[RFC PATCH] tcp: correct srtt and mdev_us calculation

[Weiping Zhang <zhangweiping@didiglobal.com>]

From the comments we can see that, rtt = 7/8 rtt + 1/8 new,
but there is an mistake,

m -= (srtt >> 3);
srtt += m;

explain:
m -= (srtt >> 3); //use t stands for new m
t = m - srtt/8;

srtt = srtt + t
= srtt + m - srtt/8
= srtt 7/8 + m

Test code:

 #include<stdio.h>

 #define u32 unsigned int

static void test_old(u32 srtt, long mrtt_us)
{
	long m = mrtt_us;
	u32 old = srtt;

	m -= (srtt >> 3);
	srtt += m;

	printf("%s old_srtt: %u mrtt_us: %ld new_srtt: %u\n", __func__,  old, mrtt_us, srtt);
}

static void test_new(u32 srtt, long mrtt_us)
{
	long m = mrtt_us;
	u32 old = srtt;

	m = ((m - srtt) >> 3);
	srtt += m;

	printf("%s old_srtt: %u mrtt_us: %ld new_srtt: %u\n", __func__,  old, mrtt_us, srtt);
}

int main(int argc, char **argv)
{
	u32 srtt = 100;
	long mrtt_us = 90;

	test_old(srtt, mrtt_us);
	test_new(srtt, mrtt_us);

	return 0;
}

./a.out
test_old old_srtt: 100 mrtt_us: 90 new_srtt: 178
test_new old_srtt: 100 mrtt_us: 90 new_srtt: 98

Signed-off-by: Weiping Zhang <zhangweiping@didiglobal.com>
---
 net/ipv4/tcp_input.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 0640453fce54..0242bb31e1ce 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -848,7 +848,7 @@ static void tcp_rtt_estimator(struct sock *sk, long mrtt_us)
 	 * that VJ failed to avoid. 8)
 	 */
 	if (srtt != 0) {
-		m -= (srtt >> 3);	/* m is now error in rtt est */
+		m = (m - srtt >> 3);	/* m is now error in rtt est */
 		srtt += m;		/* rtt = 7/8 rtt + 1/8 new */
 		if (m < 0) {
 			m = -m;		/* m is now abs(error) */
@@ -864,7 +864,7 @@ static void tcp_rtt_estimator(struct sock *sk, long mrtt_us)
 			if (m > 0)
 				m >>= 3;
 		} else {
-			m -= (tp->mdev_us >> 2);   /* similar update on mdev */
+			m = (m - tp->mdev_us >> 2);   /* similar update on mdev */
 		}
 		tp->mdev_us += m;		/* mdev = 3/4 mdev + 1/4 new */
 		if (tp->mdev_us > tp->mdev_max_us) {
-- 
2.34.1