Philippe Gerum wrote:
> Jan Kiszka wrote:
>> Philippe Gerum wrote:
>>> Jan Kiszka wrote:
>>>> Jan Kiszka wrote:
>>>>> ...
>>>>> Xenomai is loaded at this time but not yet used. Linux runs in tickless
>>>>> highres mode and obviously had programmed the host timer to fire here.
>>>>> But instead of one timer IRQ (233) + its handling, we see an additional
>>>>> early shot (about 3 µs too early here - the longer the timer is
>>>>> programmed in advance, the larger the error gets) before the xntimer
>>>>> finally expires. But at the same time, /proc/xenomai/latency reports
>>>>> 1000 (1 µs). So there must be more discrepancy between TSC and APIC
>>>>> timebases, no? Well, nothing critical, but at least suboptimal, maybe
>>>>> pointing at some hidden minor bug. Once time permits, I will check the
>>>>> APIC frequency and the delay calculation on my box and compare it to
>>>>> what Linux uses.
>>>> Looks like Xenomai is using an inaccurate APIC frequency (probably since
>>>> ages): 10.383 MHz on one of my boxes vs. 10.39591 MHz according to
>>>> Linux' calibration ( (1,000,000,000 ns * clock_event_device.mult) >>
>>>> clock_event_device.shift ), which is based on the PM-timer. As the real
>>>> frequency is higher, the APIC fires earlier than we want to. Consider,
>>>> e.g., the 4 ms host tick period => 5 µs too early! This correlates with
>>>> my LTTng traces.
>>> Oops. Once again, this proves that having a permanent trace facility in
>>> place is key to uncover bugs.
>>>> I will try to fix this issue by extending the ipipe_request_tickdev
>>>> interface so that it returns also the frequency of the requested tick
>>>> device - as long as Xenomai 2.4 is not released, such an API breakage
>>>> should not cause any hassle IMHO.
>>> You may want to have a look at ipipe_get_sysinfo() first, and track the
>>> use of the tmfreq field in Xenomai. This may be what you want to fix,
>> Nope tmfreq is not involved. The problem is:
>>      rthal_timerfreq_arg = apic_read(APIC_TMICT) * HZ;
> Actually, ports should use rthal_get_timerfreq() to get this value which
>  in turn calls into the I-pipe to determine it accurately, instead of
> approximating it by themselves (this is not to say that the I-pipe
> always does this, though). Only ARM has it right currently.
>>> IIUC. This would keep the older patches usable with the next Xenomai
>>> version, which is very desirable.
>> We could extend the information ipipe_get_sysinfo returns by the timer
>> frequency.
> sysinfo.tmfreq was meant to be such value.

Well, then it is totally mis-initialised on x86 so far, same on powerpc.
In fact, there it is the _clock_ frequency, not the timer frequency. So
what is meant by this? One comment actually call it "Timebase frequency"
(powerpc). This naming looks rather inconsistent.

>  But to play cleanly, we would have to critical_enter first,
>> look up the currently used clock_event_device, maybe even validate that
>> it was hijacked, and then return its frequency. The problem with this
>> API is, that it is by nature unsynchronised with ipipe_request_tickdev,
>> thus would not always be able to return a valid frequency.
> You get no special guarantee from getting the frequency out of the
> request_tickdev call, because if the point is to prevent anyone from
> changing/removing such device while you are using such value at Xenomai
> level, then we can't, by design.
> So we may as well read per_cpu(ipipe_tick_cpu_device) from
> ipipe_get_sysinfo() to access the current tick device installed, without
> any downside. After all, the timer is something which has to be
> considered as a reasonably stable property of the system. Btw, we don't
> currently handle any change of frequency of the underlying hw timer, so
> changing the device would not actually work, I guess.
> The next question may be, should we handle such situation? I tend to
> think that we should not, because we just cannot accept any flaky
> situation due to a misbehaving time source which would make the kernel
> downgrade the current clock device, even temporarily, anyway. We have to
> be confident in the current time source when operating.
>> Actually, we would only exclude a few patches when going the
>> ipipe_request_tickdev way: those few that were clockevent-aware up to
>> today. For all others (namely 2.6.20 on i386 and up to 2.6.23 on
>> x86_64), we would simply fall back to our current inaccurate approach. I
>> think this is more acceptable than an ipipe_get_sysinfo extension.
> The archdep section from the sysinfo struct has been meant to be
> extended, really, so I think it's actually cleaner to use it - if
> possible - for this purpose, instead of adding new ad hoc interfaces for
> dealing with a particular kernel feature.

The timer frequency is coupled with the process of requesting a specific
tick device (I'm only talking about GENERIC_CLOCKEVENTS now, because
this is what matters mid-term). Only after deciding what event source to
use, one can obtain its frequency, thus passing it along the request
process appears more than logical and clean to me. Providing this
information "out of band" is broken IMHO, specifically as we need this
value within a very well-define time window (after switching the
clock_event, before using it).

However, that may not prevent us from doing something different for
legacy kernels and not-yet CLOCKEVENT'ized archs, but only temporarily.


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