Re: [Xenomai] IO-APIC latencies

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2012-09-17 20:18, Gilles Chanteperdrix wrote:
> On 09/17/2012 08:15 PM, Jan Kiszka wrote:
> 
>> On 2012-09-17 20:12, Jan Kiszka wrote:
>>> On 2012-09-17 20:08, Gilles Chanteperdrix wrote:
>>>> On 09/17/2012 08:05 PM, Jan Kiszka wrote:
>>>>
>>>>> On 2012-09-17 19:46, Gilles Chanteperdrix wrote:
>>>>>> ipipe_end is a nop when called from primary domain, yes, but this is not
>>>>>> very different from edge irqs. Also, fasteoi become a bit like MSI: in
>>>>>> the same way as we can not mask MSI from primary domain, we should not
>>>>>> mask IO-APIC fasteoi irqs, because the cost is too prohibitive. If we
>>>>>> can live with MSI without masking them in primary mode, I guess we can
>>>>>> do the same with fasteoi irqs.
>>>>>
>>>>> MSIs are edge triggered, fasteois are still level-based. They require
>>>>> masking at the point you defer them - what we do and what Linux may even
>>>>> extend beyond that. If you mask them by raising the task priority, you
>>>>> have to keep it raised until Linux finally handled the IRQ.
>>>>
>>>>
>>>> Yes.
>>>>
>>>>> Or you
>>>>> decide to mask it at IO-APIC level again.
>>>>
>>>>
>>>> We do not want that.
>>>>
>>>>> If you keep the TPR raised,
>>>>> you will block more than what Linux wants to block.
>>>>
>>>>
>>>> The point is that if the TPR keeps raised, it means that primary domain
>>>> has preempted Linux, so, we want it to keep that way. Otherwise the TPR
>>>> gets lowered when Linux has handled the interrupt.
>>>>
>>>> A week-end of testing made me sure of one thing: it works. I assure you.
>>>
>>> Probably, in the absence of IRQF_ONESHOT Linux interrupts. No longer if
>>> you face threaded IRQs - I assure you.
>>
>> Well, it may work (if mask/unmask callbacks work as native) but the
>> benefit is gone: masking at IO-APIC level will be done again. Given that
>> threaded IRQs become increasingly popular, it will also be hard to avoid
>> them in common setups.
> 
> 
> The thing is, if we no longer use the IO-APIC spinlock from primary
> domain, we may not have to turn it into an ipipe_spinlock, and may be
> able to preempt the IO-APIC masking.

That might be true - but is the latency related to the lock or the
hardware access? In the latter case, you will still stall the CPU on it
and have to isolate the load on a non-RT CPU again.

BTW, the task priority for the RT domain is a quite important parameter.
If you put it too low, Linux can run out of vectors. If you put it too
high, the same may happen to Xenomai - on bigger boxes.

On the other hand, it may be useful as a complete mask for Linux, ie.
when applied unconditionally on entry to the RT domain and released on
exit or fasteoi ending. Already played with that model? That would have
a value beyond IO-APICs.

Jan

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