bytex的博客

主要数据结构

struct tasklet_head
{
 struct tasklet_struct *list;
};

struct tasklet_struct
{
 struct tasklet_struct *next;
 unsigned long state;
 atomic_t count;
 void (*func)(unsigned long);
 unsigned long data;
};

tasklet 是在 softirq 的基础上实现的，基于HI_SOFTIRQ（软中断0） 和 TASKLET_SOFTIRQ(软中断5)。
这两个tasklet只是在优先级上有区别。
在初始化的时候，对应的HI_SOFTIRQ 和 TASKLET_SOFTIRQ 的 action分别被设置为 tasklet_hi_action() 和 tasklet_action()。

tasklet 和 高优先级tasklet 分别存放在 PER_CPU  变量 tasklet_vec 和 tasklet_hi_vec 中，类型为 struct tasklet_head.

对于每一个CPU，维护一个tasklet的链表，如下图所示：

                                                                                             
                                                                                             
                                           |                 |                               
                                                                                             
                                           |                 |                               
                            /---------\        /---------\                                   
                            |         |    |   |         |   |                               
                            |  CPU#0  |        |  CPU#1  |         ......                    
                            |         |    |   |         |   |                               
                            \---------/        \---------/                                   
                                           |                 |                               
                                                                                             
                                           |                 |                               
                                                                                             
                                           |                 |                               
                                                                                             
                         +-----------------+-----------------+---------------                
                         |  tasklet_head   |  tasklet_head   |                               
     tasklet_vec[NR_CPUS]| +-------------+ | +-------------+ |   ......                      
                         | |   *list     | | |   *list     | |                               
                         | +-------------+ | +-------------+ |                               
                         +-----------------+-----------------+---------------                
                                                                                             
                                           |                 |                               
                                                                                             
                                           |                 |                               
                                                                                             
                         +-----------------+-----------------+---------------                
                         |  tasklet_head   |  tasklet_head   |                         
  tasklet_hi_vec[NR_CPUS]| +-------------+ | +-------------+ |   ......                
                         | |   *list     | | |   *list     | |                         
                         | +-------------+ | +-------------+ |                         
                         +-----------------+-----------------+---------------          
                                           |                 |                         
                                                                                       
                                           |                 |                         
                                                                                       
                                           |                 |                         
                                                                                       
                                                                                       
                                                                                       
                                                                                       
                                                                                       
                                                                                       
                                                                                       
                                                                                       
                                                                                       
                                                                                       
                                                                                      
                                                                                       
                                                                                       
  /---------------------------------------------------------------------------\        
  |                                                                           |        
  |   CPU#0                                                                   |        
  |                                                                           |   
  |                                                                           |   
  |                                                                           |   
  |                                                                           |   
  |    tasklet_head    /--> tasklet_struct   /--> tasklet_struct              |   
  |   +-------------+  |   +--------------+  |   +--------------+    /------\ |   
  |   |   *list     |--/   |    *next     |--/   |    *next     |--->| NULL | |   
  |   +-------------+      +--------------+      +--------------+    \------/ |   
  |                        |    state     |      |    state     |             |   
  |                        +--------------+      +--------------+             |   
  |                        |    count     |      |    count     |             |   
  |                        +--------------+      +--------------+             |   
  |                        |  (*func)()   |      |  (*func)()   |             |   
  |                        +--------------+      +--------------+             |   
  |                        |    data      |      |    data      |             |   
  |                        +--------------+      +--------------+             |   
  |                                                                           |   
  \---------------------------------------------------------------------------/   
                                                                                  
                                                                                  
                                                                                  
  /---------------------------------------------------------------------------\   
  |                                                                           |   
  |   CPU#1                                                                   |   
  |                                                                           |   
  |                                                                           |   
  |                                                                           |   
  |                                                                           |   
  |    tasklet_head    /--> tasklet_struct   /--> tasklet_struct              |   
  |   +-------------+  |   +--------------+  |   +--------------+    /------\ |   
  |   |   *list     |--/   |    *next     |--/   |    *next     |--->| NULL | |   
  |   +-------------+      +--------------+      +--------------+    \------/ |   
  |                        |    state     |      |    state     |             |   
  |                        +--------------+      +--------------+             |   
  |                        |    count     |      |    count     |             |   
  |                        +--------------+      +--------------+             |   
  |                        |  (*func)()   |      |  (*func)()   |             |   
  |                        +--------------+      +--------------+             |   
  |                        |    data      |      |    data      |             |   
  |                        +--------------+      +--------------+             |   
  |                                                                           |   
  \---------------------------------------------------------------------------/   
                                                                                  
                                                                                  
 
 

