Instead of keeping track of the fraction of time that we're

idle, keep track of the fraction of time we're not idle.  This
works better because the default processor state is idle, and
the default stat value is 0.
Keep the stat as idleFraction which is a formula that is equal
to 1 - notIdleFraction

--HG--
extra : convert_revision : 331c2e46f45ae0abda46988567ac2c4f7c42ccad

cpu/simple_cpu/simple_cpu.cc

@@ -247,7 +247,7 @@ SimpleCPU::setStatus(Status new_status)
 
       case Idle:
         assert(old_status == Running);
-        idleFraction++;
+        notIdleFraction--;
         if (tickEvent.scheduled())
             tickEvent.squash();
         break;
@@ -256,8 +256,8 @@ SimpleCPU::setStatus(Status new_status)
         assert(old_status == Idle ||
                old_status == DcacheMissStall ||
                old_status == IcacheMissComplete);
-        if (old_status == Idle && curTick != 0)
-            idleFraction--;
+        if (old_status == Idle)
+            notIdleFraction++;
 
         if (tickEvent.squashed())
             tickEvent.reschedule(curTick + 1);
@@ -304,6 +304,7 @@ SimpleCPU::regStats()
         .prereq(dcacheStallCycles)
         ;
 
+    idleFraction = constant(1.0) - notIdleFraction;
     numInsts = Statistics::scalar(numInst) - Statistics::scalar(startNumInst);
     simInsts += numInsts;
 }
@@ -312,6 +313,7 @@ void
 SimpleCPU::resetStats()
 {
     startNumInst = numInst;
+    notIdleFraction = (_status != Idle);
 }
 
 void

cpu/simple_cpu/simple_cpu.hh

@@ -193,7 +193,8 @@ class SimpleCPU : public BaseCPU
     Counter startNumLoad;
 
     // number of idle cycles
-    Statistics::Average<> idleFraction;
+    Statistics::Average<> notIdleFraction;
+    Statistics::Formula idleFraction;
 
     // number of cycles stalled for I-cache misses
     Statistics::Scalar<> icacheStallCycles;