diff --git a/arch/sim/src/up_critmon.c b/arch/sim/src/up_critmon.c
index b9ddbfca643..10ae1ac244f 100644
--- a/arch/sim/src/up_critmon.c
+++ b/arch/sim/src/up_critmon.c
@@ -44,6 +44,9 @@
  * Pre-processor Definitions
  ************************************************************************************/
 
+#undef USE_CLOCK                                   /* Too slow */
+#define USE_CLOCK_GETTIME 1                        /* Better */
+
 /* From nuttx/clock.h */
  
 #define NSEC_PER_SEC  1000000000
@@ -63,6 +66,10 @@
 
 typedef int64_t b32_t;
 
+/************************************************************************************
+ * Private Data
+ ************************************************************************************/
+
 /************************************************************************************
  * Public Functions
  ************************************************************************************/
@@ -71,15 +78,25 @@ typedef int64_t b32_t;
  * Name: up_critmon_gettime
  ************************************************************************************/
 
+#if defined(USE_CLOCK)
 uint32_t up_critmon_gettime(void)
 {
   return (uint32_t)clock() + 1;  /* Avoid returning zero which means clock-not-ready */
 }
+#else /* USE_CLOCK_GETTIME */
+uint32_t up_critmon_gettime(void)
+{
+  struct timespec ts;
+  (void)clock_gettime(CLOCK_MONOTONIC, &ts);
+  return (uint32_t)ts.tv_nsec;
+}
+#endif
 
 /************************************************************************************
  * Name: up_critmon_gettime
  ************************************************************************************/
 
+#if defined(USE_CLOCK)
 void up_critmon_convert(uint32_t elapsed, struct timespec *ts)
 {
   b32_t b32elapsed;
@@ -88,3 +105,10 @@ void up_critmon_convert(uint32_t elapsed, struct timespec *ts)
   ts->tv_sec  = b32toi(b32elapsed);
   ts->tv_nsec = NSEC_PER_SEC * b32frac(b32elapsed) / b32ONE;
 }
+#else /* USE_CLOCK_GETTIME */
+void up_critmon_convert(uint32_t elapsed, struct timespec *ts)
+{
+  ts->tv_sec  = 0;
+  ts->tv_nsec = elapsed;
+}
+#endif
diff --git a/sched/sched/sched_critmonitor.c b/sched/sched/sched_critmonitor.c
index 284689d3d11..0dc5629587e 100644
--- a/sched/sched/sched_critmonitor.c
+++ b/sched/sched/sched_critmonitor.c
@@ -121,7 +121,7 @@ void sched_critmon_preemption(FAR struct tcb_s *tcb, bool state)
 
       /* Zero means that the timer is not ready */
 
-      if (tcb->premp_start != 0)
+      if (tcb->premp_start != 0 && g_premp_start[cpu] == 0)
         {
           /* Save the global start time */
 
@@ -145,12 +145,15 @@ void sched_critmon_preemption(FAR struct tcb_s *tcb, bool state)
 
       /* Check for the global max elapsed time */
 
-      elapsed            = now - g_premp_start[cpu];
-      g_premp_start[cpu] = 0;
-
-      if (elapsed > g_premp_max[cpu])
+      if (g_premp_start[cpu] != 0)
         {
-          g_premp_max[cpu] = elapsed;
+          elapsed            = now - g_premp_start[cpu];
+          g_premp_start[cpu] = 0;
+
+          if (elapsed > g_premp_max[cpu])
+            {
+              g_premp_max[cpu] = elapsed;
+            }
         }
     }
 }
@@ -181,7 +184,7 @@ void sched_critmon_csection(FAR struct tcb_s *tcb, bool state)
 
       /* Zero means that the timer is not ready */
 
-      if (tcb->crit_start != 0)
+      if (tcb->crit_start != 0 && g_crit_start[cpu] == 0)
         {
           /* Set the global start time */
 
@@ -205,12 +208,15 @@ void sched_critmon_csection(FAR struct tcb_s *tcb, bool state)
 
       /* Check for the global max elapsed time */
 
-      elapsed           = now - g_crit_start[cpu];
-      g_crit_start[cpu] = 0;
-
-      if (elapsed > g_crit_max[cpu])
+      if (g_crit_start[cpu] != 0)
         {
-          g_crit_max[cpu] = elapsed;
+          elapsed           = now - g_crit_start[cpu];
+          g_crit_start[cpu] = 0;
+
+          if (elapsed > g_crit_max[cpu])
+            {
+              g_crit_max[cpu] = elapsed;
+            }
         }
     }
 }
@@ -243,6 +249,8 @@ void sched_critmon_resume(FAR struct tcb_s *tcb)
       tcb->premp_start = up_critmon_gettime();
       DEBUGASSERT(tcb->premp_start != 0);
 
+      /* Zero means that the timer is not ready */
+
       if (g_premp_start[cpu] == 0)
         {
           g_premp_start[cpu] = tcb->premp_start;
@@ -294,7 +302,7 @@ void sched_critmon_resume(FAR struct tcb_s *tcb)
  *
  * Description:
  *   Called when a thread suspends execution, perhaps terminating a
- *   critical section or a non-pre-emptible state.
+ *   critical section or a non-preemptible state.
  *
  * Assumptions:
  *   Called within a critical section.
@@ -305,7 +313,7 @@ void sched_critmon_suspend(FAR struct tcb_s *tcb)
 {
   uint32_t elapsed;
 
-  /* Did this task disable pre-emption? */
+  /* Did this task disable preemption? */
 
   if (tcb->lockcount > 0)
     {
@@ -336,4 +344,4 @@ void sched_critmon_suspend(FAR struct tcb_s *tcb)
     }
 }
 
-#endif
\ No newline at end of file
+#endif