score: Implement forced thread migration

The current implementation of task migration in RTEMS has some
implications with respect to the interrupt latency. It is crucial to
preserve the system invariant that a task can execute on at most one
processor in the system at a time. This is accomplished with a boolean
indicator in the task context. The processor architecture specific
low-level task context switch code will mark that a task context is no
longer executing and waits that the heir context stopped execution
before it restores the heir context and resumes execution of the heir
task. So there is one point in time in which a processor is without a
task. This is essential to avoid cyclic dependencies in case multiple
tasks migrate at once. Otherwise some supervising entity is necessary to
prevent life-locks. Such a global supervisor would lead to scalability
problems so this approach is not used. Currently the thread dispatch is
performed with interrupts disabled. So in case the heir task is
currently executing on another processor then this prolongs the time of
disabled interrupts since one processor has to wait for another
processor to make progress.

It is difficult to avoid this issue with the interrupt latency since
interrupts normally store the context of the interrupted task on its
stack. In case a task is marked as not executing we must not use its
task stack to store such an interrupt context. We cannot use the heir
stack before it stopped execution on another processor. So if we enable
interrupts during this transition we have to provide an alternative task
independent stack for this time frame. This issue needs further
investigation.

1 files changed, 11 insertions, 14 deletions

diff --git a/cpukit/score/include/rtems/score/thread.h b/cpukit/score/include/rtems/score/thread.h
index 90df3a74dc..248ae96850 100644
--- a/cpukit/score/include/rtems/score/thread.h
+++ b/cpukit/score/include/rtems/score/thread.h
@@ -504,20 +504,6 @@ struct Thread_Control_struct {
   bool                                  is_in_the_air;
 
   /**
-   * @brief This field is true if the thread is executing.
-   *
-   * A thread is executing if it executes on a processor.  An executing thread
-   * executes on exactly one processor.  There are exactly processor count
-   * executing threads in the system.  An executing thread may have a heir
-   * thread and thread dispatching is necessary.  On SMP a thread dispatch on a
-   * remote processor needs help from an inter-processor interrupt, thus it
-   * will take some time to complete the state change.  A lot of things can
-   * happen in the meantime.  This field is volatile since it is polled in
-   * _Thread_Kill_zombies().
-   */
-  volatile bool                         is_executing;
-
-  /**
    * @brief The scheduler of this thread.
    */
   const struct Scheduler_Control       *scheduler;
@@ -548,7 +534,18 @@ struct Thread_Control_struct {
   void                                 *scheduler_info;
 
 #ifdef RTEMS_SMP
+  /**
+   * @brief The processor assigned by the scheduler.
+   */
   Per_CPU_Control                      *cpu;
+
+#ifdef RTEMS_DEBUG
+  /**
+   * @brief The processor on which this thread executed the last time or is
+   * executing.
+   */
+  Per_CPU_Control                      *debug_real_cpu;
+#endif
 #endif
 
   /** This field contains information about the starting state of