Preemption issue, memory problem or something else?
Till Straumann
strauman at slac.stanford.edu
Wed Oct 19 03:11:40 UTC 2011
Indeed. A quick look at 'stanford.c' reveals that there are plenty of
globals.
You have two options:
- add a semaphore that protects the global variables and serialize
access to
them, e.g., by taking the semaphore before calling 'stanford()' and
releasing
it after return. This assumes that it is OK to share the global
state among
multiple tasks.
- modify 'stanford.c' to make it reentrant. Move all global variables
to a 'struct'
and pass a pointer to this 'context information' to 'stanford()'.
Each task has
it's own copy/instance of such a context. You probably also need to
add a routine
for initializing a context.
Note: IIRC rtems/newlib's 'rand()' is thread-safe, i.e., every thread
has it's own
'rand-state'. Otherwise you'd have to think about that one, too.
HTH
-- Till
On 10/18/2011 08:53 PM, Gedare Bloom wrote:
>
> 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 <mailto: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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