interested
>  > > in is most probably the one at the bottom of xntimer_do_tick_aperiodic.
>  > > 
>  > 
>  > Ouch, indeed. Guess I should start reading what I patch. Here comes a
>  > second try.
>  > 
>  > I'm still in favour of saving the TSC instead of re-reading it.
>  > Otherwise we would have to pave the code with #ifdefs for the case
>  > xnarch_get_cpu_tsc() is slow for a specific setup. Not that nice, is it?
> 
> I am thinking again about this patch: some handlers need to be
> rewritten, for example the posix timers handler, because the handler
> relies on the fact that it is called for every timer expiry to compute
> the overruns count. So maybe this patch should come with the addition of
> an xntimer_getoverrun service that computes the overrun count using the
> tsc ?
> 

Mmh, that gets close to hrtimer_forward now: push an overdue timer to an
expiry date that is in the future and return the number of overruns. But
do we still want this optimisation of the broken path then? It starts
getting complex, probably adding more code than it is worth. I'm
starting to vote against my own patch...

Jan


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