[Bug 1212] New: timeslice not working after task mode change

[rtems-bugs at rtems.org]

http://www.rtems.org/bugzilla/show_bug.cgi?id=1212

           Summary: timeslice not working after task mode change
           Product: RTEMS
           Version: 4.6
          Platform: All
        OS/Version: RTEMS
            Status: NEW
          Severity: major
          Priority: P3
         Component: cpukit
        AssignedTo: joel.sherrill at oarcorp.com
        ReportedBy: xudong.guan at criticalsoftware.com
                CC: xudong.guan at criticalsoftware.com


Time slicing will not happen if task mode is changed from a non-timeslicing
mode to the time slicing mode after it was started (i.e. by itself in its task
body).

An example that reproduces the bug is attached below.

Analysis: The cpu_time_budget field of the victim task is 0 after task creation
and is still 0 after task mode change to TIMESLICE. As a result,
_Thread_Tickle_timeslice() will cause an underflow of cpu_time_budget, which
practically disables timeslicing.

Patch #1 prevents this possible underflow. Patch #2 reset the cpu_time_budget
field after mode change.

Patch #1:
diff --git a/src/rtems/cpukit/score/src/threadtickletimeslice.c
b/src/rtems/cpukit/score/src/threadtickletimeslice.c
index 786f5c5..3d308bc 100644
--- a/src/rtems/cpukit/score/src/threadtickletimeslice.c
+++ b/src/rtems/cpukit/score/src/threadtickletimeslice.c
@@ -72,7 +72,7 @@ void _Thread_Tickle_timeslice( void )

     case THREAD_CPU_BUDGET_ALGORITHM_RESET_TIMESLICE:
     case THREAD_CPU_BUDGET_ALGORITHM_EXHAUST_TIMESLICE:
-      if ( --executing->cpu_time_budget == 0 ) {
+      if ( (int)(--executing->cpu_time_budget) <= 0 ) {
         _Thread_Reset_timeslice();
         executing->cpu_time_budget = _Thread_Ticks_per_timeslice;
       }

Patch #2:
diff --git a/src/rtems/cpukit/rtems/src/taskmode.c
b/src/rtems/cpukit/rtems/src/taskmode.c
index 142812e..77f3a03 100644
--- a/src/rtems/cpukit/rtems/src/taskmode.c
+++ b/src/rtems/cpukit/rtems/src/taskmode.c
@@ -85,9 +85,10 @@ rtems_status_code rtems_task_mode(
     executing->is_preemptible = _Modes_Is_preempt(mode_set) ? TRUE : FALSE;

   if ( mask & RTEMS_TIMESLICE_MASK ) {
-    if ( _Modes_Is_timeslice(mode_set) )
+    if ( _Modes_Is_timeslice(mode_set) ) {
       executing->budget_algorithm =
THREAD_CPU_BUDGET_ALGORITHM_RESET_TIMESLICE;
-    else
+      executing->cpu_time_budget  = _Thread_Ticks_per_timeslice;
+    } else
       executing->budget_algorithm = THREAD_CPU_BUDGET_ALGORITHM_NONE;
   }


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