man uptime
DESCRIPTION
...
System load averages is the average number of processes that are either in a runnable or uninterruptable  state.   A  process  in  a
runnable  state  is  either  using  the  CPU  or waiting to use the CPU.  A process in uninterruptable state is waiting for some I/O
access, eg waiting for disk.  The averages are taken over the three time intervals.  Load averages are not normalized for the number
of CPUs in a system, so a load average of 1 means a single CPU system is loaded all the time while on a 4 CPU system it means it was
idle 75% of the time. 翻译：

#define FSHIFT   11		/* nr of bits of precision */
#define FIXED_1  (1<<FSHIFT)	/* 1.0 as fixed-point */
#define LOAD_FREQ (5*HZ)	/* 5 sec intervals */
#define EXP_1  1884		/* 1/exp(5sec/1min) as fixed-point */
#define EXP_5  2014		/* 1/exp(5sec/5min) */
#define EXP_15 2037		/* 1/exp(5sec/15min) */

#define CALC_LOAD(load,exp,n) \
   load *= exp; \
   load += n*(FIXED_1-exp); \
   load >>= FSHIFT;

unsigned long avenrun[3];

static inline void calc_load(unsigned long ticks)
{
   unsigned long active_tasks; /* fixed-point */
   static int count = LOAD_FREQ;

   count -= ticks;
   if (count < 0) {
      count += LOAD_FREQ;
      active_tasks = count_active_tasks();
      CALC_LOAD(avenrun[0], EXP_1, active_tasks);
      CALC_LOAD(avenrun[1], EXP_5, active_tasks);
      CALC_LOAD(avenrun[2], EXP_15, active_tasks);
   }
}

1 HZ = 100 ticks
5 HZ = 500 ticks
1 tick = 10 milliseconds
500 ticks = 5000 milliseconds (or 5 seconds)
So, 5 HZ means that CALC_LOAD is called every 5 seconds.

read_lock (&tasklist_lock);
for_each_task(p) {
    if ( (p->state == TASK_RUNNING ||
            (p->state & TASK_UNINTERRUPTIBLE ) ) )
    nr += FIXED_1;
    }
read_unlock(&tasklist_lock);
return nr; }