详解sotfirq和tasklet

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softirq

软中断是在硬件中断推出后执行。软中断不能被自己打断(即单个cpu上软中断不能嵌套执行)，只能被硬件中断打断（上半部）。

可以并发运行在多个CPU上(同一类型也可以)。所以软中断必须设计为可重入的函数（允许多个CPU同时操作），因此也需要使用自旋锁来保其数据结构。

gic_handle_irq __handle_domain_irq generic_handle_irq generic_handle_irq_desc(desc); desc->handle_irq(desc); irq_exit

判断是否在中断中

包括中断和软中断 ；下面的__local_bh_disable_ip(_RET_IP_, SOFTIRQ_OFFSET);就是标记正在执行软中断

#define in_interrupt() (irq_count()) #define irq_count() (preempt_count() & (HARDIRQ_MASK | SOFTIRQ_MASK \ | NMI_MASK)) static __always_inline void __local_bh_disable_ip(unsigned long ip, unsigned int cnt) { preempt_count_add(cnt); barrier(); } void irq_exit(void) { #ifndef __ARCH_IRQ_EXIT_IRQS_DISABLED local_irq_disable(); #else WARN_ON_ONCE(!irqs_disabled()); #endif account_irq_exit_time(current); preempt_count_sub(HARDIRQ_OFFSET); if (!in_interrupt() && local_softirq_pending()) invoke_softirq(); tick_irq_exit(); rcu_irq_exit(); trace_hardirq_exit(); /* must be last! */ }

invoke_softirq

1.如果ksoftirqd线程执行中，直接退出

2.force_irqthreads=0执行__do_softirq处理软终端；=1直接执行ksoftirqd

static inline void invoke_softirq(void) { if (ksoftirqd_running(local_softirq_pending())) return; if (!force_irqthreads) { #ifdef CONFIG_HAVE_IRQ_EXIT_ON_IRQ_STACK /* * We can safely execute softirq on the current stack if * it is the irq stack, because it should be near empty * at this stage. */ __do_softirq(); #else /* * Otherwise, irq_exit() is called on the task stack that can * be potentially deep already. So call softirq in its own stack * to prevent from any overrun. */ do_softirq_own_stack(); #endif } else { wakeup_softirqd(); } } #ifdef CONFIG_IRQ_FORCED_THREADING # ifdef CONFIG_PREEMPT_RT # define force_irqthreads (true) # else extern bool force_irqthreads; # endif #else #define force_irqthreads (0) #endif

__do_softirq

1.调用local_softirq_pending函数取得目前有哪些位存在软件中断。

2.调用__local_bh_disable关闭软中断，其实就是设置正在处理软件中断标记，在同一个CPU上使得不能重入__do_softirq函数。

3.set_softirq_pending重新设置软中断标记为0，在之后重新开启中断之后又可以设置软件中断位。

4.调用local_irq_enable，开启硬件中断。

5.之后在一个循环中，遍历pending标志的每一位且调用软件中断的处理函数。在这个过程中硬件中断是开启的，随时可以打断软件中断。这样保证硬件中断不会丢失。

6.之后关闭硬件中断(local_irq_disable)，查看是否又有软件中断处于pending状态，如果是，并且在本次调用__do_softirq函数过程中没有累计重复进入软件中断处理的次数超过max_restart=10次，就可以重新调用软件中断处理。如果超过了10次，就调用wakeup_softirqd()唤醒内核的一个进程来处理软件中断。设立10次的限制，也是为了避免影响系统响应时间。

