Re: [Xenomai] xenomai/ipipe arm64 port

[Philippe Gerum <rpm@xenomai.org>]

On 09/01/2015 09:11 PM, Jorge Ramirez Ortiz wrote:
> On 09/01/2015 02:27 PM, Dmitriy Cherkasov wrote:
>> We have seen this before, but so far have not found a way to consistently
>> reproduce it.
>>
>> There are some caveats with the current way context switching is done in Xenomai.
>>
>> On armv7, there was a completely separate implementation of xnarch_switch_to(),
>> as well as separate fpu and TLS switching. At this stage for armv8 however, we
>> are doing a direct call to the kernel's __switch_to(), which then performs the
>> relevant register switches.
>>
>> Another possibility is that this may be lurking in the mm switch. During calls 
>> to check_and_switch_context(), the interrupt state may not always
>> match what is expected according to the comments in the original
>> version of that function. 
>>
>> Overall, this mechanism could definitely benefit from further review.
>>
> 
> 
> Thanks Dimitry.
> I'll also try to look into this for this specific platform (however I am hopping
> that either Gilles or Philippe can spend some time on this soon)

No arm64 hw here, so unless this is easily reproducible on a vm, I can
only help with code review. Did you enable
CONFIG_XENO_ARCH_UNLOCKED_SWITCH? If so, any change when turning it off?

> 
> Just for completeness, as things stand this is pretty much that value I am
> seeing over consecutive runs.
> 
> RTD|      4.167|      6.099|     11.667|       0|     0|      3.333|     66.667

This latency seems high for such class of hardware, especially with no
load. We'll need the tracer to dig this too.

-- 
Philippe.