【内核调度、负载计算】【calc_global_load_tick】

最新推荐文章于 2022-11-15 17:23:20 发布

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本文深入探讨了Linux系统中负载计算的实现机制，详细解析了calc_global_load_tick、calc_load_fold_active及calc_global_load等关键函数的工作原理，揭示了多CPU环境下系统负载统计的细节。

calc_global_load_tick是计算整个系统的负载，这个函数在scheduler_tick中被周期性的调用

calc_global_load_tick

/*
 * Called from scheduler_tick() to periodically update this CPU's
 * active count.
 */
void calc_global_load_tick(struct rq *this_rq)
{
	long delta;

	if (time_before(jiffies, this_rq->calc_load_update))
		return;

	delta  = calc_load_fold_active(this_rq, 0);
	/* 	全局变量：calc_load_tasks
	是整系统多个cpu(nr_running+nr_uninterruptible)任务数量的总和，
	多cpu在访问calc_load_tasks变量时使用原子操作来互斥*/
	if (delta)
		atomic_long_add(delta, &calc_load_tasks);

	this_rq->calc_load_update += LOAD_FREQ;
}

calc_load_fold_active

long calc_load_fold_active(struct rq *this_rq, long adjust)
{
	long nr_active, delta = 0;

	nr_active = this_rq->nr_running - adjust;
	nr_active += (long)this_rq->nr_uninterruptible;

	if (nr_active != this_rq->calc_load_active) {
		delta = nr_active - this_rq->calc_load_active;
		this_rq->calc_load_active = nr_active;
	}

	return delta;
}

多个cpu更新calc_load_tasks，但是计算load只由一个cpu来完成，这个cpu就是tick_do_timer_cpu，同时tick_do_timer_cpu也会调用do_timer() -> calc_global_load()来计算系统负载

calc_global_load

/*
 * calc_load - update the avenrun load estimates 10 ticks after the
 * CPUs have updated calc_load_tasks.
 *
 * Called from the global timer code.
 */
void calc_global_load(unsigned long ticks)
{
	unsigned long sample_window;
	long active, delta;

	sample_window = READ_ONCE(calc_load_update);
	/* (1) 计算的间隔时间为5s + 10tick，
        加10tick的目的就是让所有cpu都更新完calc_load_tasks，
        tick_do_timer_cpu再来计算
     */
	if (time_before(jiffies, sample_window + 10))
		return;

	/*
	 * Fold the 'old' NO_HZ-delta to include all NO_HZ cpus.
	 */
	delta = calc_load_nohz_fold();
	if (delta)
		atomic_long_add(delta, &calc_load_tasks);

	/* (2) 读取全局统计变量 */
	active = atomic_long_read(&calc_load_tasks);
	active = active > 0 ? active * FIXED_1 : 0;

	/* (3) 计算1分钟、5分钟、15分钟的负载 */
	avenrun[0] = calc_load(avenrun[0], EXP_1, active);
	avenrun[1] = calc_load(avenrun[1], EXP_5, active);
	avenrun[2] = calc_load(avenrun[2], EXP_15, active);

	WRITE_ONCE(calc_load_update, sample_window + LOAD_FREQ);

	/*
	 * In case we went to NO_HZ for multiple LOAD_FREQ intervals
	 * catch up in bulk.
	 */
	calc_global_nohz();
}