7.调用_local_bh_enable开启软中断。

asmlinkage __visible void __softirq_entry __do_softirq(void) { unsigned long end = jiffies + MAX_SOFTIRQ_TIME; unsigned long old_flags = current->flags; int max_restart = MAX_SOFTIRQ_RESTART; struct softirq_action *h; bool in_hardirq; __u32 pending; int softirq_bit; /* * Mask out PF_MEMALLOC as the current task context is borrowed for the * softirq. A softirq handled, such as network RX, might set PF_MEMALLOC * again if the socket is related to swapping. */ current->flags &= ~PF_MEMALLOC; pending = local_softirq_pending(); //#define local_softirq_pending() (__this_cpu_read(local_softirq_pending_ref)) account_irq_enter_time(current); __local_bh_disable_ip(_RET_IP_, SOFTIRQ_OFFSET); in_hardirq = lockdep_softirq_start(); restart: /* Reset the pending bitmask before enabling irqs */ set_softirq_pending(0); local_irq_enable(); h = softirq_vec; while ((softirq_bit = ffs(pending))) { unsigned int vec_nr; int prev_count; h += softirq_bit - 1; vec_nr = h - softirq_vec; prev_count = preempt_count(); kstat_incr_softirqs_this_cpu(vec_nr); trace_softirq_entry(vec_nr); h->action(h); trace_softirq_exit(vec_nr); if (unlikely(prev_count != preempt_count())) { pr_err("huh, entered softirq %u %s %p with preempt_count %08x, exited with %08x?\n", vec_nr, softirq_to_name[vec_nr], h->action, prev_count, preempt_count()); preempt_count_set(prev_count); } h++; pending >>= softirq_bit; } if (__this_cpu_read(ksoftirqd) == current) rcu_softirq_qs(); local_irq_disable(); pending = local_softirq_pending(); if (pending) { if (time_before(jiffies, end) && !need_resched() && --max_restart) goto restart; wakeup_softirqd(); } lockdep_softirq_end(in_hardirq); account_irq_exit_time(current); __local_bh_enable(SOFTIRQ_OFFSET); WARN_ON_ONCE(in_interrupt()); current_restore_flags(old_flags, PF_MEMALLOC); }

ksoftirqd

下面是软中断是否应该执行的判断条件，也即有没软中断pending， 以及run_ksoftirqd线程函数来执行一次软中断

static int ksoftirqd_should_run(unsigned int cpu) { return local_softirq_pending(); } static void run_ksoftirqd(unsigned int cpu) { local_irq_disable(); if (local_softirq_pending()) { __do_softirq(); rcu_note_context_switch(cpu); local_irq_enable(); cond_resched(); return; } local_irq_enable(); } static struct smp_hotplug_thread softirq_threads = { .store = &ksoftirqd, .thread_should_run = ksoftirqd_should_run, .thread_fn = run_ksoftirqd, .thread_comm = "ksoftirqd/%u", }; static __init int spawn_ksoftirqd(void) { register_cpu_notifier(&cpu_nfb); BUG_ON(smpboot_register_percpu_thread(&softirq_threads)); return 0; } early_initcall(spawn_ksoftirqd);

smpboot_register_percpu_thread

watchdog_enable_all_cpus ,cpu_stop_init等接口也会使用这个函数来为每个cpu创建对应线程

int smpboot_register_percpu_thread(struct smp_hotplug_thread *plug_thread) { unsigned int cpu; int ret = 0; mutex_lock(&smpboot_threads_lock); for_each_online_cpu(cpu) { ret = __smpboot_create_thread(plug_thread, cpu); if (ret) { smpboot_destroy_threads(plug_thread); goto out; } smpboot_unpark_thread(plug_thread, cpu); } list_add(&plug_thread->list, &hotplug_threads); out: mutex_unlock(&smpboot_threads_lock); return ret; } static int __smpboot_create_thread(struct smp_hotplug_thread *ht, unsigned int cpu) { struct task_struct *tsk = *per_cpu_ptr(ht->store, cpu); struct smpboot_thread_data *td; if (tsk) return 0; td = kzalloc_node(sizeof(*td), GFP_KERNEL, cpu_to_node(cpu)); if (!td) return -ENOMEM; td->cpu = cpu; td->ht = ht; tsk = kthread_create_on_cpu(smpboot_thread_fn, td, cpu, ht->thread_comm); if (IS_ERR(tsk)) { kfree(td); return PTR_ERR(tsk); } get_task_struct(tsk); *per_cpu_ptr(ht->store, cpu) = tsk; if (ht->create) { /* * Make sure that the task has actually scheduled out * into park position, before calling the create * callback. At least the migration thread callback * requires that the task is off the runqueue. */ if (!wait_task_inactive(tsk, TASK_PARKED)) WARN_ON(1); else ht->create(cpu); } return 0; }

