Re: pps_capture() and pps_fetch()

On 02.06.22 06:59, Konstantin Belousov wrote:
> On Wed, Jun 01, 2022 at 01:03:19PM +0200, Sebastian Huber wrote:
>> Hello Poul-Henning,
>>
>> On 01/06/2022 09:25, Poul-Henning Kamp wrote:
>>> Sebastian Huber writes:
>>>
>>>> I try to understand how the PPS synchronization works in FreeBSD. It
>>>> seems that pps_capture() starts a transaction and pps_event() completes
>>>> the transaction if nothing interfered in the meantime.
>>> The answer to most of your questions are in ./i386/i386/elan-mmcr.c
>>>
>>> The PPS capture in the Soekris 4501 used two hardware counters, counting at the same rate.
>>>
>>> The first of the two were the timecounter, the other was started by the hardware signal.
>>>
>>> Sometime later the hardware signals interrupt processing would happen.
>>>
>>> By reading read both counters as close to instantaneously as possible, and compensated for the interrupt latency by subtracting the event-started counter from the timecounter.
>>>
>>> (See also:http://phk.freebsd.dk/soekris/pps/)
>>
>> thanks for the background information.
>>
>> What I don't understand is why the th_generation is checked three times
>> (pps_capture(): 1, pps_event(): 2) and not only once in pps_event() right
>> before we use the captured time.
> 
> I am not sure about supposed use of the pps(9) API, but for me it looks
> like generation rechecks cut potentially CPU-consuming computation in the
> interrupt handler context.  For instance, pps_event() seems to be used _some_
> time after pps_capture(), and needs to do a lot.  So there is a chance
> that the data from pps_capture() is already not relevant due to timecounter
> advance.
> 
> On the other hand, single read of the generation should be quite cheap,
> the cache line should be hot and we spent it to avoid larger waste.  In
> other words, doing it probably slighly improves system latency.
> 
> Note that I did not wrote the code and speculataion above is a speculation.

I just ask because we may want to use this code in a spacecraft and 
every "if" increases the time to specify and test the implementation.

The expensive part in pps_event() is after the th_generation checks. I 
think from a performance point of view, the checks can be reduced to 
just one th_generation check. I am more concerned if this would 
introduce a subtle functional change.

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