本文共 2436 字，大约阅读时间需要 8 分钟。

在kernel中会注册一个软件中断来做负载均衡__init void init_sched_fair_class(void){#ifdef CONFIG_SMP	open_softirq(SCHED_SOFTIRQ, run_rebalance_domains);endif /* SMP */}这个软件中断SCHED_SOFTIRQ 对应的回调函数是run_rebalance_domainsstatic __latent_entropy void run_rebalance_domains(struct softirq_action *h){	struct rq *this_rq = this_rq();	#判断当前rg是否是idle	enum cpu_idle_type idle = this_rq->idle_balance ?						CPU_IDLE : CPU_NOT_IDLE;	#开始负载均衡，从这里可以知道负载均衡是在一个domain 内的cpu来做的	rebalance_domains(this_rq, idle);}static void rebalance_domains(struct rq *rq, enum cpu_idle_type idle){		#负载均衡的周期，这里很根据当前rq是否idle 会对应不同的时间，也就是说	#负载均衡的周期有两个值	interval = get_sd_balance_interval(sd, idle != CPU_IDLE);	#遍历domain中的每个cpu	for_each_domain(cpu, sd) {			if (time_after_eq(jiffies, sd->last_balance + interval)) {		#周期到后开始做负载均衡			if (load_balance(cpu, rq, sd, idle, &continue_balancing)) {				/*				 * The LBF_DST_PINNED logic could have changed				 * env->dst_cpu, so we can't know our idle				 * state even if we migrated tasks. Update it.				 */				idle = idle_cpu(cpu) ? CPU_IDLE : CPU_NOT_IDLE;			}			sd->last_balance = jiffies;			interval = get_sd_balance_interval(sd, idle != CPU_IDLE);		}}这里首先看看负载均衡的两个周期是如何计算的static inline unsigned longget_sd_balance_interval(struct sched_domain *sd, int cpu_busy){	unsigned long interval = sd->balance_interval;	#关键是这一步，如果cpu busy的话，则做负载均衡的时间会变长	if (cpu_busy)		interval *= sd->busy_factor;	/* scale ms to jiffies */	interval = msecs_to_jiffies(interval);	#负载均衡的最大值不能超过1UL和max_load_balance_interval 之间，也就是负载均衡的最大周期不能超过max_load_balance_interval	interval = clamp(interval, 1UL, max_load_balance_interval);	return interval;}搞清楚负载均衡的周期后，开始看看处理负载均衡的函数load_balancestatic int load_balance(int this_cpu, struct rq *this_rq,			struct sched_domain *sd, enum cpu_idle_type idle,			int *continue_balancing){		#找到domain中最忙的group	group = find_busiest_group(&env);	if (!group) {		schedstat_inc(sd->lb_nobusyg[idle]);		goto out_balanced;	}	#找到最忙group中最忙的rq	busiest = find_busiest_queue(&env, group);	if (!busiest) {		schedstat_inc(sd->lb_nobusyq[idle]);		goto out_balanced;	}	BUG_ON(busiest == env.dst_rq);	schedstat_add(sd->lb_imbalance[idle], env.imbalance);	env.src_cpu = busiest->cpu;	env.src_rq = busiest;	ld_moved = 0;	#如果确实存在最忙的rq	if (busiest->nr_running > 1) {		 #将task从最忙cpu中的rq迁移出来		cur_ld_moved = detach_tasks(&env);		rq_unlock(busiest, &rf);		if (cur_ld_moved) {		#将前面从最忙cpu rq中迁出的task 加入到目标cpu上			attach_tasks(&env);			ld_moved += cur_ld_moved;		}}

转载地址：http://hjnmi.baihongyu.com/