smpboot_thread_fn

就是根据对应线程的thread_should_run来决定当前的软件中断线程是否需要执行，不需要执行则放弃时间片

static int smpboot_thread_fn(void *data) { struct smpboot_thread_data *td = data; struct smp_hotplug_thread *ht = td->ht; while (1) { set_current_state(TASK_INTERRUPTIBLE); //关闭内核抢占机制 preempt_disable(); if (kthread_should_stop()) { __set_current_state(TASK_RUNNING); preempt_enable(); if (ht->cleanup) ht->cleanup(td->cpu, cpu_online(td->cpu)); kfree(td); return 0; } if (kthread_should_park()) { __set_current_state(TASK_RUNNING); preempt_enable(); if (ht->park && td->status == HP_THREAD_ACTIVE) { BUG_ON(td->cpu != smp_processor_id()); ht->park(td->cpu); td->status = HP_THREAD_PARKED; } kthread_parkme(); /* We might have been woken for stop */ continue; } BUG_ON(td->cpu != smp_processor_id()); /* Check for state change setup */ switch (td->status) { case HP_THREAD_NONE: __set_current_state(TASK_RUNNING); preempt_enable(); if (ht->setup) ht->setup(td->cpu); td->status = HP_THREAD_ACTIVE; continue; case HP_THREAD_PARKED: __set_current_state(TASK_RUNNING); preempt_enable(); if (ht->unpark) ht->unpark(td->cpu); td->status = HP_THREAD_ACTIVE; continue; } /* * 就是通过调用ksoftirqd_should_run 这是在一开始定义的softirq_threads中指定的，检查当前CPU上维护的软件中断数组中是否有中断 * 的置起了从而决定当前的软件中断线程是否需要执行，不需要执行则放弃时间片 */ if (!ht->thread_should_run(td->cpu)) { 　　　　　　　/* 　　　　　　　*没有需要的软件中断需要执行，则放弃时间片 　　　　　　　*/ preempt_enable_no_resched(); schedule(); } else { /* * 有中断需要执行则直接调用 run_ksoftirqd 执行软件中断注册的接口的调用 */ __set_current_state(TASK_RUNNING); preempt_enable(); //这个接口在上面初始化时绑定为run_ksoftirqd ht->thread_fn(td->cpu); } } }

symbol

软中断描述符

struct softirq_action{ void (*action)(struct softirq_action *);}; 描述每一种类型的软中断，其中void(*action)是软中断触发时的执行函数。

软中断全局数据和类型

static struct softirq_action softirq_vec[NR_SOFTIRQS] __cacheline_aligned_in_smp; enum { HI_SOFTIRQ=0, /*用于高优先级的tasklet*/ TIMER_SOFTIRQ, /*用于定时器的下半部*/ NET_TX_SOFTIRQ, /*用于网络层发包*/ NET_RX_SOFTIRQ, /*用于网络层收报*/ BLOCK_SOFTIRQ, BLOCK_IOPOLL_SOFTIRQ, TASKLET_SOFTIRQ, /*用于低优先级的tasklet*/ SCHED_SOFTIRQ, HRTIMER_SOFTIRQ, RCU_SOFTIRQ, /* Preferable RCU should always be the last softirq */ NR_SOFTIRQS };

注册软中断

void open_softirq(int nr, void (*action)(struct softirq_action *))即注册对应类型的处理函数到全局数组softirq_vec中。例如网络发包对应类型为NET_TX_SOFTIRQ的处理函数net_tx_action.

触发软中断

void raise_softirq(unsigned int nr)实际上即以软中断类型nr作为偏移量置位每cpu变量irq_stat[cpu_id]的成员变量__softirq_pending，这也是同一类型软中断可以在多个cpu上并行运行的根本原因

示例

定义软中断的每cpu变量

static DEFINE_PER_CPU(struct tasklet_head, tasklet_vec); static DEFINE_PER_CPU(struct tasklet_head, tasklet_hi_vec);

注册软中断处理函数

void __init softirq_init(void) { int cpu; for_each_possible_cpu(cpu) { per_cpu(tasklet_vec, cpu).tail = &per_cpu(tasklet_vec, cpu).head; per_cpu(tasklet_hi_vec, cpu).tail = &per_cpu(tasklet_hi_vec, cpu).head; } open_softirq(TASKLET_SOFTIRQ, tasklet_action); open_softirq(HI_SOFTIRQ, tasklet_hi_action); }

tasklet软中断的处理函数的实现的具体实现

tasklet_trylock通过原子操作标记正在运行，来保证只有一个执行单元(一个cpu)执行此类型的tasklet的func；然后依次遍历tasklet_vec或者tasklet_hi_vec链表里的元素

