linux-kernel - Re: [PATCH net 1/2] tcp_metrics: set maximum cwnd from the dst entry

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Message-ID: <da990565-b8ec-4d34-9739-cf13a2a7d2b3@redhat.com>
Date: Tue, 17 Jun 2025 13:00:53 +0200
From: Paolo Abeni <pabeni@...hat.com>
To: Petr Tesarik <ptesarik@...e.com>, "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>
Cc: David Ahern <dsahern@...nel.org>, Jakub Kicinski <kuba@...nel.org>,
 open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net 1/2] tcp_metrics: set maximum cwnd from the dst entry

On 6/13/25 12:20 PM, Petr Tesarik wrote:
> diff --git a/net/ipv4/tcp_metrics.c b/net/ipv4/tcp_metrics.c
> index 4251670e328c8..dd8f3457bd72e 100644
> --- a/net/ipv4/tcp_metrics.c
> +++ b/net/ipv4/tcp_metrics.c
> @@ -477,6 +477,9 @@ void tcp_init_metrics(struct sock *sk)
>  	if (!dst)
>  		goto reset;
>  
> +	if (dst_metric_locked(dst, RTAX_CWND))
> +		tp->snd_cwnd_clamp = dst_metric(dst, RTAX_CWND);
> +
>  	rcu_read_lock();
>  	tm = tcp_get_metrics(sk, dst, false);
>  	if (!tm) {
> @@ -484,9 +487,6 @@ void tcp_init_metrics(struct sock *sk)
>  		goto reset;
>  	}
>  
> -	if (tcp_metric_locked(tm, TCP_METRIC_CWND))
> -		tp->snd_cwnd_clamp = tcp_metric_get(tm, TCP_METRIC_CWND);
> -
>  	val = READ_ONCE(net->ipv4.sysctl_tcp_no_ssthresh_metrics_save) ?
>  	      0 : tcp_metric_get(tm, TCP_METRIC_SSTHRESH);
>  	if (val) {

It's unclear to me why you drop the tcp_metric_get() here. It looks like
the above will cause a functional regression, with unlocked cached
metrics no longer taking effects?

/P