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

[Eric Dumazet]

On Thu, Sep 3, 2026 at 10:08 AM Paolo Abeni <pabeni@xxxxxxxxxx> wrote:
>
> 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.

I agree, this is a 'legacy comment' that should probably stay for one
more century :)