Re: [Xenomai-core] Synchronising TSC and periodic timer

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

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Anders Blomdell wrote:
> Jan Kiszka wrote:
>> Jan Kiszka wrote:
>>
>>> Hi,
>>>
>>> for those who haven't followed the endless "RTDM and Timer functions"
>>> thread: we are currently discussing a way to provide high-resolution
>>> timestamps in periodic mode for RTDM users. It was suggested to use the
>>> TSC for this, but I noted that this source will not be in sync with the
>>> periodic system timer and may even be out of sync across multiple CPUs.
>>>
>>> A straight-forward approach to overcome this might be to record the
>>> current TSC value together with the current jiffies in
>>> xntimer_do_tick_periodic(). This tuple (per CPU) could then be provided
>>> to skin implementers in order to let them offer a high-resolution
>>> timestamp source even in periodic mode. Hmm, sounds too simple actually,
>>> so I'm waiting now for someone pointing out the pitfalls.
>>
>>
>> Likely too simple: The periodic IRQ seems to pop up on every CPU so that
>> the TSC could be recorded, but will this happen synchronously? At least
>> we will see (IRQ) jitters, and those jitters could already create in the
>> single-CPU case a non-monotonic clock...
> Returning a struct with { jiffies, cpu#, tsc, clockscaling, ... } and
> routines to compare ( lt | equal | gt | unordered ) and calculate
> differences { diff, accuracy }. And then people (of course) will send
> them over the network and compare items emanating from different systems!
> 

Well, I think it's more obvious that timestamps taken on box A will not
necessarily match timestamps of box B. Moreover, tasks on box A will not
that often get migrated automatically to box B, while this can happen
inside SMP boxes if you do not explicitly bind to a certain CPU.

For the local usage I have this scheme in mind:

local_irqs_off();
read_cpu_tsc();
get_last_cpu_tsc_offset();
local_irqs_on();
calc_corrected_time();

So, nothing has to be exported to the skin user.

Or did I misunderstand what you wanted to express?

Jan


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