removing taskvares (was Re: rtems_filesystem_current issues)

[Chris Johns]

Eric Norum wrote:
> 
> For example, here's the task-variable support code in the task context 
> switch (in rtems/src/tasks.c)  that would be removed.
 >

Yes. The same goes for ASR and POSIX signals. I have a solution in mind 
for all these, but that is something for another day.

 >
> Sure it's only a 
> couple of tests and conditional branches if neither task uses task 
> variables, but this is a very heavily-used piece of code......

It is worse, as this code runs as part of another loop of switch 
handlers. My aim is all this overhead goes including the top loop.

> 
> Can you expand a little on how you see RPC being supported?
> 

I have not looked into the RPC code any further than the couple of files 
meantioned here. I do not know the exact tasking structure, but this 
sort of change is a start and I hope shows how things can be made to work.

Change cpukit/librpc/src/rpc/rtems_rpc.c with something like:

static RTEMS_BOOLEAN has_extension;
rtems_id             rpc_extension_id;
int                  rtems_rpc_extension_index;

static rtems_extension
rtems_rpc_delete_task (rtems_tcb* current_task,
                        rtems_tcb* deleted_task)
{
   /* what ever is needed here, something like ... */
   void* rpc_vars =
          rtems_get_tcb_task_ext(deleted_task,
                           rtems_rpc_extension_index);
   if (rpc_vars)
   {
      free (rpc_vars);
   }
}

static rtems_extension
rtems_rpc_exitted_task (rtems_tcb* exitted_task)
{
   /* what ever is needed here, something like ... */
   void* rpc_vars =
          rtems_get_tcb_task_ext(exitted_task,
                           rtems_rpc_extension_index);
   if (rpc_vars)
   {
      free (rpc_vars);
   }
}

int rtems_rpc_task_init (void)
{
   if (!has_extension)
   {
      /* this can be and should be local */
      rtems_extensions_table rpc_extensions;

      rpc_extensions.thread_create  = NULL;
      rpc_extensions.thread_start   = NULL;
      rpc_extensions.thread_restart = NULL;
      rpc_extensions.thread_delete  = rtems_rpc_delete_task;
      rpc_extensions.thread_switch  = NULL;
      rpc_extensions.thread_begin   = NULL;
      rpc_extensions.thread_exitted = rtems_rpc_exitted_task;
      rpc_extensions.fatal          = NULL;

      rtems_extension_create (rtems_build_name ('R', 'P', 'C', 'v'), 

                  &rpc_extensions, &rpc_extension_id);

      rtems_rpc_extension_index =
                rtems_get_index(rpc_extension_id);
      has_extension = 1;
   }
}

Then change cpukit/librpc/include/rpc/rpc.h:

extern int rtems_rpc_extension_index;

#define rtems_rpc_task_variables \
   (rtems_get_self_task_ext(rtems_rpc_extension_index))

which turns into:

_Executing_Thread->extensions[rtems_rpc_extension_index]

The extensions plus index calculations uses a few more cycles than I 
would have liked, but I cannot see a leaner way.

The RTEMS reent structure would have a similar macro.

-- 
  Chris Johns, cjohns at cybertec.com.au