netdev - Re: [PATCH v6 net-next 2/2] tcp: switch rtt estimations to usec resolution

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Date:	Tue, 25 Feb 2014 12:46:40 -0500
From:	Neal Cardwell <ncardwell@...gle.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Stephen Hemminger <stephen@...workplumber.org>,
	Julian Anastasov <ja@....bg>,
	David Miller <davem@...emloft.net>,
	Yuchung Cheng <ycheng@...gle.com>,
	netdev <netdev@...r.kernel.org>, Larry Brakmo <brakmo@...gle.com>
Subject: Re: [PATCH v6 net-next 2/2] tcp: switch rtt estimations to usec resolution

On Tue, Feb 25, 2014 at 1:22 AM, Eric Dumazet <eric.dumazet@...il.com> 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.
...
> ---
> v6: Do not use ktime_get() but skb_mstamp, as Stephen Hemminger pointed
>     out some platforms have slow ktime_get()
...
>
> -                       ca_ops->pkts_acked(sk, pkts_acked, rtt_us);
> -               }
> -       } else if (skb && rtt_update && sack_rtt >= 0 &&
> -                  sack_rtt > (s32)(now - TCP_SKB_CB(skb)->when)) {
> +       } else if (skb && rtt_update && sack_rtt_us >= 0 &&
> +                  sack_rtt_us > skb_mstamp_us_delta(&now, &skb->skb_mstamp)) {

This gets rid of the (s32) cast for the result of the time
subtraction. Is that safe?

The rest looks great to me!

neal
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