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

[Paolo Abeni]

On 9/1/26 4:35 AM, 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.
> 
> Signed-off-by: Ziran Zhang <zhangcoder@xxxxxxxx>
> ---
>  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
> +						 */

I'm sorry, I have the feeling the above does not clarify much. Also
AFAICS it's not a 'sharp' drop but any negative delta.

/P