From: Ziran Zhang <zhangcoder@yeah.net>
To: Eric Dumazet <edumazet@google.com>,
Neal Cardwell <ncardwell@google.com>,
"David S . Miller" <davem@davemloft.net>,
Jakub Kicinski <kuba@kernel.org>, Paolo Abeni <pabeni@redhat.com>
Cc: Kuniyuki Iwashima <kuniyu@google.com>,
Simon Horman <horms@kernel.org>,
linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
Ziran Zhang <zhangcoder@yeah.net>
Subject: [PATCH net-next] tcp: fix mdev comment in tcp_rtt_estimator()
Date: Tue, 18 Aug 2026 16:51:53 +0800 [thread overview]
Message-ID: <20260818085153.18509-1-zhangcoder@yeah.net> (raw)
The old comment "mdev = 3/4 mdev + 1/4 new" only describes the
common case. However, when RTT drops sharply, the code applies
a finer gain of 1/32 instead of 1/4, resulting in:
mdev = 31/32 mdev + 1/32 * new.
Express both cases with a unified formula using variable
gain g.
Signed-off-by: Ziran Zhang <zhangcoder@yeah.net>
---
net/ipv4/tcp_input.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 0f60a1dbf..2c9feb181 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -1108,7 +1108,10 @@ static void tcp_rtt_estimator(struct sock *sk, long mrtt_us)
} else {
m -= (tp->mdev_us >> 2); /* similar update on mdev */
}
- tp->mdev_us += m; /* mdev = 3/4 mdev + 1/4 new */
+ tp->mdev_us += m; /*
+ * mdev = (1-g)*mdev + g*new, g=1/4 normally,
+ * g=1/32 on sharp RTT drop
+ */
if (tp->mdev_us > tp->mdev_max_us) {
tp->mdev_max_us = tp->mdev_us;
if (tp->mdev_max_us > tp->rttvar_us)
--
2.51.0
reply other threads:[~2026-08-18 8:59 UTC|newest]
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