分析從Linux源碼看TIME_WAIT的持續時間

#define TCP_TIMEWAIT_LEN (60*HZ) /* how long to wait to destroy TIME-WAIT
				  * state, about 60 seconds	*/

/* 這邊TCP_TIMEWAIT_LEN 60 * HZ */
inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN,
					 TCP_TIMEWAIT_LEN);

void inet_twsk_schedule(struct inet_timewait_sock *tw,struct inet_timewait_death_row *twdr,const int timeo, const int timewait_len)
{
	......
	// 計算時間輪的slot
	slot = (timeo + (1 << INET_TWDR_RECYCLE_TICK) - 1) >> INET_TWDR_RECYCLE_TICK;
	......
	// 慢時間輪的邏輯，由於沒有開啟TCP\_TW\_RECYCLE,timeo總是60*HZ(60s)
	// 所有都走slow_timer邏輯 
	if (slot >= INET_TWDR_RECYCLE_SLOTS) {
		/* Schedule to slow timer */
		if (timeo >= timewait_len) {
			slot = INET_TWDR_TWKILL_SLOTS - 1;
		} else {
			slot = DIV_ROUND_UP(timeo, twdr->period);
			if (slot >= INET_TWDR_TWKILL_SLOTS)
				slot = INET_TWDR_TWKILL_SLOTS - 1;
		}
		tw->tw_ttd = jiffies + timeo;
		// twdr->slot當前正在處理的slot
		// 在TIME_WAIT_LEN下，這個邏輯一般7
		slot = (twdr->slot + slot) & (INET_TWDR_TWKILL_SLOTS - 1);
		list = &twdr->cells[slot];
	} else{
		// 走短時間定時器，由於篇幅原因，不在這裡贅述
		......
	}
	......
	/* twdr->period 60/8=7.5 */
	if (twdr->tw_count++ == 0)
		mod_timer(&twdr->tw_timer, jiffies + twdr->period);
	spin_unlock(&twdr->death_lock);
}

/*參數中的tcp_death_row即為承載時間輪處理函數的結構體*/
inet_twsk_schedule(tw,&tcp_death_row,TCP_TIMEWAIT_LEN,TCP_TIMEWAIT_LEN)
/* 具體的處理結構體 */
struct inet_timewait_death_row tcp_death_row = {
	......
	/* slow_timer時間輪處理函數 */
	.tw_timer	= TIMER_INITIALIZER(inet_twdr_hangman, 0,
					    (unsigned long)&tcp_death_row),
	/* slow_timer時間輪輔助處理函數*/
	.twkill_work	= __WORK_INITIALIZER(tcp_death_row.twkill_work,
					     inet_twdr_twkill_work),
	/* 短時間輪處理函數 */
	.twcal_timer	= TIMER_INITIALIZER(inet_twdr_twcal_tick, 0,
					    (unsigned long)&tcp_death_row),				
};

void inet_twdr_hangman(unsigned long data)
{
	struct inet_timewait_death_row *twdr;
	unsigned int need_timer;

	twdr = (struct inet_timewait_death_row *)data;
	spin_lock(&twdr->death_lock);

	if (twdr->tw_count == 0)
		goto out;

	need_timer = 0;
	// 如果此slot處理的time_wait socket已經達到瞭100個，且還沒處理完
	if (inet_twdr_do_twkill_work(twdr, twdr->slot)) {
		twdr->thread_slots |= (1 << twdr->slot);
		// 將餘下的任務交給work queue處理
		schedule_work(&twdr->twkill_work);
		need_timer = 1;
	} else {
		/* We purged the entire slot, anything left?  */
		// 判斷是否還需要繼續處理
		if (twdr->tw_count)
			need_timer = 1;
		// 如果當前slot處理完瞭，才跳轉到下一個slot
		twdr->slot = ((twdr->slot + 1) & (INET_TWDR_TWKILL_SLOTS - 1));
	}
	// 如果還需要繼續處理，則在7.5s後再運行此函數
	if (need_timer)
		mod_timer(&twdr->tw_timer, jiffies + twdr->period);
out:
	spin_unlock(&twdr->death_lock);
}

public class TimeWaitSimulator {

    public static void main(String[] args) {
        double delta = (60) * 1.0 / 8;

        // 0表示開始清理，1表示清理完畢
        // 清理完畢之後slot向前推進
        int startPurge = 0;
        double sum = 0;
        int slot = 0;
        while (slot < 8) {
            if (startPurge == 0) {
                sum += delta;
                startPurge = 1;
                if (slot == 7) {
                    // 因為假設進入work_queue之後，很快就會清理完
                    // 所以在slot為7的時候並不需要等最後的那個purge過程7.5s
                    System.out.println("slot " + slot + " has reach the last " + sum);
                    break;
                }
            }
            if (startPurge == 1) {
                sum += delta;
                startPurge = 0;
                System.out.println("slot " + "move to next at time " + sum);
                // 清理完之後，slot才應該向前推進
                slot++;
            }
        }
    }
}

enum tcp_tw_status
tcp_timewait_state_process(struct inet_timewait_sock *tw, struct sk_buff *skb,
			   const struct tcphdr *th)
{
	......
	if (paws_reject)
		NET_INC_STATS_BH(twsk_net(tw), LINUX_MIB_PAWSESTABREJECTED);
		
	if (!th->rst) {
		/* In this case we must reset the TIMEWAIT timer.
		 *
		 * If it is ACKless SYN it may be both old duplicate
		 * and new good SYN with random sequence number <rcv_nxt.
		 * Do not reschedule in the last case.
		 */
		/* 如果有回繞校驗失敗的包到達的情況下,或者其實ack包 
		 * 重置定時器到新的60s之後
		 * /
		if (paws_reject || th->ack)
			inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN,
					   TCP_TIMEWAIT_LEN);

		/* Send ACK. Note, we do not put the bucket,
		 * it will be released by caller.
		 */
		/* 向對端發送當前time wait狀態應該返回的ACK */
		return TCP_TW_ACK;
	}
	inet_twsk_put(tw);
	/* 註意，這邊通過paws校驗的包，會返回tcp_tw_success,使得time_wait狀態的
	 * socket五元組也可以三次握手成功重新復用
	 * /
	return TCP_TW_SUCCESS;
}