netdev - RE: [net-next,v3] tcp: Improve setsockopt() TCP_USER

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Date:   Tue, 10 Jul 2018 09:17:27 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Jon Maxwell' <jmaxwell37@...il.com>,
        "davem@...emloft.net" <davem@...emloft.net>
CC:     "edumazet@...gle.com" <edumazet@...gle.com>,
        "eric.dumazet@...il.com" <eric.dumazet@...il.com>,
        "ncardwell@...gle.com" <ncardwell@...gle.com>,
        "kuznet@....inr.ac.ru" <kuznet@....inr.ac.ru>,
        "yoshfuji@...ux-ipv6.org" <yoshfuji@...ux-ipv6.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "jmaxwell@...hat.com" <jmaxwell@...hat.com>
Subject: RE: [net-next,v3] tcp: Improve setsockopt() TCP_USER_TIMEOUT accuracy

From: Jon Maxwell
> Sent: 10 July 2018 07:52
...
> +static __u32 tcp_clamp_rto_to_user_timeout(struct sock *sk)
> +{
> +	struct inet_connection_sock *icsk = inet_csk(sk);
> +	__u32 elapsed, user_timeout;
> +	u32 start_ts;
> +
> +	start_ts = tcp_retransmit_stamp(sk);
> +	if (!icsk->icsk_user_timeout || !start_ts)
> +		return icsk->icsk_rto;
> +	elapsed = tcp_time_stamp(tcp_sk(sk)) - start_ts;
> +	user_timeout = jiffies_to_msecs(icsk->icsk_user_timeout);
> +	if (elapsed >= user_timeout)
> +		return 1; /* user timeout has passed; fire ASAP */
> +	else
> +		return (icsk->icsk_rto < msecs_to_jiffies(user_timeout - elapsed)) ?
> +			icsk->icsk_rto : msecs_to_jiffies(user_timeout - elapsed);

You've not looked at what the above actually does - something like:
msecs_to_jiffies() probably rounds up, the old code rounded down
so dividing by HZ is probably right.

	int new_tmo = icsk->icsk_user_timeout - (tcp_time_stamp(tcp_sk(sk)) - start_ts)/HZ;

	if (new_tmo > (int) icsk->icsk_rto;
		return icsk->icsk_rto;
	return new_tmo <= 0 ? 1 : new_tmo;
}