fix lost context switches in GHCi (fixes test 3429(ghci))

diff --git a/rts/Schedule.c b/rts/Schedule.c
index c61bbc7..66af8be 100644 (file)
--- a/rts/Schedule.c
+++ b/rts/Schedule.c
@@ -1188,36 +1188,32 @@ scheduleHandleStackOverflow (Capability *cap, Task *task, StgTSO *t)
 static rtsBool
 scheduleHandleYield( Capability *cap, StgTSO *t, nat prev_what_next )
 {
-    // Reset the context switch flag.  We don't do this just before
-    // running the thread, because that would mean we would lose ticks
-    // during GC, which can lead to unfair scheduling (a thread hogs
-    // the CPU because the tick always arrives during GC).  This way
-    // penalises threads that do a lot of allocation, but that seems
-    // better than the alternative.
-    cap->context_switch = 0;
-    
     /* put the thread back on the run queue.  Then, if we're ready to
      * GC, check whether this is the last task to stop.  If so, wake
      * up the GC thread.  getThread will block during a GC until the
      * GC is finished.
      */
-#ifdef DEBUG
-    if (t->what_next != prev_what_next) {
-       debugTrace(DEBUG_sched,
-                  "--<< thread %ld (%s) stopped to switch evaluators", 
-                  (long)t->id, what_next_strs[t->what_next]);
-    }
-#endif
-    
+
     ASSERT(t->_link == END_TSO_QUEUE);
     
     // Shortcut if we're just switching evaluators: don't bother
     // doing stack squeezing (which can be expensive), just run the
     // thread.
-    if (t->what_next != prev_what_next) {
+    if (cap->context_switch == 0 && t->what_next != prev_what_next) {
+       debugTrace(DEBUG_sched,
+                  "--<< thread %ld (%s) stopped to switch evaluators", 
+                  (long)t->id, what_next_strs[t->what_next]);
        return rtsTrue;
     }
 
+    // Reset the context switch flag.  We don't do this just before
+    // running the thread, because that would mean we would lose ticks
+    // during GC, which can lead to unfair scheduling (a thread hogs
+    // the CPU because the tick always arrives during GC).  This way
+    // penalises threads that do a lot of allocation, but that seems
+    // better than the alternative.
+    cap->context_switch = 0;
+    
     IF_DEBUG(sanity,
             //debugBelch("&& Doing sanity check on yielding TSO %ld.", t->id);
             checkTSO(t));