[PATCH net-next v2] tcp: fix mdev comment in tcp_rtt_estimator()
From: Ziran Zhang <hidden>
Date: 2026-09-03 10:44:46
Also in:
lkml
Subsystem:
networking [general], networking [tcp], the rest · Maintainers:
"David S. Miller", Eric Dumazet, Jakub Kicinski, Paolo Abeni, Neal Cardwell, Linus Torvalds
The old comment "mdev = 3/4 mdev + 1/4 new" only describes the common case. However, when RTT decreases and |rtt - srtt| > mdev/4, 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 <redacted> --- net/ipv4/tcp_input.c | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-)
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 0f60a1dbf..b4a6aba21 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c@@ -1108,7 +1108,11 @@ 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 RTT decrease when + * abs(rtt - srtt) > mdev/4 + */ if (tp->mdev_us > tp->mdev_max_us) { tp->mdev_max_us = tp->mdev_us; if (tp->mdev_max_us > tp->rttvar_us)
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2.51.0