Re: AMP on an SMP system

[Marco Stornelli <marco.stornelli@gmail.com>]

Il 02/08/2013 14:13, Michael Schnell ha scritto:
> On 08/02/2013 01:42 PM, Robert Schwebel wrote:
>> Before hacking around (which might also lead to interesting solutions),
>> I would start using a kernel with preempt-rt support and play with the
>> cpu affinity:
>>
>> http://lxr.linux.no/#linux+v3.10.4/Documentation/kernel-parameters.txt#L1257
>>
>>
>
> Robert !
> Nice to see you here (I do own your "Embedded Linux Handbuch für
> Entwickler" :-) )
>
> Thanks for the pointer !
>
> I do already know "preempt-rt", but I was not aware of cpu affinity.
>
> So this might help.
>
> In fact I need a way to do very guaranteed low latency. regarding the
> high clock rate (about 1 GHz) modern ARM chips can provide, maybe
> preempt-rt with the cpu affinity might be a decent way to go.
>

Just to be clear: at the moment there isn't an easy way to dedicate 
"completely" a cpu for a task. The last time I tried (some years ago 
actually) to use exclusive cpu set, the scheduler didn't do a good work 
because it was designed for SMP, not SMP minus some piece. However you 
can try and you can report your results. It would be interesting.

> The raining questions include
>   - how to calculate the maximum latency that can be guaranteed ? (i.e.
> does the Kernel impose any spinlocks and interrupt disables on the would
> be AMP subsystem ?)

No. You can use full dyn tick for example to disable timer interrupt, 
but it has got some pros and cons, especially with very low latency 
requirement.

>   - how to assign an interrupt (e.g. a dedicated timer) to the subsystem ?

Interrupt handler are kernel thread, so you can schedule your kernel 
thread on your "normal" cpu.

>   - Do the interrupts immediately call the ISR of the cpu "under
> affinity" or is some additional latency imposed by the Kernel

AFAIC, no latency for cpu "under affinity".

> (and how
> many cpu cycles at max are needed to enter the ISR) ?

It's difficult to answer to this question because the performance 
depends on your system. From my last statistics I saw that with an rt 
linux kernel you can stay below 50us for the interrupt latency.

Marco