软中断介绍
一、 软中断产生的原因
软中断:用于有效的实现内核的延期操作,也是底半部机制tasklet的基础
二、 数据结构
1)softirq_action
该结构是软中断的核心数据结构,代表软中断处理函数。
struct softirq_action
{
void (*action)(struct softirq_action *);
};
2)下面是软中断的类型,目前支持10个软中断类型。
/* PLEASE, avoid to allocate new softirqs, if you need not _really_ high
frequency threaded job scheduling. For almost all the purposes
tasklets are more than enough. F.e. all serial device BHs et
al. should be converted to tasklets, not to softirqs.
*/
enum
{
HI_SOFTIRQ=0,
TIMER_SOFTIRQ,
NET_TX_SOFTIRQ,
NET_RX_SOFTIRQ,
BLOCK_SOFTIRQ,
BLOCK_IOPOLL_SOFTIRQ,
TASKLET_SOFTIRQ,
SCHED_SOFTIRQ,
HRTIMER_SOFTIRQ,
RCU_SOFTIRQ, /* Preferable RCU should always be the last softirq */
NR_SOFTIRQS
};
3)softirq_vec
该数组存储了已注册的软中断类型对应的处理函数,该数组非常重要
static struct softirq_action softirq_vec[NR_SOFTIRQS] __cacheline_aligned_in_smp;
4)irq_cpustat_t
该结构体主要记录哪一个软中断需要进行处理
typedef struct {
unsigned int __softirq_pending;
} ____cacheline_aligned irq_cpustat_t;
三、 软中断的注册
要在内核中使用一个软中断,首先要进行注册,所谓注册,其实就是在数组softirq_vec中增加相应软中断的处理函数,调用函数定义如下:
/*
注册一个软中断,即在软中断向量数据中,增加软中断number为nr的处理函数
*/
void open_softirq(int nr, void (*action)(struct softirq_action *))
{
softirq_vec[nr].action = action;
}
四、 启动软中断
如果要启动某个软中断,则需要置位irq_stat[cpu].__softirq_pending中的相应位,而后续的处理工作则由do_softirq处理。
而设置__softirq_pending的操作则由函数__raise_softirq_irqoff来实现。
可以通过以下两种方法启动软中断:
1)在中断上下文中,通过调用函数raise_softirq,置位irq_stat[cpu].__softirq_pending中的相应软中断位,则会在中断结束后在函数irq_exit中调用invoke_softirq,实现软中断处理
2)在非中断上下文中,通过调用raise_softirq_irqoff,置位irq_stat[cpu].__softirq_pending中的相应软中断位,并唤醒软中断守护进程,通过软中断守护进程实现软中断的处理
3)在__do_softirq中,当该函数执行完时还有未决的软中断,则唤醒软中断守护进程,由软中断守护进程继续处理未决的软中断
以上3种方法中,不管是通过调用函数invoke_softirq,还是通过软中断守护进程来处理软中断,最终都会调用函数do_softirq、__do_softirq。
do_softirq主要完成以下工作
1) 首先判断当前是否在中断上下文中,若是则直接返回
2) 关闭irq,读取当前cpu的irq_stat[cpu].__softirq_pending
3) 判断__softirq_pending的值是否为0,若不为0则说明有软中断待处理,则调用__do_softirq
asmlinkage void do_softirq(void)
{
__u32 pending;
unsigned long flags;
if (in_interrupt())
return;
local_irq_save(flags);
pending = local_softirq_pending();
if (pending)
__do_softirq();
local_irq_restore(flags);
}
__do_softirq的定义如下:
其主要完成的工作有:
1) 获取irq_stat[cpu].__softirq_pending的值
2) 重置irq_stat[cpu].__softirq_pending的值为0,并开启软中断
3) 获取软中断向量数据softirq_vec
4) 在一个while循环中,对于每一个未处理的软中断,执行softirq_vec中相对应的action处理函数
5) 关闭中断,重新读取irq_stat[cpu].__softirq_pending的值,若该值不为0则
6) 在重复执行的次数没有超过MAX_SOFTIRQ_RESTART,且irq_stat[cpu].__softirq_pending的值不为0时,重新执行上述2、3、4、5的操作
7) 若已超过MAX_SOFTIRQ_RESTART,则调用wakeup_softirqd,唤醒软中断守护进程,由软中断守护进程继续处理
#define MAX_SOFTIRQ_RESTART 10
asmlinkage void __do_softirq(void)
{
struct softirq_action *h;
__u32 pending;
int max_restart = MAX_SOFTIRQ_RESTART;
int cpu;
pending = local_softirq_pending();
account_system_vtime(current);
__local_bh_disable((unsigned long)__builtin_return_address(0));
lockdep_softirq_enter();
cpu = smp_processor_id();
restart:
/* Reset the pending bitmask before enabling irqs */
set_softirq_pending(0);
local_irq_enable();
h = softirq_vec;
do {
if (pending & 1) {
int prev_count = preempt_count();
kstat_incr_softirqs_this_cpu(h - softirq_vec);
trace_softirq_entry(h, softirq_vec);
h->action(h);
trace_softirq_exit(h, softirq_vec);
if (unlikely(prev_count != preempt_count())) {
printk(KERN_ERR "huh, entered softirq %td %s %p"
"with preempt_count %08x,"
" exited with %08x?\n", h - softirq_vec,
softirq_to_name[h - softirq_vec],
h->action, prev_count, preempt_count());
preempt_count() = prev_count;
}
rcu_bh_qs(cpu);
}
h++;
pending >>= 1;
} while (pending);
local_irq_disable();
pending = local_softirq_pending();
if (pending && --max_restart)
goto restart;
if (pending)
wakeup_softirqd();
lockdep_softirq_exit();
account_system_vtime(current);
_local_bh_enable();
}