netdev - Re: [PATCH net-next] tcp: switch rtt estimations to usec resolution

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Message-ID: <alpine.LFD.2.11.1402230908210.1460@ja.home.ssi.bg>
Date:	Sun, 23 Feb 2014 09:36:30 +0200 (EET)
From:	Julian Anastasov <ja@....bg>
To:	Eric Dumazet <eric.dumazet@...il.com>
cc:	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	Yuchung Cheng <ycheng@...gle.com>,
	Neal Cardwell <ncardwell@...gle.com>,
	Larry Brakmo <brakmo@...gle.com>
Subject: Re: [PATCH net-next] tcp: switch rtt estimations to usec
 resolution


	Hello,

On Sat, 22 Feb 2014, Eric Dumazet wrote:

> From: Eric Dumazet <edumazet@...gle.com>
> 
> Upcoming congestion controls for TCP require usec resolution for RTT
> estimations. Millisecond resolution is simply not enough these days.

...

> --- a/net/ipv4/tcp_metrics.c
> +++ b/net/ipv4/tcp_metrics.c

...

> @@ -408,10 +395,10 @@ void tcp_update_metrics(struct sock *sk)
>  	 */
>  	if (!tcp_metric_locked(tm, TCP_METRIC_RTT)) {
>  		if (m <= 0)
> -			rtt = tp->srtt;
> +			rtt = tp->srtt_us;
>  		else
>  			rtt -= (m >> 3);
> -		tcp_metric_set_msecs(tm, TCP_METRIC_RTT, rtt);
> +		tcp_metric_set(tm, TCP_METRIC_RTT, rtt);

	What is the way to handle this change of
resolution in TCP_METRICS_ATTR_VALS? The ip tool has
code in ip/tcp_metrics.c process_msg() that needs to
know what we get, ms or us. Do we need to add some
new attribute as flag or version? Or we need to increase
TCP_METRICS_GENL_VERSION in kernel and to check it in
process_msg ?

> -		tcp_metric_set_msecs(tm, TCP_METRIC_RTTVAR, var);
> +		tcp_metric_set(tm, TCP_METRIC_RTTVAR, var);

Regards

--
Julian Anastasov <ja@....bg>
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