[PATCH net-next] tcp: fix mdev comment in tcp_rtt_estimator()

[PATCH net-next] tcp: fix mdev comment in tcp_rtt_estimator()

[Ziran Zhang]

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

Re: [PATCH net-next] tcp: fix mdev comment in tcp_rtt_estimator()

[Jakub Kicinski]

On Tue, 18 Aug 2026 16:51:53 +0800 Ziran Zhang wrote:
> 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.

## Form letter - net-next-closed

The merge window for v7.3 has started, and therefore net-next is closed
for new drivers, features, code refactoring and optimizations.
We will only consider applying net-next patches which were posted
before the announcement:

https://lore.kernel.org/20260816155953.072d73da@kernel.org

Fixes are obviously welcome at any time. net-next patches may be sent
for review and discussion only with an RFC tag.

Please repost when net-next reopens.

See: https://www.kernel.org/doc/html/next/process/maintainer-netdev.html#development-cycle
-- 
pw-bot: defer
pv-bot: closed

Re: [PATCH net-next] tcp: fix mdev comment in tcp_rtt_estimator()

[Ziran Zhang]

On Tue, 18 Aug 2026 08:13:46 -0700 Jakub Kicinski wrote:
> ## Form letter - net-next-closed
> 
> The merge window for v7.3 has started, and therefore net-next is closed
> for new drivers, features, code refactoring and optimizations.
> We will only consider applying net-next patches which were posted
> before the announcement:
> 
> https://lore.kernel.org/20260816155953.072d73da@kernel.org
> 
> Fixes are obviously welcome at any time. net-next patches may be sent
> for review and discussion only with an RFC tag.
> 
> Please repost when net-next reopens.
> 
> See: https://www.kernel.org/doc/html/next/process/maintainer-netdev.html#development-cycle
> -- 
> pw-bot: defer
> pv-bot: closed

Hi Jakub,

Thanks for your reply.

I understand that net-next is currently closed. I will wait for it to reopen 
and then resubmit the patch.

Thanks,
Ziran Zhang

Re: [PATCH net-next] tcp: fix mdev comment in tcp_rtt_estimator()

[Jakub Kicinski]

On Tue, 18 Aug 2026 23:42:53 +0800 Ziran Zhang wrote:
> Thanks for your reply.
> 
> I understand that net-next is currently closed. I will wait for it to reopen 
> and then resubmit the patch.

TBH I'm not sure this patch is worth the review overhead in the first
place.