Preemption issue, memory problem or something else?
Gedare Bloom
gedare at gwmail.gwu.edu
Wed Oct 19 01:53:45 UTC 2011
Hi,
The most likely answer is that global variables in the stanford.c file are
shared and not protected e.g. by semaphores.
On Oct 18, 2011 5:31 PM, "Constantine "chicky" Giotopoulos" <
kotsosgiotopoulos at gmail.com> wrote:
> 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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> rtems-users at rtems.org
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>
>
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