path: root/cpukit/score/src/threaddispatch.c

/**
 * @file
 * 
 * @brief Dispatch Thread
 * @ingroup ScoreThread
 */

/*
 *  COPYRIGHT (c) 1989-2009.
 *  On-Line Applications Research Corporation (OAR).
 *
 *  The license and distribution terms for this file may be
 *  found in the file LICENSE in this distribution or at
 *  http://www.rtems.com/license/LICENSE.
 */

#if HAVE_CONFIG_H
#include "config.h"
#endif

#include <rtems/score/threaddispatch.h>
#include <rtems/score/apiext.h>
#include <rtems/score/isr.h>
#include <rtems/score/threadimpl.h>
#include <rtems/score/tod.h>
#include <rtems/score/userextimpl.h>
#include <rtems/score/wkspace.h>

void _Thread_Dispatch( void )
{
  Thread_Control   *executing;
  Thread_Control   *heir;
  ISR_Level         level;

  #if defined(RTEMS_SMP)
    /*
     * WARNING: The SMP sequence has severe defects regarding the real-time
     * performance.
     *
     * Consider the following scenario.  We have three tasks L (lowest
     * priority), M (middle priority), and H (highest priority).  Now let a
     * thread dispatch from M to L happen.  An interrupt occurs in
     * _Thread_Dispatch() here:
     *
     * void _Thread_Dispatch( void )
     * {
     *   [...]
     *
     * post_switch:
     *
     *   _ISR_Enable( level );
     *
     *   <-- INTERRUPT
     *   <-- AFTER INTERRUPT
     *
     *   _Thread_Unnest_dispatch();
     *
     *   _API_extensions_Run_post_switch();
     * }
     *
     * The interrupt event makes task H ready.  The interrupt code will see
     * _Thread_Dispatch_disable_level > 0 and thus doesn't perform a
     * _Thread_Dispatch().  Now we return to position "AFTER INTERRUPT".  This
     * means task L executes now although task H is ready!  Task H will execute
     * once someone calls _Thread_Dispatch().
     */
    _Thread_Disable_dispatch();

    /*
     *  If necessary, send dispatch request to other cores.
     */
    _SMP_Request_other_cores_to_dispatch();
  #else
    _Thread_Dispatch_set_disable_level( 1 );
  #endif

  /*
   *  Now determine if we need to perform a dispatch on the current CPU.
   */
  executing   = _Thread_Executing;
  _ISR_Disable( level );
  while ( _Thread_Dispatch_necessary == true ) {
    heir = _Thread_Heir;
    #if defined(RTEMS_SMP)
      executing->is_executing = false;
      heir->is_executing = true;
    #endif
    _Thread_Dispatch_necessary = false;
    _Thread_Executing = heir;

    /*
     *  When the heir and executing are the same, then we are being
     *  requested to do the post switch dispatching.  This is normally
     *  done to dispatch signals.
     */
    if ( heir == executing )
      goto post_switch;

    /*
     *  Since heir and executing are not the same, we need to do a real
     *  context switch.
     */
#if __RTEMS_ADA__
    executing->rtems_ada_self = rtems_ada_self;
    rtems_ada_self = heir->rtems_ada_self;
#endif
    if ( heir->budget_algorithm == THREAD_CPU_BUDGET_ALGORITHM_RESET_TIMESLICE )
      heir->cpu_time_budget = _Thread_Ticks_per_timeslice;

    _ISR_Enable( level );

    #ifndef __RTEMS_USE_TICKS_FOR_STATISTICS__
      {
        Timestamp_Control uptime, ran;
        _TOD_Get_uptime( &uptime );
        _Timestamp_Subtract(
          &_Thread_Time_of_last_context_switch,
          &uptime,
          &ran
        );
        _Timestamp_Add_to( &executing->cpu_time_used, &ran );
        _Thread_Time_of_last_context_switch = uptime;
      }
    #else
      {
        _TOD_Get_uptime( &_Thread_Time_of_last_context_switch );
        heir->cpu_time_used++;
      }
    #endif

#if !defined(__DYNAMIC_REENT__)
    /*
     * Switch libc's task specific data.
     */
    if ( _Thread_libc_reent ) {
      executing->libc_reent = *_Thread_libc_reent;
      *_Thread_libc_reent = heir->libc_reent;
    }
#endif

    _User_extensions_Thread_switch( executing, heir );

    /*
     *  If the CPU has hardware floating point, then we must address saving
     *  and restoring it as part of the context switch.
     *
     *  The second conditional compilation section selects the algorithm used
     *  to context switch between floating point tasks.  The deferred algorithm
     *  can be significantly better in a system with few floating point tasks
     *  because it reduces the total number of save and restore FP context
     *  operations.  However, this algorithm can not be used on all CPUs due
     *  to unpredictable use of FP registers by some compilers for integer
     *  operations.
     */

#if ( CPU_HARDWARE_FP == TRUE ) || ( CPU_SOFTWARE_FP == TRUE )
#if ( CPU_USE_DEFERRED_FP_SWITCH != TRUE )
    if ( executing->fp_context != NULL )
      _Context_Save_fp( &executing->fp_context );
#endif
#endif

    _Context_Switch( &executing->Registers, &heir->Registers );

#if ( CPU_HARDWARE_FP == TRUE ) || ( CPU_SOFTWARE_FP == TRUE )
#if ( CPU_USE_DEFERRED_FP_SWITCH == TRUE )
    if ( (executing->fp_context != NULL) &&
         !_Thread_Is_allocated_fp( executing ) ) {
      if ( _Thread_Allocated_fp != NULL )
        _Context_Save_fp( &_Thread_Allocated_fp->fp_context );
      _Context_Restore_fp( &executing->fp_context );
      _Thread_Allocated_fp = executing;
    }
#else
    if ( executing->fp_context != NULL )
      _Context_Restore_fp( &executing->fp_context );
#endif
#endif

    executing = _Thread_Executing;

    _ISR_Disable( level );
  }

post_switch:
  #ifndef RTEMS_SMP
    _Thread_Dispatch_set_disable_level( 0 );
  #endif

  _ISR_Enable( level );

  #ifdef RTEMS_SMP
    _Thread_Unnest_dispatch();
  #endif

  _API_extensions_Run_post_switch( executing );
}