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, 65 insertions, 3 deletions

diff --git a/cpukit/score/include/rtems/score/threadimpl.h b/cpukit/score/include/rtems/score/threadimpl.h
index 4efc85d8f1..2be5cc56fa 100644
--- a/cpukit/score/include/rtems/score/threadimpl.h
+++ b/cpukit/score/include/rtems/score/threadimpl.h
@@ -454,6 +454,22 @@ RTEMS_INLINE_ROUTINE bool _Thread_Is_executing (
   return ( the_thread == _Thread_Executing );
 }
 
+#if defined(RTEMS_SMP)
+/**
+ * @brief Returns @true in case the thread executes currently on some processor
+ * in the system, otherwise @a false.
+ *
+ * Do not confuse this with _Thread_Is_executing() which checks only the
+ * current processor.
+ */
+RTEMS_INLINE_ROUTINE bool _Thread_Is_executing_on_a_processor(
+  const Thread_Control *the_thread
+)
+{
+  return _CPU_Context_Get_is_executing( &the_thread->Registers );
+}
+#endif
+
 /**
  * This function returns true if the_thread is the heir
  * thread, and false otherwise.
@@ -491,7 +507,7 @@ RTEMS_INLINE_ROUTINE void _Thread_Restart_self( Thread_Control *executing )
 
   _Giant_Release();
 
-  _Per_CPU_ISR_disable_and_acquire( _Per_CPU_Get(), level );
+  _ISR_Disable_without_giant( level );
   ( void ) level;
 #endif
 
@@ -590,7 +606,7 @@ RTEMS_INLINE_ROUTINE void _Thread_Request_dispatch_if_executing(
 )
 {
 #if defined(RTEMS_SMP)
-  if ( thread->is_executing ) {
+  if ( _Thread_Is_executing_on_a_processor( thread ) ) {
     const Per_CPU_Control *cpu_of_executing = _Per_CPU_Get();
     Per_CPU_Control *cpu_of_thread = _Thread_Get_CPU( thread );
 
@@ -611,7 +627,7 @@ RTEMS_INLINE_ROUTINE void _Thread_Signal_notification( Thread_Control *thread )
     _Thread_Dispatch_necessary = true;
   } else {
 #if defined(RTEMS_SMP)
-    if ( thread->is_executing ) {
+    if ( _Thread_Is_executing_on_a_processor( thread ) ) {
       const Per_CPU_Control *cpu_of_executing = _Per_CPU_Get();
       Per_CPU_Control *cpu_of_thread = _Thread_Get_CPU( thread );
 
@@ -624,6 +640,39 @@ RTEMS_INLINE_ROUTINE void _Thread_Signal_notification( Thread_Control *thread )
   }
 }
 
+/**
+ * @brief Gets the heir of the processor and makes it executing.
+ *
+ * The thread dispatch necessary indicator is cleared as a side-effect.
+ *
+ * @return The heir thread.
+ *
+ * @see _Thread_Dispatch(), _Thread_Start_multitasking() and
+ * _Scheduler_SMP_Update_heir().
+ */
+RTEMS_INLINE_ROUTINE Thread_Control *_Thread_Get_heir_and_make_it_executing(
+  Per_CPU_Control *cpu_self
+)
+{
+  Thread_Control *heir;
+
+  cpu_self->dispatch_necessary = false;
+
+#if defined( RTEMS_SMP )
+  /*
+   * It is critical that we first update the dispatch necessary and then the
+   * read the heir so that we don't miss an update by
+   * _Scheduler_SMP_Update_heir().
+   */
+  _Atomic_Fence( ATOMIC_ORDER_SEQ_CST );
+#endif
+
+  heir = cpu_self->heir;
+  cpu_self->executing = heir;
+
+  return heir;
+}
+
 RTEMS_INLINE_ROUTINE void _Thread_Update_cpu_time_used(
   Thread_Control *executing,
   Timestamp_Control *time_of_last_context_switch
@@ -736,6 +785,19 @@ RTEMS_INLINE_ROUTINE bool _Thread_Is_life_changing(
   return ( life_state & THREAD_LIFE_RESTARTING_TERMINTING ) != 0;
 }
 
+RTEMS_INLINE_ROUTINE void _Thread_Debug_set_real_processor(
+  Thread_Control  *the_thread,
+  Per_CPU_Control *cpu
+)
+{
+#if defined(RTEMS_SMP) && defined(RTEMS_DEBUG)
+  the_thread->debug_real_cpu = cpu;
+#else
+  (void) the_thread;
+  (void) cpu;
+#endif
+}
+
 #if !defined(__DYNAMIC_REENT__)
 /**
  * This routine returns the C library re-enterant pointer.