From: John Heffner <jheffner@psc.edu>
To: Stephen Hemminger <shemminger@osdl.org>
Cc: David Miller <davem@davemloft.net>, netdev <netdev@vger.kernel.org>
Subject: Re: simplify microsecond rtt sampling
Date: Wed, 27 Sep 2006 12:21:45 +0100 [thread overview]
Message-ID: <451A5EC9.6090001@psc.edu> (raw)
[-- Attachment #1: Type: text/plain, Size: 488 bytes --]
About commit 2d2abbab63f6726a147ae61ada39bf2c9ee0db9a:
It looks like this patch bypassed the enforcement of Karn's algorithm in
tcp_ack_no_tstamp() for the purposes of usec RTT sampling used by
congestion control modules. This will give them bad RTT data when there
are retransmits. I haven't actually observed this, but it seems like it
would be the case. ;) Please correct me if I'm wrong.
Here's a patch that should be a fix.
Signed-off-by: John Heffner <jheffner@psc.edu>
[-- Attachment #2: rtt_sample_fix.patch --]
[-- Type: text/plain, Size: 1061 bytes --]
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 159fa3f..725c868 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -2306,8 +2306,6 @@ static int tcp_clean_rtx_queue(struct so
seq_rtt = -1;
} else if (seq_rtt < 0) {
seq_rtt = now - scb->when;
- if (rtt_sample)
- (*rtt_sample)(sk, tcp_usrtt(skb));
}
if (sacked & TCPCB_SACKED_ACKED)
tp->sacked_out -= tcp_skb_pcount(skb);
@@ -2320,8 +2318,6 @@ static int tcp_clean_rtx_queue(struct so
}
} else if (seq_rtt < 0) {
seq_rtt = now - scb->when;
- if (rtt_sample)
- (*rtt_sample)(sk, tcp_usrtt(skb));
}
tcp_dec_pcount_approx(&tp->fackets_out, skb);
tcp_packets_out_dec(tp, skb);
@@ -2333,6 +2329,8 @@ static int tcp_clean_rtx_queue(struct so
if (acked&FLAG_ACKED) {
tcp_ack_update_rtt(sk, acked, seq_rtt);
tcp_ack_packets_out(sk, tp);
+ if (rtt_sample && !(acked & FLAG_RETRANS_DATA_ACKED))
+ (*rtt_sample)(sk, tcp_usrtt(skb));
if (icsk->icsk_ca_ops->pkts_acked)
icsk->icsk_ca_ops->pkts_acked(sk, pkts_acked);
next reply other threads:[~2006-09-27 11:22 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2006-09-27 11:21 John Heffner [this message]
2006-09-27 15:13 ` simplify microsecond rtt sampling John Heffner
2006-09-28 12:35 ` John Heffner
2006-09-28 12:47 ` John Heffner
2006-09-28 16:36 ` Stephen Hemminger
2006-09-28 21:49 ` David Miller
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