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/**
* @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 );
}
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