linux-kernel - Re: [PATCH net 2/2] tcp_metrics: use ssthresh value from dst if there is no metrics

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Message-ID: <20250617135612.26aed53d@mordecai.tesarici.cz>
Date: Tue, 17 Jun 2025 13:56:12 +0200
From: Petr Tesarik <ptesarik@...e.com>
To: Paolo Abeni <pabeni@...hat.com>
Cc: "David S. Miller" <davem@...emloft.net>, Eric Dumazet
 <edumazet@...gle.com>, Neal Cardwell <ncardwell@...gle.com>, Kuniyuki
 Iwashima <kuniyu@...gle.com>, "open list:NETWORKING [TCP]"
 <netdev@...r.kernel.org>, David Ahern <dsahern@...nel.org>, Jakub Kicinski
 <kuba@...nel.org>, open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net 2/2] tcp_metrics: use ssthresh value from dst if
 there is no metrics

On Tue, 17 Jun 2025 12:48:30 +0200
Paolo Abeni <pabeni@...hat.com> wrote:

> On 6/13/25 12:20 PM, Petr Tesarik wrote:
> > @@ -537,6 +537,9 @@ void tcp_init_metrics(struct sock *sk)
> >  
> >  		inet_csk(sk)->icsk_rto = TCP_TIMEOUT_FALLBACK;
> >  	}
> > +
> > +	if (tp->snd_ssthresh > tp->snd_cwnd_clamp)
> > +		tp->snd_ssthresh = tp->snd_cwnd_clamp;  
> 
> I don't think we can do this unconditionally, as other parts of the TCP
> stack check explicitly for TCP_INFINITE_SSTHRESH.

Good catch! I noticed that the condition can never be true unless the
congestion window is explicitly clamped, but you're right that it is a
valid combination to lock the maximum cwnd but keep the initial TCP Slow
Start.

I'll fix that in v2.

Thank you,
Petr T