netdev - Re: [PATCH net-next 2/2] tcp: add net.ipv4.tcp_rtt

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Message-ID: <9e1011a8-70bd-468d-96b2-a306039b97f9@redhat.com>
Date: Wed, 19 Nov 2025 09:59:45 +0100
From: Paolo Abeni <pabeni@...hat.com>
To: Eric Dumazet <edumazet@...gle.com>
Cc: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski
 <kuba@...nel.org>, Simon Horman <horms@...nel.org>,
 Neal Cardwell <ncardwell@...gle.com>, Kuniyuki Iwashima <kuniyu@...gle.com>,
 netdev@...r.kernel.org, eric.dumazet@...il.com
Subject: Re: [PATCH net-next 2/2] tcp: add net.ipv4.tcp_rtt_threshold sysctl

On 11/18/25 10:22 PM, Eric Dumazet wrote:
> I would perhaps use 8 senders, and force all receivers on one cpu (cpu
> 4 in the following run)
> 
> for i in {1..8}
> do
>  netperf -H host -T,4 -l 100 &
> done
> 
> This would I think show what can happen when receivers can not keep up.

Thanks for the suggestion. I should have understood the receiver needs
to be under stress in the relevant scenario.

With the above setup, on vanilla kernel, the rcvbuf I see is:

min 2134391 max 33554432 avg 12085941

with multiple connections hitting tcp_rmem[2]

with the patched kernel:

min 1192472 max 33554432 avg 4247351

there is a single outlier hitting tcp_rmem[2], and in that case the
connection observes for some samples a rtt just above tcp_rtt_threshold
sysctl/tcp_rcvbuf_low_rtt.

FWIW I guess you can add:

Tested-by: Paolo Abeni <pabeni@...hat.com>

Thanks,

Paolo