TCP拥塞控制算法 — CUBIC的补丁（三）

描述

以下是提交者Stephen Hemminger对这个patch的描述：

fix clock dependency

The hystart code was written with assumption that HZ=1000.

Replace the use of jiffies with bictcp_clock as a millisecond real time clock.

代码

--- a/net/ipv4/tcp_cubic.c
+++ b/net/ipv4/tcp_cubic.c
@@ -88,7 +88,7 @@ struct bictcp {
 	u32	last_time;	/* time when updated last_cwnd */
 	u32	bic_origin_point;/* origin point of bic function */
 	u32	bic_K;		/* time to origin point from the beginning of the current epoch */
-	u32	delay_min;	/* min delay，以前的单位为jiffies*/
+	u32	delay_min;	/* min delay (msec << 3)，现在的单位为毫秒*/
 	u32	epoch_start;	/* beginning of an epoch */
 	u32	ack_cnt;	/* number of acks */
 	u32	tcp_cwnd;	/* estimated tcp cwnd */
@@ -98,7 +98,7 @@ struct bictcp {
 	u8	found;		/* the exit point is found? */
 	u32	round_start;	/* beginning of each round */
 	u32	end_seq;	/* end_seq of the round */
-	u32	last_jiffies;	/* last time when the ACK spacing is close，单位为jiffies */
+	u32	last_ack;	/* last time when the ACK spacing is close，单位为毫秒 */
 	u32	curr_rtt;	/* the minimum rtt of current round */
 };
 
@@ -119,12 +119,21 @@ static inline void bictcp_reset(struct bictcp *ca)
 	ca->found = 0;
 }
             
+static inline u32 bictcp_clock(void)
+{
+#if HZ < 1000 /* NZ小于1000，返回的是实际时间，单位为毫秒 */
+	return ktime_to_ms(ktime_get_real());
+#else
+	return jiffies_to_msecs(jiffies); /* 返回的是jiffies时间，单位为毫秒*/
+#endif
+}
+
 static inline void bictcp_hystart_reset(struct sock *sk)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
 	struct bictcp *ca = inet_csk_ca(sk);
 
-	ca->round_start = ca->last_jiffies = jiffies;
+	ca->round_start = ca->last_ack = bictcp_clock();
 	ca->end_seq = tp->snd_nxt;
 	ca->curr_rtt = 0;
 	ca->sample_cnt = 0;
@@ -239,8 +248,8 @@ static inline void bictcp_update(struct bictcp *ca, u32 cwnd)
 	 */
 
 	/* change the unit from HZ to bictcp_HZ */
-	t = ((tcp_time_stamp + (ca->delay_min>>3) - ca->epoch_start)
-	     << BICTCP_HZ) / HZ;
+	t = ((tcp_time_stamp + msecs_to_jiffies(ca->delay_min>>3)
+	      - ca->epoch_start) << BICTCP_HZ) / HZ;
 
 	if (t < ca->bic_K)		/* t - K */
 		offs = ca->bic_K - t;
@@ -342,14 +351,12 @@ static void hystart_update(struct sock *sk, u32 delay)
 	struct bictcp *ca = inet_csk_ca(sk);
 
 	if (!(ca->found & hystart_detect)) {
-		u32 curr_jiffies = jiffies;
+		u32 now = bictcp_clock();
 
 		/* first detection parameter - ack-train detection */
-		if ((s32)(curr_jiffies - ca->last_jiffies) <=
-		    msecs_to_jiffies(hystart_ack_delta)) {
-			ca->last_jiffies = curr_jiffies;
-			if ((s32) (curr_jiffies - ca->round_start) >
-			    ca->delay_min >> 4)
+		if ((s32)(now - ca->last_ack) <= hystart_ack_delta) {
+			ca->last_ack = now;
+			if ((s32)(now - ca->round_start) > ca->delay_min >> 4)
 				ca->found |= HYSTART_ACK_TRAIN;
 		}
 
@@ -396,7 +403,7 @@ static void bictcp_acked(struct sock *sk, u32 cnt, s32 rtt_us)
 	if ((s32)(tcp_time_stamp - ca->epoch_start) < HZ)
 		return;
 
-	delay = usecs_to_jiffies(rtt_us) << 3;
+	delay = (rtt_us << 3) / USEC_PER_MSEC;
 	if (delay == 0)
 		delay = 1;

分析

#ifndef _STRUCT_TIMESPEC
#define _STRUCT_TIMESPEC
struct timespec {
    __kernel_time_t tv_sec; /* seconds，long*/
    long tv_nsec; /* nanoseconds */
};
#endif

union ktime {
    s64 tv64;

#if BITS_PER_LONG != 64 && !defined(CONFIG_KTIME_SCALAR)
    struct {
#ifdef __BIG_ENDIAN
    s32 sec, nsec;
#else
    s32 nsec, sec;
#endif
    } tv;
#endif
};

typedef union ktime ktime_t;

/**
 * ktime_get_real - get the real (wall) time in ktime_t format
 * returns the time in ktime_t format
 */
ktime_t ktime_get_real(void)
{
    struct timespec now;
    getnstimeofday(&now);
    return timespec_to_ktime(now);
}

static inline s64 ktime_to_ms(const ktime_t kt)
{
    struct timeval tv = ktime_to_timeval(kt);
    return (s64) tv.tv_sec * MSEC_PER_SEC + tv.tv_usec / USEC_PER_MSEC;
}

/**
 * ktime_to_timeval - convert a ktime_t variable to timeval format
 * @kt: the ktime_t variable to convert
 * Returns the timeval representation of the ktime value
 */
static inline struct timeval ktime_to_timeval (const ktime_t kt)
{
    return (struct timeval) {
        .tv_sec = (time_t) kt.tv.sec,
        .tv_usec = (suseconds_t) (kt.tv.nsec / NSEC_PER_USEC) };
}

评价

原来的bictcp结构中的round_start、delay_min和last_jiffies的单位为jiffies，现在考虑到1jiffies不

一定代表1ms（如果HZ不为1000），所以这三个变量就采用毫秒为单位来计算。原来的版本也没

有错误，现在只是改了下度量单位。

Author

zhangskd @ csdn blog