Re: ARM Linux Suitability for Real-time Application

["Eugeny S. Mints" <emints@ru.mvista.com>]

Andrew Lewis wrote:
> I have recently been using Linux on the AT91RM9200 as a data 
> concentrator for a small (as yet unreleased) communications application.
> 
[snip]
> In order to run the radio successfully I need to be able to handle 
> interrupts within around 50us, and have interrupts run constantly for 
> between 20us and 200us (mostly at the low end).  I would expect there to 
> be three or four of these interrupts every 1000us, probably using about 
> 25% of the CPU power just to manage the radio deck.
> 
> This would makeup the real-time control element of the application.  If 
> I was writing this on bare metal, I would lock the high level interrupts 
> out to perform communications between the mainline and the interrupt 
> control paths for no more than 10us.
> 
[snip]

> The question is, would I be able to meet these time contraints if I 
> wrote a device driver running under Linux?  Especially important is 
> keeping the latency of handling the interrupts below 50us, and not 
> breaking anything by running 200us long interrupts.
> 
> I just wanted a general indication that such a driver is possible or 
> impossible before embarking on a more detailed investigation.
The rough answer is that it's impossible with vanilla kernel and 
possible   (as a rough approximation) with Ingo Molnar's real-time patch 
  http://redhat.com/~mingo/realtime-preempt/ which includes arm 
real-time port and hardirq-disable removal feature by Daniel Walker.

All linux real-time related discussions are going on LKML so first of 
all you might want to use LKML instead arm linux list for all real-time 
questions (thus I changed list in the header of the message).

As to concrete numbers you are interested in please take a look at 
"PREEMPT_RT vs I-PIPE: the numbers, part 2" thread on LKML which I 
believe presents most recent numbers.

The bottom line is that interrupt and preemption latencies for a kernel 
with RT patch are inbetween 15-150us. But of course even with real-time 
patch you have to perform _real_ fine tuning of your system to achieve 
such hard constraints you identified.

	Eugeny
> Best Regards,
> Andrew Lewis
> 
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