From mboxrd@z Thu Jan 1 00:00:00 1970 From: Stephen Hemminger Subject: Re: [PATCH] tcp_cubic: enable TCP timestamps Date: Tue, 8 Mar 2011 11:17:57 -0800 Message-ID: <20110308111757.7db14e58@nehalam> References: <20110308080926.GA22641@xanadu.blop.info> <20110308104211.3567526f@nehalam> <20110308.105503.104060436.davem@davemloft.net> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Cc: lucas.nussbaum@loria.fr, netdev@vger.kernel.org, sha2@ncsu.edu To: David Miller Return-path: Received: from mail.vyatta.com ([76.74.103.46]:57422 "EHLO mail.vyatta.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754438Ab1CHTSA (ORCPT ); Tue, 8 Mar 2011 14:18:00 -0500 In-Reply-To: <20110308.105503.104060436.davem@davemloft.net> Sender: netdev-owner@vger.kernel.org List-ID: On Tue, 08 Mar 2011 10:55:03 -0800 (PST) David Miller wrote: Complete the sentence... > on old desktops. But without it enabling RTT_STAMP, packets that get acked in less than a jiffie (1 - 10 ms) will be processed with no rtt value (-1); this causes hystart never to be invoked on local connections. Maybe the following would be better, it passes 0 as RTT when not using RTT_STAMP for quick packets. It turns out the Cubic ends up converting rtt_us back to jiffies anyway, so RTT_STAMP is not really needed. ---- Subject: tcp: fix RTT for quick packets in congestion control In the congestion control interface, the callback for each ACK includes an estimated round trip time in microseconds. Some algorithms need high resolution (Vegas style) but most only need jiffie resolution. If RTT is not accurate (like a retransmission) -1 is used as a flag value. When doing coarse resolution if RTT is less than a a jiffie then 0 should be returned rather than no estimate. Otherwise algorithms that expect good ack's to trigger slow start (like CUBIC Hystart) will be confused. Signed-off-by: Stephen Hemminger --- a/net/ipv4/tcp_input.c 2011-03-08 11:11:26.093183654 -0800 +++ b/net/ipv4/tcp_input.c 2011-03-08 11:11:46.641404939 -0800 @@ -3350,7 +3350,7 @@ static int tcp_clean_rtx_queue(struct so net_invalid_timestamp())) rtt_us = ktime_us_delta(ktime_get_real(), last_ackt); - else if (ca_seq_rtt > 0) + else if (ca_seq_rtt >= 0) rtt_us = jiffies_to_usecs(ca_seq_rtt); }