On Wed, 2007-06-20 at 20:52 +0200, Jan Kiszka wrote:
> Gilles Chanteperdrix wrote:
> > Philippe Gerum wrote:
> >  > On Mon, 2007-06-18 at 10:27 +0200, Jan Kiszka wrote: 
> >  > > Jan Kiszka wrote:
> >  > > > ...
> >  > > > The answer I found is to synchronise all time bases as good as 
> > possible.
> >  > > > That means if one base changes its wall clock offset, all others 
> > need to
> >  > > > be adjusted as well. At this chance, we would also implement
> >  > > > synchronisation of the time bases on the system clock when they get
> >  > > > started. Because skins may work with different type width to 
> > represent
> >  > > > time, relative changes have to be applied, i.e. the core API changes
> >  > > > from xntbase_set_time(new_time) to 
> > xntbase_adjust_time(relative_change).
> >  > > > The patch (global-wallclock.patch) finally touches more parts than I 
> > was
> >  > > > first hoping. Here is the full list:
> >  > > > 
> >  > > >  - synchronise slave time bases on the master on xntbase_start
> >  > > >  - xntbase_set_time -> xntbase_adjust_time, fixing all time bases
> >  > > >    currently registered
> >  > > >  - make xnarch_start_timer return the nanos since the last host tick
> >  > > >    (only ia64 affected, all others return 0 anyway, causing one tick
> >  > > >    off when synchronising on system time -- but this fiddling becomes
> >  > > >    pointless on the long term due to better clocksourses on all 
> > archs)
> >  > 
> >  > Support for 2.4 kernels will be still around for the Xenomai 2.x series
> >  > though, and those will likely never support clocksources. Support for
> >  > Linux 2.4 will be discontinued starting from x3.
> >  > 
> >  > > >  - adapt vrtx, vxworks, and psos+ skin to new scheme, fixing 
> > sc_sclock
> >  > > >    at this chance
> >  > > >  - make xnarch_get_sys_time internal, no skin should (need to) touch
> >  > > >    this anymore
> >  > > 
> >  > 
> >  > This interface has not been meant to be part of the skin building
> >  > interface, but for internal support code that needs to get the host
> >  > time. For instance, one may want this information for obscure data
> >  > logging from within a module, independently of any wallclock offset
> >  > fiddling Xenomai may do on its timebases (so nktbase is not an option
> >  > here if timebases start being tighly coupled). And this should work in
> >  > real execution mode, or in virtual simulation mode. IOW,
> >  > xnarch_get_sys_time() has to remain part of the exported internal
> >  > interface (even if as some inline routine, that's not the main issue
> >  > here).
> >  > 
> >  > > Forgot to mention two further aspects:
> >  > > 
> >  > >  - The semantic of XNTBSET was kept time base-local. But I wonder if
> >  > >    this flag is still required. Unless it was introduced to emulated
> >  > >    some special RTOS behaviour, we now have the time bases 
> > automatically
> >  > >    set on startup. Comments welcome.
> >  > > 
> >  > 
> >  > That might be a problem wrt pSOS for instance. In theory, tm_set() has
> >  > to be issued to set the initial time, so there is indeed the notion of
> >  > unset/invalid state for the pSOS wallclock time when the system starts.
> >  > This said, in the real world, such initialization better belongs to the
> >  > BSP rather than to the application itself, and in our case, the BSP is
> >  > Linux/Xenomai's business, so this would still make sense to assume that
> >  > a timebase has no unset state from the application POV, and XNTBSET
> >  > could therefore go away.
> >  > 
> >  > The main concern I have right now regarding this patch is that it
> >  > changes a key aspect of Xenomai's current time management scheme:
> >  > timebases would be tighly coupled, whilst they aren't right now. For
> >  > instance, two timebases could have a very different idea of the Epoch in
> >  > the current implementation, and this patch is precisely made to kill
> >  > that aspect. This is a key issue if one considers how Xenomai should
> >  > deal with concurrent skins: either 1) as isolated virtual RTOS machines
> >  > with only a few bridges allowing very simple interfaces, or 2) as
> >  > possibly cooperating interfaces. It's all a matter of design; actually,
> >  > user/customer experience I know of clearly proves that #2 makes a lot of
> >  > sense, but still, this point needs to be discussed if needed.
> >  > 
> >  > So, two questions arise:
> >  > 
> >  > - what's the short term impact on the common - or not that common - use
> >  > case involving multiple concurrent skins? I tend to think that not that
> >  > many people are actually leveraging the current decoupling between
> >  > timebases. But, would some do, well, then they should definitely speak
> >  > up NOW.
> > 
> > There is a special concern with the POSIX spec: it states that when the
> > time is set, absolute timers should keep their absolute tick date (so,
> > when the time is set to a later date, absolute timers that should have
> > elapsed in the interval should elapse asap), and relative timers should
> > be changed to elapse at the correct date (new_elapse_date = new_date +
> > previous_elapse_date - old_date). The fact that the nucleus did
> > not implement relative and absolute timers (now it does) and that
> > xnpod_settime does not do what the posix spec wants is the reason why
> > clock_settime is still not implemented. Now, if another skin is allowed
> > to change the nucleus time, I guess it should trigger the posix
> > behaviour as well.
> > 
> > So, IMHO, if we take Jan's patch (which I am in favor of) we should
> > implement xnpod_settime the way posix wants it, after all, posix spec is
> > just common sense (with regard to this specific problem, I
> > mean). CLOCK_MONOTONIC timeouts would be implemented as relative
> > timeouts so that they would not be affected by CLOCK_REALTIME changes.
> > 
> 
> I was afraid you would insist on this support. ;)
> 
> There are two ways to implement this:
> 
>  A) The poor man's variant
> 
>     On xntbase_adjust_time() (the code will change again, pay attention!
>     ;) ), iterate over all pending timers (or over all timers in the
>     base that POSIX uses?) and fix those which do not have the recently
>     introduced XNTIMER_MONOTONIC flag set. "Poor man" because it's
>     simple, but it scales poorly.
> 
>  B) The scalable but complex one
> 
>     Introduce a second time base for each existing one (or for the one
>     that POSIX uses?), put in all the adjustable (realtime) timers. We
>     then only need to play with the base's clock offset on adjustment,
>     but we would also have to include that offset into timeout
>     considerations inside the timer interrupt handler.
> 
> I wonder now if the number of use cases where people are playing with
> the wallclock all over the time while a significant amounts of timers
> are pending is actually worth the troubles of B)... What do you think?
> 

Changing the epoch frequently enough and under any load condition, so
that the system could be impacted due to having a lot of outstanding
timers to resync, all this would be just silly. I'm with the old UN*X
mantra here again: do not penalize 99% of regular users for 1% of
weirdos by implementing overkill stuff everyone would have to suffer
from, just to make the weirdos happy. Scalability yes, but not for
braindamage behaviours. So A) looks like sufficient, and should likely
be confined to timebases holding absolute timers.

> Jan
> 
-- 
Philippe.



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