Interrupt latency problems on MPC860

[Joel Sherrill]

Phil Torre wrote:
> 
> We are using RTEMS 4.5.0 on a 50MHz MPC860.  Our application relies
> on a particular periodic interrupt being serviced with no more
> than 78 microseconds of latency.  We keep having problems because
> that interrupt isn't being serviced on time.
> 
> There's not much else going on in the system currently.  Several
> tasks are asleep or blocking on I/O, but none of them should be
> waking up and causing a context switch.  (We've got things really
> stripped down trying to pin down the latency problem, so practically
> everything but our critical interrupt is commented out.)
> 
> Is 78us of latency for maskable interrupts a reasonable goal, or is
> that outside of RTEMS' performance envelope?  Any comments are welcome.

In general terms, this is not a problem for RTEMS itself.  However,
application code and the PowerPC architecture itself can make this
hard to achieve.

First, on a PPC you have to go hunting for the interrupt source 
and that could be bad on some boards.  One I know of had a very complicated
procedure to track down what happened.

Moreover, this really may be tough to attain on the PowerPC
architecture without VERY special care.  The PowerPC by default does
not really have interrupt priority or (without work) nesting of 
interrupts.  So all you have to do is take > 79 usecs to process
an interrupt and you are nailed.  Since (I am pretty sure about this)
interrupts are disabled from the time a handler is vectored by HW
until the entire ISR returns, you have a long window there.  

Any other PowerPC experts out there want to comment?

> Thanks,
> -Phil
> 
> --
> 
> =====================================================================
> Phil Torre                               phone: 425-820-6363 x234
> Design Engineer                          email: ptorre at zetron.com
> Switching Systems Group                    fax: 425-820-7031
> Zetron, Inc.                               web: http://www.zetron.com

-- 
Joel Sherrill, Ph.D.             Director of Research & Development
joel at OARcorp.com                 On-Line Applications Research
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