Preemption issue, memory problem or something else?
Constantine "chicky" Giotopoulos
kotsosgiotopoulos at gmail.com
Tue Oct 18 21:00:29 UTC 2011
Hello everyone.
I am using RTEMS to create a number of tasks and run a set of functions
performing mostly mathematical operations. These are included in a file
named "stanford.c" (
http://classes.engineering.wustl.edu/cse465/docs/BCCExamples/stanford.c ).
The problem is that when I enable preemption, for some reason the functions
do not output the expected results.
I initially create 100 tasks using the following lines of code:
----------------------------------------------------------------------------------------------------
rtems_task Init( rtems_task_argument ignored )
{
rtems_status_code status; rtems_id id;
int i;
for (i=0;i<100;i++)
{
srand(i*3);
status = rtems_task_create(
rtems_build_name( 'T', 'A', '2', ' ' ), rand()%254+1,
RTEMS_MINIMUM_STACK_SIZE, RTEMS_DEFAULT_MODES,
RTEMS_DEFAULT_ATTRIBUTES | RTEMS_FLOATING_POINT, &id
);
assert( !status );
status = rtems_task_start( id, task2, id%1000 );
assert( !status );
}
status = rtems_task_delete( RTEMS_SELF );
exit( 0 );
}
-----------------------------------------------------------------------------------------------------------------------------
Then I state the functionality of the tasks.Each task executes the functions
included in the "stanford.c" 5 times using a "for" loop. Each task is
triggered with a "wake_after" directive that is set randomly before each
execution and the priority is changed also randomly after each execution.The
point in that is to get various tasks triggering at a random manner and
taking over the CPU in a (somewwhat) random way:
---------------------------------------------------------------------------------------------------------------------------
rtems_task task2( rtems_task_argument id_arg )
{
rtems_status_code status;
int i;
for (i=0;i<5;i++)
{
srand(id_arg*i);
rtems_task_wake_after(rand()%1000);
printf("Task2-stanford with id=%d is executing. Iteration:%d\n", id_arg,
i);
stanford();
printf("Task2-stanford with id=%d iteration:%d has finished.\n\n\n",
id_arg, i);
rtems_task_set_priority(RTEMS_SELF, rand()%254+1, 1);
}
rtems_task_delete( RTEMS_SELF );
}
My configuration options are:
#define CONFIGURE_APPLICATION_NEEDS_CONSOLE_DRIVER
#define CONFIGURE_APPLICATION_NEEDS_CLOCK_DRIVER
#define CONFIGURE_RTEMS_INIT_TASKS_TABLE
#define CONFIGURE_MICROSECONDS_PER_TICK 100
#define CONFIGURE_MAXIMUM_TASKS 105
#define CONFIGURE_INIT
#include <rtems/confdefs.h>
--------------------------------------------------------------------------------------------------------------------------
I have attached the code that is being executed (test.c-makefile)
If I enable non-preemption, then the execution output is as expected, no
error messages or anything out of the ordinary, for example:
------------------------------------------------------------------------------------------------------------------------
Task2-stanford with id=698 is executing. Iteration:0
Starting
Perm Towers Queens Intmm Mm Puzzle Quick Bubble Tree
FFT
0 0 0 0 0 0 0 0
0 0
Nonfloating point composite is 0
Floating point composite is 0
Task2-stanford with id=698 iteration:0 has finished.
Task2-stanford with id=714 is executing. Iteration:0
Starting
Perm Towers Queens Intmm Mm Puzzle Quick Bubble Tree
FFT
0 0 0 0 0 0 0 0
0 0
Nonfloating point composite is 0
Floating point composite is 0
Task2-stanford with id=714 iteration:0 has finished.
Task2-stanford with id=730 is executing. Iteration:0
Starting
Perm Towers Queens Intmm Mm Puzzle Quick Bubble Tree
FFT
0 0 0 0 0 0 0 0
0 0
Nonfloating point composite is 0
Floating point composite is 0
Task2-stanford with id=730 iteration:0 has finished.
However, when I enable preemption, tasks start interrupting each
other (depending on their priority). Especially when it comes to functions
that use recursion, I am presented with errors. These do not occur everytime
these functions are executed, but occasionally. These errors occur mostly
when a task is interrupted by another task(s) of higher priority. The error
messages are somewhat like that:
" Perm Towers Error in Towers: nothing to pop
Error in Towers: nothing to pop
Error in Towers: disc size error
Error in Towers: disc size error
Error in Towers: nothing to pop
Error in Towers: nothing to pop
Error in Towers: disc size error"
"Task2-stanford with id=714 is executing. Iteration:2
Starting
Task2-stanford with id=745 is executing. Iteration:3
Starting
Perm Towers Queens Intmm Mm Puzzle Quick Bubble Tree
FFT
0 0 0 0 0 0 0 0
0 0
Nonfloating point composite is 0
Floating point composite is 0
Task2-stanford with id=745 iteration:3 has finished.
* Perm Error in Perm.
* Towers Queens Intmm Mm Puzzle Quick Bubble Tree FFT
0 0 0 0 0 0 0 0
0 0
Nonfloating point composite is 0
Floating point composite is 0
Task2-stanford with id=714 iteration:2 has finished.
* Perm Error in Perm. *
Task2-stanford with id=714 is executing. Iteration:3
Starting
Perm Towers Queens Intmm Mm Puzzle Quick Bubble Tree
FFT
0 0 0 0 0 0 0 0
0 0
Nonfloating point composite is 0
Floating point composite is 0
Task2-stanford with id=714 iteration:3 has finished."
-------------------------------------------------------------------------------------------------------------------------------------------------
My question is, why are these errors produced when I enable preemption and
are not produced when I disable preemption? Does the fact that this concerns
only the recursive functions have to do anything with the problem? Could for
some reason, when there is a context switch, the data in the registers of
the task being interrupted are not stored correctly in the memory? Or that
they are not being correctly transfered back to the registers when it's
their turn to be executed again?
Or could it be a memory issue? I have tried initializing the tasks with
(RTEMS_MINIMUM_STACK_SIZE * 2) but the errors keep coming. I also tried to
allocate more memory to the task stack by using the
"CONFIGURE_EXTRA_TASK_STACKS", but when I checked if with the
"rtems_stack_checker_report_usage" directive, for some reason it didn't have
an impact at the available memory being used by the tasks.
What I need, is for the program to execute with preemption enabled without
presenting these errors.
What am I missing out? Are there any options I could utilize, or debugging
with gdb is the way to continue (I'd like to avoid that if possible)?
Thank you in advance.
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