static __latent_entropy void tasklet_action(struct softirq_action *a) { tasklet_action_common(a, this_cpu_ptr(&tasklet_vec), TASKLET_SOFTIRQ); } static __latent_entropy void tasklet_hi_action(struct softirq_action *a) { tasklet_action_common(a, this_cpu_ptr(&tasklet_hi_vec), HI_SOFTIRQ); } static void tasklet_action_common(struct softirq_action *a, struct tasklet_head *tl_head, unsigned int softirq_nr) { struct tasklet_struct *list; local_irq_disable(); list = tl_head->head; tl_head->head = NULL; tl_head->tail = &tl_head->head; local_irq_enable(); while (list) { struct tasklet_struct *t = list; list = list->next; if (tasklet_trylock(t)) { if (!atomic_read(&t->count)) { if (!test_and_clear_bit(TASKLET_STATE_SCHED, &t->state)) BUG(); t->func(t->data); tasklet_unlock(t); continue; } tasklet_unlock(t); } local_irq_disable(); t->next = NULL; *tl_head->tail = t; tl_head->tail = &t->next; __raise_softirq_irqoff(softirq_nr); local_irq_enable(); } }

tasklet

由于软中断必须使用可重入函数，这就导致设计上的复杂度变高，作为设备驱动程序的开发者来说，增加了负担。

而如果某种应用并不需要在多个CPU上并行执行，那么软中断其实是没有必要的。因此诞生了弥补以上两个要求的tasklet。它具有以下特性：

1. 一种特定类型的tasklet只能运行在一个CPU上，不能并行，只能串行执行。多个不同类型的tasklet可以并行在多个CPU上。

2. 软中断是静态分配的，在内核编译好之后，就不能改变。但tasklet就灵活许多，可以在运行时改变（比如添加模块时）。

tasklet是在两种软中断类型的基础上实现的(高优先级、低优先级)，因此如果不需要软中断的并行特性，tasklet就是最好的选择。

相关宏和结构体

struct tasklet_struct { struct tasklet_struct *next; unsigned long state; atomic_t count; void (*func)(unsigned long); unsigned long data; }; struct tasklet_head { struct tasklet_struct *head; struct tasklet_struct tail; }; #define DECLARE_TASKLET(name, func, data) \ struct tasklet_struct name = { NULL, 0, ATOMIC_INIT(0), func, data } #define DECLARE_TASKLET_DISABLED(name, func, data) \ struct tasklet_struct name = { NULL, 0, ATOMIC_INIT(1), func, data }

示例

主要包括tasklet的定义和调度

static void kgdb_tasklet_bpt(unsigned long ing) { kgdb_breakpoint(); atomic_set(&kgdb_break_tasklet_var, 0); } static DECLARE_TASKLET(kgdb_tasklet_breakpoint, kgdb_tasklet_bpt, 0); void kgdb_schedule_breakpoint(void) { if (atomic_read(&kgdb_break_tasklet_var) || atomic_read(&kgdb_active) != -1 || atomic_read(&kgdb_setting_breakpoint)) return; atomic_inc(&kgdb_break_tasklet_var); tasklet_schedule(&kgdb_tasklet_breakpoint); } EXPORT_SYMBOL_GPL(kgdb_schedule_breakpoint);

调度tasklet

1. 先通过__tasklet_schedule_common将tasklet加入tasklet_vec或者tasklet_hi_vec链表

2. 最终调用wakeup_softirqd唤醒软中断线程

static inline void tasklet_schedule(struct tasklet_struct *t) { if (!test_and_set_bit(TASKLET_STATE_SCHED, &t->state)) __tasklet_schedule(t); } static inline void tasklet_hi_schedule(struct tasklet_struct *t) { if (!test_and_set_bit(TASKLET_STATE_SCHED, &t->state)) __tasklet_hi_schedule(t); } void __tasklet_schedule(struct tasklet_struct *t) { __tasklet_schedule_common(t, &tasklet_vec, TASKLET_SOFTIRQ); } EXPORT_SYMBOL(__tasklet_schedule); void __tasklet_hi_schedule(struct tasklet_struct *t) { __tasklet_schedule_common(t, &tasklet_hi_vec, HI_SOFTIRQ); } EXPORT_SYMBOL(__tasklet_hi_schedule); static void __tasklet_schedule_common(struct tasklet_struct *t, struct tasklet_head __percpu *headp, unsigned int softirq_nr) { struct tasklet_head *head; unsigned long flags; local_irq_save(flags); head = this_cpu_ptr(headp); t->next = NULL; *head->tail = t; head->tail = &(t->next); raise_softirq_irqoff(softirq_nr); local_irq_restore(flags); } inline void raise_softirq_irqoff(unsigned int nr) { __raise_softirq_irqoff(nr); /* * If we're in an interrupt or softirq, we're done * (this also catches softirq-disabled code). We will * actually run the softirq once we return from * the irq or softirq. * * Otherwise we wake up ksoftirqd to make sure we * schedule the softirq soon. */ if (!in_interrupt()) wakeup_softirqd(); }