netdev - RE: [PATCH net-next v2 1/2] tcp: derive delack_max with tcp_rto

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Date: Sat, 1 Jun 2024 14:56:17 +0000
From: David Laight <David.Laight@...LAB.COM>
To: 'Kevin Yang' <yyd@...gle.com>, David Miller <davem@...emloft.net>, "Eric
 Dumazet" <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>
CC: "netdev@...r.kernel.org" <netdev@...r.kernel.org>, "ncardwell@...gle.com"
	<ncardwell@...gle.com>, "ycheng@...gle.com" <ycheng@...gle.com>,
	"kerneljasonxing@...il.com" <kerneljasonxing@...il.com>, "pabeni@...hat.com"
	<pabeni@...hat.com>, "tonylu@...ux.alibaba.com" <tonylu@...ux.alibaba.com>
Subject: RE: [PATCH net-next v2 1/2] tcp: derive delack_max with tcp_rto_min
 helper

From: Kevin Yang
> Sent: 30 May 2024 16:35
> To: David Miller <davem@...emloft.net>; Eric Dumazet <edumazet@...gle.com>; Jakub Kicinski
> 
> Rto_min now has multiple souces, ordered by preprecedence high to
> low: ip route option rto_min, icsk->icsk_rto_min.
> 
> When derive delack_max from rto_min, we should not only use ip
> route option, but should use tcp_rto_min helper to get the correct
> rto_min.
...
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index f97e098f18a5..b44f639a9fa6 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -4163,16 +4163,9 @@ EXPORT_SYMBOL(tcp_connect);
> 
>  u32 tcp_delack_max(const struct sock *sk)
>  {
> -	const struct dst_entry *dst = __sk_dst_get(sk);
> -	u32 delack_max = inet_csk(sk)->icsk_delack_max;
> -
> -	if (dst && dst_metric_locked(dst, RTAX_RTO_MIN)) {
> -		u32 rto_min = dst_metric_rtt(dst, RTAX_RTO_MIN);
> -		u32 delack_from_rto_min = max_t(int, 1, rto_min - 1);
> +	u32 delack_from_rto_min = max_t(int, 1, tcp_rto_min(sk) - 1);

That max_t() is more horrid than most.
Perhaps:
		= max(tcp_rto_min(sk), 2) - 1;

> 
> -		delack_max = min_t(u32, delack_max, delack_from_rto_min);
> -	}
> -	return delack_max;
> +	return min_t(u32, inet_csk(sk)->icsk_delack_max, delack_from_rto_min);

Can that just be a min() ??

	David

>  }
> 
>  /* Send out a delayed ack, the caller does the policy checking
> --
> 2.45.1.288.g0e0cd299f1-goog
> 

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