[PATCH net v2 0/2] tcp: preserve RACK tracking across partial undo

[PATCH net v2 0/2] tcp: preserve RACK tracking across partial undo

[nramaswamy]

From: Neil Ramaswamy <nramaswamy@openai.com>

Partial undo can clear the TCPCB_LOST flag on segments already removed
from RACK's list, which prevents subsequent RACK loss detection, leading
to segments only being retransmitted after the RTO. Restoring them to
the RACK list as part of partial undo makes sure we can properly
reconsider them for fast retransmission in the future.

To do this, we first sort (by transmission time) the segments whose
TCPCB_LOST flag is being cleared and then linearly insert those back
into the RACK list, which is sorted by transmission time.

My investigation started from seeing repeated TCP stalls in prod and the
mitigation that seemed to prevent these stalls was limiting SO_SNDBUF to
96 KiB. It also seems like others have seen similar symptoms before [1].

[1]
https://lore.kernel.org/netdev/35A4DDAA-7E8D-43CB-A1F5-D1E46A4ED42E@gmail.com/

Changes in v2:
- Added production symptoms
- Removed the redundant TCPCB_LOST comparison
- Fixed lint/long lines

Patch 2 is unchanged.

v1:
https://lore.kernel.org/netdev/20260926002520.42955-4-nramaswamy@openai.com/

Neil Ramaswamy (2):
  tcp: restore RACK list membership when undoing loss
  selftests: net: packetdrill: test RACK after partial undo

 net/ipv4/tcp_input.c                          | 35 ++++++++++++
 ...tcp_partial_undo-restores-to-rack-list.pkt | 54 +++++++++++++++++++
 2 files changed, 89 insertions(+)
 create mode 100644 tools/testing/selftests/net/packetdrill/tcp_partial_undo-restores-to-rack-list.pkt


base-commit: 11536ee3d3e0b1bd35b6f3f8df55a6053eb0c71d
-- 
2.55.0

[PATCH net v2 1/2] tcp: restore RACK list membership when undoing loss

[nramaswamy]

From: Neil Ramaswamy <nramaswamy@openai.com>

Partial undo can clear the TCPCB_LOST flag on segments already removed
from RACK's list, which prevents subsequent RACK loss detection, leading
to segments only being retransmitted after the RTO. Restoring them to
the RACK list as part of partial undo makes sure we can properly
reconsider them for fast retransmission in the future.

To do this, we first sort (by transmission time) the segments whose
TCPCB_LOST flag is being cleared and then linearly insert those back
into the RACK list, which is sorted by transmission time.

My investigation started from seeing repeated TCP stalls in prod and the
mitigation that seemed to prevent these stalls was limiting SO_SNDBUF to
96 KiB. It also seems like others have seen similar symptoms before [1].

[1]
https://lore.kernel.org/netdev/35A4DDAA-7E8D-43CB-A1F5-D1E46A4ED42E@gmail.com/

Fixes: 043b87d7599e ("tcp: more efficient RACK loss detection")
Signed-off-by: Neil Ramaswamy <nramaswamy@openai.com>
Assisted-by: LLM sparse
---
 net/ipv4/tcp_input.c | 35 +++++++++++++++++++++++++++++++++++
 1 file changed, 35 insertions(+)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 92bc60716f33..51f04c7474dc 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -69,6 +69,7 @@
 #include <linux/module.h>
 #include <linux/sysctl.h>
 #include <linux/kernel.h>
+#include <linux/list_sort.h>
 #include <linux/prefetch.h>
 #include <linux/bitops.h>
 #include <net/dst.h>
@@ -2840,16 +2841,50 @@ static void DBGUNDO(struct sock *sk, const char *msg)
 #endif
 }
 
+static int tcp_rack_skb_cmp(void *priv, const struct list_head *a,
+			    const struct list_head *b)
+{
+	const struct sk_buff *skb_a = list_entry(a, struct sk_buff,
+					       tcp_tsorted_anchor);
+	const struct sk_buff *skb_b = list_entry(b, struct sk_buff,
+					       tcp_tsorted_anchor);
+
+	return tcp_skb_sent_after(tcp_skb_timestamp_us(skb_a),
+				  tcp_skb_timestamp_us(skb_b),
+				  TCP_SKB_CB(skb_a)->end_seq,
+				  TCP_SKB_CB(skb_b)->end_seq);
+}
+
 static void tcp_undo_cwnd_reduction(struct sock *sk, bool unmark_loss)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
 
 	if (unmark_loss) {
+		LIST_HEAD(restored);
 		struct sk_buff *skb;
 
