Patch "tcp: allow TLP in ECN CWR" has been added to the 4.15-stable tree

[<gregkh@linuxfoundation.org>]

This is a note to let you know that I've just added the patch titled

    tcp: allow TLP in ECN CWR

to the 4.15-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     tcp-allow-tlp-in-ecn-cwr.patch
and it can be found in the queue-4.15 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


>From foo@baz Thu Mar 22 14:03:39 CET 2018
From: Neal Cardwell <ncardwell@google.com>
Date: Mon, 11 Dec 2017 15:42:53 -0800
Subject: tcp: allow TLP in ECN CWR

From: Neal Cardwell <ncardwell@google.com>


[ Upstream commit b4f70c3d4ec32a2ff4c62e1e2da0da5f55fe12bd ]

This patch enables tail loss probe in cwnd reduction (CWR) state
to detect potential losses. Prior to this patch, since the sender
uses PRR to determine the cwnd in CWR state, the combination of
CWR+PRR plus tcp_tso_should_defer() could cause unnecessary stalls
upon losses: PRR makes cwnd so gentle that tcp_tso_should_defer()
defers sending wait for more ACKs. The ACKs may not come due to
packet losses.

Disallowing TLP when there is unused cwnd had the primary effect
of disallowing TLP when there is TSO deferral, Nagle deferral,
or we hit the rwin limit. Because basically every application
write() or incoming ACK will cause us to run tcp_write_xmit()
to see if we can send more, and then if we sent something we call
tcp_schedule_loss_probe() to see if we should schedule a TLP. At
that point, there are a few common reasons why some cwnd budget
could still be unused:

(a) rwin limit
(b) nagle check
(c) TSO deferral
(d) TSQ

For (d), after the next packet tx completion the TSQ mechanism
will allow us to send more packets, so we don't really need a
TLP (in practice it shouldn't matter whether we schedule one
or not). But for (a), (b), (c) the sender won't send any more
packets until it gets another ACK. But if the whole flight was
lost, or all the ACKs were lost, then we won't get any more ACKs,
and ideally we should schedule and send a TLP to get more feedback.
In particular for a long time we have wanted some kind of timer for
TSO deferral, and at least this would give us some kind of timer

Reported-by: Steve Ibanez <sibanez@stanford.edu>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Reviewed-by: Nandita Dukkipati <nanditad@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 net/ipv4/tcp_output.c |    9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -2440,15 +2440,12 @@ bool tcp_schedule_loss_probe(struct sock
 
 	early_retrans = sock_net(sk)->ipv4.sysctl_tcp_early_retrans;
 	/* Schedule a loss probe in 2*RTT for SACK capable connections
-	 * in Open state, that are either limited by cwnd or application.
+	 * not in loss recovery, that are either limited by cwnd or application.
 	 */
 	if ((early_retrans != 3 && early_retrans != 4) ||
 	    !tp->packets_out || !tcp_is_sack(tp) ||
-	    icsk->icsk_ca_state != TCP_CA_Open)
-		return false;
-
-	if ((tp->snd_cwnd > tcp_packets_in_flight(tp)) &&
-	     !tcp_write_queue_empty(sk))
+	    (icsk->icsk_ca_state != TCP_CA_Open &&
+	     icsk->icsk_ca_state != TCP_CA_CWR))
 		return false;
 
 	/* Probe timeout is 2*rtt. Add minimum RTO to account


Patches currently in stable-queue which might be from ncardwell@google.com are

queue-4.15/tcp-allow-tlp-in-ecn-cwr.patch