add CPU usage example code.

git-svn-id: https://rt-thread.googlecode.com/svn/trunk@1282 bbd45198-f89e-11dd-88c7-29a3b14d5316

examples/kernel/cpuusage.c

+#include <rtthread.h>
+#include <rthw.h>
+
+#define CPU_USAGE_CALC_TICK	10
+#define CPU_USAGE_LOOP		100
+
+static rt_uint8_t  cpu_usage_major = 0, cpu_usage_minor= 0;
+static rt_uint32_t total_count = 0;
+
+static void cpu_usage_idle_hook()
+{
+	rt_tick_t tick;
+	rt_uint32_t count;
+	volatile rt_uint32_t loop;
+
+	if (total_count == 0)
+	{
+		loop = 0;
+
+		/* get total count */
+		rt_enter_critical();
+		tick = rt_tick_get();
+		while(rt_tick_get() - tick < CPU_USAGE_CALC_TICK)
+		{
+			total_count ++;
+			while (loop < CPU_USAGE_LOOP) loop ++;
+		}
+		rt_exit_critical();
+	}
+
+	count = 0;
+	loop  = 0;
+	/* get CPU usage */
+	tick = rt_tick_get();
+	while (rt_tick_get() - tick < CPU_USAGE_CALC_TICK)
+	{
+		count ++;
+		while (loop < CPU_USAGE_LOOP) loop ++;
+	}
+
+	/* calculate major and minor */
+	if (count < total_count)
+	{
+		count = total_count - count;
+		cpu_usage_major = (count * 100) / total_count;
+		cpu_usage_minor = ((count * 100) % total_count) * 100 / total_count;
+	}
+	else
+	{
+		total_count = count;
+
+		/* no CPU usage */
+		cpu_usage_major = 0;
+		cpu_usage_minor = 0;
+	}
+}
+
+void cpu_usage_get(rt_uint8_t *major, rt_uint8_t *minor)
+{
+	RT_ASSERT(major != RT_NULL);
+	RT_ASSERT(minor != RT_NULL);
+
+	*major = cpu_usage_major;
+	*minor = cpu_usage_minor;
+}
+
+void cpu_usage_init()
+{
+	/* set idle thread hook */
+	rt_thread_idle_sethook(cpu_usage_idle_hook);
+}