在runnable_avg_period + 1中，我为什么要加1？

Question

内核/排程/ fair.c

static inline void __update_task_entity_contrib(struct sched_entity *se)
{
u32 contrib;

/* avoid overflowing a 32-bit type w/ SCHED_LOAD_SCALE */
contrib = se->avg.runnable_avg_sum * scale_load_down(se->load.weight);
contrib /= (se->avg.runnable_avg_period + 1);
se->avg.load_avg_contrib = scale_load(contrib);
trace_sched_task_load_contrib(task_of(se), se->avg.load_avg_contrib);
contrib = se->avg.runnable_avg_sum * scale_load_down(NICE_0_LOAD);
contrib /= (se->avg.runnable_avg_period + 1);
.....

此代码计算以下等式。

                        runnable_avg_sum
load_avg_contrib =  -----------------------------  * weight
                      runnable_avg_period + 1

和

                        runnable_avg_sum
load_avg_ratio =  -----------------------------  * NICE_0_LOAD
                      runnable_avg_period + 1

我们为什么要在这个等式中使用runnable_avg_period + 1？ 为什么我们不能使用runnable_avg_period？