Support an asynchronous, 'external' clock to get better CPU load measurements

4 files changed, 60 insertions, 5 deletions

diff --git a/nuttx/ChangeLog b/nuttx/ChangeLog
index 400173b4e..d71a05a1e 100644
--- a/nuttx/ChangeLog
+++ b/nuttx/ChangeLog
@@ -6680,3 +6680,7 @@
 	* fs/procfs/fs_procfscpuload.c and fs_procfsproc.c:  Remove the newline
 	  at the end of the percentage so that the returned strings can be
 	  printed on the same line as other values (2014-2-27).
+	* sched/Kconfig, sched_cpuload.c, and nuttx/sched/sched_processtimer.c:
+	  An asynchronous, "external" clock may now be used to drive the CPU
+	  load calculations for more accurate load measurements when needed
+	  (2014-2-27).
diff --git a/nuttx/sched/Kconfig b/nuttx/sched/Kconfig
index a6b661e05..cd6b200e7 100644
--- a/nuttx/sched/Kconfig
+++ b/nuttx/sched/Kconfig
@@ -75,12 +75,51 @@ config SCHED_CPULOAD
 
 if SCHED_CPULOAD
 
+config SCHED_CPULOAD_EXTCLK
+	bool "Use external clock"
+	default n
+	---help---
+		The CPU load measurements are determined by sampling the active
+		tasks periodically at the occurrence to a timer expiration.  By
+		default, the system clock is used to do that sampling.
+
+		There is a serious issue for the accuracy of measurements if the
+		system clock is used, however.  NuttX threads are often started at
+		the time of the system timer expiration.  Others may be stopped at
+		the time of the system timer expiration (if round-robin time-slicing
+		is enabled).  Such thread behavior occurs synchronously with the
+		system timer and, hence, is not randomly sampled.  As a consequence,
+		the CPU load attributed to these threads that run synchronously with
+		they system timer may be grossly in error.
+
+		The solution is to use some other clock that runs at a different
+		rate and has timer expirations that are asynchronous with the
+		system timer.  Then truly accurate load measurements can be
+		achieved.  This option enables use of such an "external" clock.  The
+		implementation of the clock must be provided by platform-specific
+		logic; that platform-specific logic must call the system function
+		sched_process_cpuload() at each timer expiration with interrupts
+		disabled.
+
+config SCHED_CPULOAD_TICKSPERSEC
+	int "External clock rate"
+	default 100
+	depends on SCHED_CPULOAD_EXTCLK
+	---help---
+		If an external clock is used to drive the sampling for the CPU load
+		calculations, then this value must be provided.  This value provides
+		the rate of the external clock in units of ticks per second.  The
+		default value of 100 corresponds to 100Hz clock.  NOTE: that 100Hz
+		is the default frequency of the system time and, hence, the worst
+		possible choice in most cases.
+
 config SCHED_CPULOAD_TIMECONSTANT
 	int "CPU load time constant"
 	default 2
 	---help---
 		The accumulated CPU count is divided by two when the accumulated
-		tick count exceeds this time constant.
+		tick count exceeds this time constant.  This time constant is in
+		units of seconds.
 
 endif # SCHED_CPULOAD
 
diff --git a/nuttx/sched/sched_cpuload.c b/nuttx/sched/sched_cpuload.c
index 0f5495750..95a8b59e2 100644
--- a/nuttx/sched/sched_cpuload.c
+++ b/nuttx/sched/sched_cpuload.c
@@ -52,7 +52,19 @@
 /************************************************************************
  * Pre-processor Definitions
  ************************************************************************/
- 
+/* Are we using the system timer, or an external clock?  Get the rate
+ * of the sampling in ticks per second for the selected timer.
+ */
+
+#ifdef CONFIG_SCHED_CPULOAD_EXTCLK
+#  ifndef CONFIG_SCHED_CPULOAD_TICKSPERSEC
+#    error CONFIG_SCHED_CPULOAD_TICKSPERSEC is not defined
+#  endif
+#  define CPULOAD_TICKSPERSEC CONFIG_SCHED_CPULOAD_TICKSPERSEC
+#else
+#  define CPULOAD_TICKSPERSEC CLOCKS_PER_SEC
+#endif
+
 /************************************************************************
  * Private Type Declarations
  ************************************************************************/
@@ -119,7 +131,7 @@ void weak_function sched_process_cpuload(void)
    * constant, then shift the accumulators.
    */
 
-  if (++g_cpuload_total > (CONFIG_SCHED_CPULOAD_TIMECONSTANT * CLOCKS_PER_SEC))
+  if (++g_cpuload_total > (CONFIG_SCHED_CPULOAD_TIMECONSTANT * CPULOAD_TICKSPERSEC))
     {
       uint32_t total = 0;
 
diff --git a/nuttx/sched/sched_processtimer.c b/nuttx/sched/sched_processtimer.c
index d65112948..6ca5981fb 100644
--- a/nuttx/sched/sched_processtimer.c
+++ b/nuttx/sched/sched_processtimer.c
@@ -182,9 +182,9 @@ static inline void sched_process_timeslice(void)
 
 void sched_process_timer(void)
 {
+#ifndef CONFIG_DISABLE_CLOCK
   /* Increment the system time (if in the link) */
 
-#ifndef CONFIG_DISABLE_CLOCK
 #ifdef CONFIG_HAVE_WEAKFUNCTIONS
   if (clock_timer != NULL)
 #endif
@@ -193,11 +193,11 @@ void sched_process_timer(void)
     }
 #endif
 
+#if defined(CONFIG_SCHED_CPULOAD) && !defined(CONFIG_SCHED_CPULOAD_EXTCLK)
   /* Perform CPU load measurements (before any timer-initiated context switches
    * can occur)
    */
 
-#ifdef CONFIG_SCHED_CPULOAD
 #ifdef CONFIG_HAVE_WEAKFUNCTIONS
   if (sched_process_cpuload != NULL)
 #endif