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, 16 insertions, 0 deletions

diff --git a/cpukit/score/cpu/arm/rtems/score/cpu.h b/cpukit/score/cpu/arm/rtems/score/cpu.h
index cb9dc7c409..dc57a78a67 100644
--- a/cpukit/score/cpu/arm/rtems/score/cpu.h
+++ b/cpukit/score/cpu/arm/rtems/score/cpu.h
@@ -216,6 +216,14 @@
   #define ARM_CONTEXT_CONTROL_D8_OFFSET 48
 #endif
 
+#ifdef RTEMS_SMP
+  #ifdef ARM_MULTILIB_VFP_D32
+    #define ARM_CONTEXT_CONTROL_IS_EXECUTING_OFFSET 112
+  #else
+    #define ARM_CONTEXT_CONTROL_IS_EXECUTING_OFFSET 48
+  #endif
+#endif
+
 #define ARM_EXCEPTION_FRAME_SIZE 76
 
 #define ARM_EXCEPTION_FRAME_REGISTER_SP_OFFSET 52
@@ -280,6 +288,9 @@ typedef struct {
   uint64_t register_d14;
   uint64_t register_d15;
 #endif
+#ifdef RTEMS_SMP
+  volatile bool is_executing;
+#endif
 } Context_Control;
 
 typedef struct {
@@ -410,6 +421,11 @@ void _CPU_Context_Initialize(
 #define _CPU_Context_Get_SP( _context ) \
   (_context)->register_sp
 
+#ifdef RTEMS_SMP
+  #define _CPU_Context_Get_is_executing( _context ) \
+    (_context)->is_executing
+#endif
+
 #define _CPU_Context_Restart_self( _the_context ) \
    _CPU_Context_restore( (_the_context) );