[Xenomai-help] IRQ-Latency when in idle State on ARM

["Karl Tyss" <tyss.k@domain.hid>]

Hi,

I am running Xenomai 2.4.7 and linux-2.6.28 on ATMELs AT91SAM9G20-EK. I
noticed a strange thing while measuring IRQ latency with a logic
analyser. Basically I just insert a module with insmod into the kernel.
The module creates an interrupt with "rt_intr_create" and the isr only
triggers a gpio pin up and down.

I also turned I- and D-caches off to avoid the effects of cache-flushes
on a context switch. If the kernel runs the idle process (cpu_idle) the
latency is about 2us larger than with a load. It seems be irrelevant
what kind of load I generate. To generate load I am now using a "stress"
application I found. The simplest case is that it runs a loop and calls
the rand() function.

The "idle"-latency is about 25us in this case and the "stressed" one
about 23 us.

If i run the same test on a kernel without the xenomai extensions i
dont get an difference between the latencies.

Has anyone any ideas why this is happening?

Best Regards,
Karl Tyss




Mit Freundlichen Grüßen,
Karl Tyss




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