[Bug 2180] New: _TOD_Get_with_nanoseconds() is broken on SMP

[Gedare Bloom]

On Thu, Aug 21, 2014 at 3:03 AM, Sebastian Huber
<sebastian.huber at embedded-brains.de> wrote:
> On 20/08/14 16:05, Joel Sherrill wrote:
>>
>>
>> On 8/20/2014 8:59 AM, Sebastian Huber wrote:
>>>
>>> On 20/08/14 15:53, Joel Sherrill wrote:
>>>>
>>>> On 8/20/2014 8:12 AM, Daniel Cederman wrote:
>>>>>>
>>>>>> Hi,
>>>>>>
>>>>>> We are currently experiencing this bug reported by Sebastian Huber and
>>>>>> I
>>>>>> could not find any discussion on it on the list. I'm guessing that the
>>>>>> simple solution of moving the call to Clock_driver_support_at_tick()
>>>>>> to
>>>>>> the critical section of _TOD_Tickle_ticks() is not an acceptable
>>>>>> solution, given that it would be a call from score to the BSP?
>>>>
>>>> I don't have a problem with it. Some documentation may need to be
>>>> updated to
>>>> reflect the environment in which the bsp hook is called but I think it
>>>> is pretty
>>>> obvious it is called when an accurate timestamp is wanted.
>>>
>>> Its not that easy.  This Clock_driver_support_at_tick() is part of
>>> clockdrv_shell.h.  What we need is a proper clock driver API.
>>>
>> Grrr.. I should have actually re-read the PR and not tried to remember
>> the issue.
>> Sorry. :(
>>
>> It looks like the entire sequence needs to be locked.  What that exposes
>> is
>> another matter.
>>
>
> The clock handling is entirely broken on SMP and not scalable.  Our clock
> infrastructure is only good enough for uni-processor systems, but even here
> the usage of nanosecond timestamps is suboptimal.  I would rather use
> something like the FreeBSD time hands:
>
> http://phk.freebsd.dk/pubs/timecounter.pdf
>
Nice.  It looks like it would take:
1) Convert the 64-bit timestamps into bintime: a split time_t
'seconds' + uint64_t 'frac' format. The math is shown in the paper.
2) A lock-free ring buffer for the timehands.
3) The timecounter structure that describes the hw clock source, and a
way to register timecounters from BSPs.
4) Rewrite tick handling to do the refresh operation.

Did I miss anything?

> Anyway I don't have a budget for this, so for the current ESA project I
> would disable the nanoseconds support in case this bug hurts.
>
>
> --
> Sebastian Huber, embedded brains GmbH
>
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>
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