初始化一个tasklet通过函数 tasklet_init() ，该函数会初始化 tasklet_struct 中的各个成员：
void tasklet_init(struct tasklet_struct *t,
    void (*func)(unsigned long), unsigned long data)
{
 t->next = NULL;
 t->state = 0;
 atomic_set(&t->count, 0);
 t->func = func;
 t->data = data;
}

调度一个tasklet 通过函数 tasklet_hi_schedule() 或者 tasklet_schedule()。
他们做的事情类似，以下本文将以tasklet_schedule()为例来说明。

tasklet_schedule() 首先会判断该 tasklet 是否已经被调度过，对于同一个tasklet，在同一时刻只能被调度一次，这通过 tasklet的 state中的 TASKLET_STATE_SCHED 位来标记。
然后会将 tasklet 插入到当前CPU的tasklet_vec 的list中。
然后触发taskt的softirq

当触发一个tasklet的时候，对应的softirq会被执行，即会执行 tasklet_hi_action() 或者 tasklet_action().
而这两个函数，就会遍历该CPU上的tasklet链表，依次执行各个tasklet的func函数，执行的tasklet会从链表上删除掉。
tasklet 在执行的时候, state中的 TASKLET_STATE_RUN 位会被置位，然后会清除 state 的 TASKLET_STATE_SCHED 位。
执行完毕后，state中的 TASKLET_STATE_RUN 位会被清除掉。
如果在执行前， state中的 TASKLET_STATE_RUN 位会被置位，说明当前类型的tasklet正在另一个CPU上被执行，此时需要等待其在另一个CPU上执行完。

出现这种情况的原因是：
对于一个tasklet， TASKLET_STATE_SCHED 标记该tasklet是否被调度，TASKLET_STATE_RUN 标记该 tasklet 是否正在执行。
tasklet的状态顺序如下图所示：

                                                    
                
                
                
                                                     
                                   init               
                      +---------------------------+   
                      | clear TASKLET_STATE_SCHED |   
                      | clear TASKLET_STATE_RUN   |   
                      +---------------------------+   
                                   ||                 
                                   ||                 
                                   ||                 
                                   ||                 
                                   ||                 
                                   ||                 
                                   \/     
                             /------------\
                             |    idle    |
                             \------------/
                                   ||     
                                   ||                    
                                   ||                    
                                   ||                    
                                   ||                    
                                   ||                    
                                   \/     
                                                            
                           tasklet_schedule                 
               +-----------------------------------------+                 
               |                                         |                 
               |     /------------------------------\    |   /--------\    
               |     |                              | no |   |        |    
               |     |  TASKLET_STATE_SCHED == 0 ?  |----|-->| return |    
               |     |                              |    |   |        |    
               |     \------------------------------/    |   \--------/    
               |                   |                     |                 
               |                   |yes                  |
               |                   |                     |
               |                   v                     |
               |        +-------------------------+      |    
               |        | set TASKLET_STATE_SCHED |      |    
               |        +-------------------------+      |    
               |                  ||                     |    
               +------------------||---------------------+    
                                  ||                          
                                  ||                          
                                  ||                             
                                  ||                             
                                  \/         
                                             
                           tasklet_action    
               +-----------------------------------------+
               |                                         |
               |     /------------------------------\    |
               |     |                              |    |
               |     |    TASKLET_STATE_RUN == 0 ?  |<------------\
               |     |                              |    |        |
               |     \------------------------------/    |        |
               |                  |       |              |        |
               |               yes|       |no            |        |
               |                  |       |              |        |
               |                  |       \----------->-----------/
               |                  v                      |
               |      +--------------------------+       |
               |      |  set TASKLET_STATE_RUN   |       |
               |      +--------------------------+       |
               |                  |                      |
               |                  |                      |
               |                  |                      |
               |                  |                      |
               |                  v                      |
               |      +---------------------------+      |
               |      | clear TASKLET_STATE_SCHED |      |
               |      +---------------------------+      |
               |                  |                      |
               |                  |                      |
               |                  |                      |
               |                  |                      |
               |                  v                      |
               |      +--------------------------+       |
               |      |   run tasklet function   |       |
               |      +--------------------------+       |
               |                  |                      |
               |                  |                      |
               |                  |                      |            
               |                  |                      |            
               |                  v                      |            
               |      +--------------------------+       |            
               |      | clear TASKLET_STATE_RUN  |       |            
               |      +--------------------------+       |            
               |                 ||                      |            
               |                 ||                      |            
               +-----------------||----------------------+
                                 ||
                                 ||
                                 ||    
                                 \/    
                             /--------\
                             |        |
                             | return |
                             |        |
                             \--------/             
                                                    
                                                    
                                                    
注意：图中的 return表明函数返回，此时tasklet的状态并不一定是idle的。