Gilles Chanteperdrix wrote: > Jan Kiszka wrote: >> Gilles Chanteperdrix wrote: >>> Wolfgang Mauerer wrote: >>>> Gilles Chanteperdrix wrote: >>>>> Wolfgang Mauerer wrote: >>>>>>> On the one hand you make complicated code (which will be costly on low >>>>>>> end hardware) to avoid shutting interrupts around a few assignments, but >>>>>>> on the other hand you leave an architecture specific function pointer >>>>>>> call where we want a fast behaviour on average (remember, we do all this >>>>>>> to avoid a system call, which is only a few hundreds nanoseconds on your >>>>>>> big iron x86), and where we have a generic fast replacement. Sometimes, >>>>>>> I do not understand your logic. >>>>>> But using the same argument, you could get rid of Linux vsyscall based >>>>>> gettimeofday()... >>>>> I do not see your point, the Linux code does not go a long way to make >>>>> lockless code, it simply turns off interrupts around the gtod data >>>>> update, which is really reasonable given the size of the masking >>>>> section. The reading is lockless, the writing is not. >>>>> >>>> I was referring to the argument that system calls are so fast that >>>> replacing gtod with a syscall-less version that uses function >>>> pointer dereferencing instead does not make much of a difference. >>> That is not what I said. I compared the weight of a function pointer >>> call with the one of four asignments with irqs off. And yes syscalls are >>> fast on x86, do the measurements yourself, you may be surprised. >>> >>>> Be it as it may, I need to check how far our budget can cover >>>> the (much more comprehensive) modifications required for the >>>> solution suggested by you. Let's see. >>> I do not think there is that much work involved. The way I see it, we >>> would need to replace our tsc reading function with one returning >>> "ntp-corrected" tsc (that is, essentially a subset of the gettimeofday >>> function you implemented, without conversion to ns and to CLOCK_REALTIME). >>> >>> Changes in this monotonic clock would trigger a recomputation of the >>> next timer event date. >>> Changes in monotonic to real-time conversion would trigger a call to >>> xnpod_set_time. >> If all works out well, it might be that simple. But this is timer/clock >> stuff, the heart of the system, and easy to get subtly wrong. >> >> For that reason the plan was to gain more confidence in the externally >> corrected clock source, collect experience in other use cases, and then >> work on the core for its optional use. >> >> So far our customer is using this clock for important but not yet >> critical jobs. Making it the RT clock source is of a different quality, >> and for now without a use case. > > To be quite frank about use cases, I do not really understand in what > use case the patch Wolfgang sent is useful. An application can not use > the timestamps returned by this gettimeofday syscall for anything > useful. And if we talk about things breaking subtly, an application that > would use these timestamps with Xenomai services would be subtly broken > too.
Just like it seems to be the case for Steve (unless I misunderstood his reply), it is very useful for us being able to time-stamp events in RT context that need to be correlated with events stamped in non-RT (including non-Xenomai) parts or even on other systems: (offline) data fusion, logging, tracing. I even bet that this is currently the major use case for synchronized clocks, only a smaller part already has the need to synchronize timed activities on a common clock source. But there is huge potential in the second part once we can provide a stable infrastructure. > > Avoiding the drift between Xenomai clock and Linux clock by making them > synchronized by design, on the other hand, albeit probably solving a > corner case, looks more useful, and some people asked for it (it looks > to me like what Steve asked for is that, not to have a third timebase It is surely more useful, but also more complex. Nothing unsolvable, but asking for more care. > accessible in real-time context, and Steve asked for a solution for > powerpc, not for x86). Even a "third clock" would have to be delivered for more archs than x86, no question. We would basically need a generic but slow syscall variant and per-arch syscall-less optimizations (where feasible). > > So, my feeling about all this is that Wolfgang's patch is not useful for > anyone else than your customer. > I think your feeling is a bit too pessimistic on this general approach and a bit too optimistic regarding the complexity of a complete solution. But I wouldn't mind being proven wrong, specifically regarding the latter. However, let's see if we can do some uncontroversial steps in this direction. Jan
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