Scheduler bug?

Leon Pollak leonp at
Sun May 10 12:48:22 UTC 2009

Hello, all.

My customer reported the unit resets sometimes in the very old unit running 
RTEMS version from 2003. They changed the cpu clock to faster and the reset 

Although I do not believe to myself, but my investigations showed that I very 
probably encounter some bug in RTEMS scheduler (see below). The question is if 
the bug is corrected in any new version? I mean, will the upgrade to 4.9 help?

As the end customer (the customer of my customer) is a military organization, 
they will be really upset when I will say "upgrading the RTOS"...:-) without 
being sure...

So, please, advice...:-)
A lot of thanks ahead.


The problem description:
The application has only 2 tasks: 

- WD - priority 50, task to reset HW watchdog, sleeps for 10 ticks, puts the 
reset line to high, sleeps for 10 clocks, puts the reset line to low;

- MB - priority 100, task waits for event from interrupt with 1(!!!) tick 
timeout. If timeout occurs, it refreshes some variables and waits for event 
again. Interrupt frequency is about 50Hz (20ms).

The WD tasks stops working rather fast (30-50s) when tick=2ms.
Debugger shows, entering rtems_clock_tick() routine and further 
_Watchdog_Tickle() routine, that
the_watchdog->Node.delta_interval=0xFFFFFeXX for the WD task.

This obviously causes the HW watchdog to reset the system very soon...:-)

-Increasing the MB task event waiting timeout even to 2 ticks seems to 
eliminate the problem.
-Masking the incoming interrupts seems to eliminate the problem.
-Increasing tick significantly reduces the problem probability.

Other changes seem not to influence the situation.

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