scheduler clock for MXS [Was: Re: Wakeup latency measured with SCHED_TRACER depends on HZ]

[Stanislav Meduna]

On 05.11.2012 14:46, Shawn Guo wrote:

>>> From my quick testing on imx23 with printk timestamp, it's not OK,
>>> so we may need to leave imx23 out there.
>>
> I should say it's practically not OK since it wraps in such a short
> period.  But it actually works as expected.
> 
>> Hmm, does it wrap after 2 seconds?
> 
> Yes, it does wrap after ~2 seconds.

This is weird. AFAIK the printk should be using sched_clock(),
which is a weak symbol overridden in arch/arm/kernel/sched_clock.c
and it should take care of the extension to never-ever-wrapping
64-bit timestamp. Looks that it does not and if it does not,
I think there is more to be worried of than just printk timestamps.

I don't see anything wrong with the code; bumping the 64-bit
part so that it is no more than wrap time away should take
place in sched_clock_timer which is called at 90% of the
time interval.

BTW this patch deserves IMHO looking at
  https://patchwork.kernel.org/patch/1193631/
but it is probably not the problem here.

In case it matters I'm working with the current 3.4 with
RT patches (none of them touches arch/arm/mach-mxs) - sorry
not to mention it earlier, I forgot to write it again after
I included additional mailing lists to the discussion.


Regards
-- 
                                            Stano