寫一個system call，讓process可以是用他得到進入waiting queue的次數。
在include/linux中的task_struct新增一個變數enter_queue_counter，用來計算總共進入waiting queue幾次。 :::info 如果將變數加到task_struct中的任意位置，會產生問題。因為context switch是靠固定的位移記憶體大小來得到task_struct其中的變數，交換process的 :::
...
#ifdef CONFIG_SECURITY
/* Used by LSM modules for access restriction: */
void *security;
#endif
unsigned int enter_queue_counter; //here
/*
* New fields for task_struct should be added above here, so that
* they are included in the randomized portion of task_struct.
*/
randomized_struct_fields_end
...
在/kernel/fork.c的copy_mm中進行enter_queue_counter的初始化
static int copy_mm(unsigned long clone_flags, struct task_struct *tsk)
{
struct mm_struct *mm, *oldmm;
int retval;
tsk->enter_queue_counter = 0; //here
tsk->min_flt = tsk->maj_flt = 0;
tsk->nvcsw = tsk->nivcsw = 0;
#ifdef CONFIG_DETECT_HUNG_TASK
tsk->last_switch_count = tsk->nvcsw + tsk->nivcsw;
#endif
...
欲計算進入waiting queue的次數，等同於計算有多少個process(task)執行wake up。 trace 路徑: default_wake_function->try_to_wake_up->ttwu_queue->ttwu_do_activate->ttwu_do_wakeup。 每次執行wake up時，counter+1。
/*
* Mark the task runnable and perform wakeup-preemption.
*/
static void ttwu_do_wakeup(struct rq *rq, struct task_struct *p, int wake_flags,
struct rq_flags *rf)
{
check_preempt_curr(rq, p, wake_flags);
p->enter_queue_counter++; //here
p->state = TASK_RUNNING;
trace_sched_wakeup(p);
#ifdef CONFIG_SMP
...
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