 		skb_rbtree_walk(skb, &sk->tcp_rtx_queue) {
+			if (TCP_SKB_CB(skb)->sacked & TCPCB_LOST)
+				list_move_tail(&skb->tcp_tsorted_anchor,
+					       &restored);
 			TCP_SKB_CB(skb)->sacked &= ~TCPCB_LOST;
 		}
+		if (!list_empty(&restored)) {
+			struct list_head *pos = &tp->tsorted_sent_queue;
+
+			/* Ensure lost skbs are added in transmission order */
+			list_sort(NULL, &restored, tcp_rack_skb_cmp);
+			while (!list_empty(&restored)) {
+				struct list_head *entry = restored.next;
+
+				while (pos->next != &tp->tsorted_sent_queue &&
+				       !tcp_rack_skb_cmp(NULL, pos->next,
+						 entry))
+					pos = pos->next;
+				list_move(entry, pos);
+				pos = entry;
+			}
+		}
 		tp->lost_out = 0;
 		tcp_clear_all_retrans_hints(tp);
 	}
-- 
2.55.0

[PATCH net v2 2/2] selftests: net: packetdrill: test RACK after partial undo

[nramaswamy]

From: Neil Ramaswamy <nramaswamy@openai.com>

Reproduces a partial undo bug where a segment is unmarked as lost but is
never returned to RACK's timestamp sorted list, which makes it ineligible
for future fast retransmission.

Signed-off-by: Neil Ramaswamy <nramaswamy@openai.com>
Assisted-by: LLM sparse
---
 ...tcp_partial_undo-restores-to-rack-list.pkt | 54 +++++++++++++++++++
 1 file changed, 54 insertions(+)
 create mode 100644 tools/testing/selftests/net/packetdrill/tcp_partial_undo-restores-to-rack-list.pkt

diff --git a/tools/testing/selftests/net/packetdrill/tcp_partial_undo-restores-to-rack-list.pkt b/tools/testing/selftests/net/packetdrill/tcp_partial_undo-restores-to-rack-list.pkt
new file mode 100644
index 000000000000..c07a2f8a5cbd
--- /dev/null
+++ b/tools/testing/selftests/net/packetdrill/tcp_partial_undo-restores-to-rack-list.pkt
@@ -0,0 +1,54 @@
+// SPDX-License-Identifier: GPL-2.0
+//
+// Test that a segment unmarked as lost during partial undo is eligible
+// for future fast retransmission.
+
+`./defaults.sh`
+
+// Establish a connection with a 100 ms RTT and a 1000-byte payload MSS.
+// Linux subtracts the 12-byte timestamp options from the advertised MSS.
+    0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
+   +0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
+   +0 bind(3, ..., ...) = 0
+   +0 listen(3, 1) = 0
+
+  +.1 < S 0:0(0) win 20000 <mss 1012,sackOK,TS val 1000 ecr 0>
+   +0 > S. 0:0(0) ack 1 <mss 1460,sackOK,TS val 100 ecr 1000>
+  +.1 < . 1:1(0) ack 1 win 20000 <nop,nop,TS val 1100 ecr 100>
+   +0 accept(3, ..., ...) = 4
+
+// Send A, B, C and D, then E 47 ms after D.
+ +.01 write(4, ..., 1000) = 1000
+   +0 > P. 1:1001(1000) ack 1 <nop,nop,TS val 210 ecr 1100>
++.001 write(4, ..., 1000) = 1000
+   +0 > P. 1001:2001(1000) ack 1 <...>
++.001 write(4, ..., 1000) = 1000
+   +0 > P. 2001:3001(1000) ack 1 <...>
++.001 write(4, ..., 1000) = 1000
+   +0 > P. 3001:4001(1000) ack 1 <...>
++.047 write(4, ..., 1000) = 1000
+   +0 > P. 4001:5001(1000) ack 1 <...>
+
+// SACK C and E together. RACK marks A, B and D lost. D is old enough
+// to be retransmitted, but this ACK reports only two newly delivered
+// segments, allowing A and B to be retransmitted while D waits.
+ +.12 < . 1:1(0) ack 1 win 20000 <TS val 1280 ecr 100,sack 4001:5001 2001:3001>
+   +0 > P. 1:1001(1000) ack 1 <...>
+   +0 > P. 1001:2001(1000) ack 1 <...>
+   +0 %{
+assert tcpi_ca_state == TCP_CA_Recovery, tcpi_ca_state
+assert tcpi_lost == 3, tcpi_lost
+assert tcpi_retrans == 2, tcpi_retrans
+}%
+
+// Deliver retransmitted B and then the original A. D stays missing,
+// and we ACK through C with A's original timestamp, which is before
+// retransmission started. This triggers partial undo, but D should
+// remain eligible for fast retransmission (critically, the timeout
+// retransmission counter should be 0).
+ +.07 < . 1:1(0) ack 3001 win 20000 <TS val 1350 ecr 210,sack 4001:5001>
+ +0~+.05 > P. 3001:4001(1000) ack 1 <nop,nop,TS val 450 ecr 1350>
+   +0 %{ assert tcpi_retransmits == 0, tcpi_retransmits }%
+
+// Acknowledge all five segments.
+ +.01 < . 1:1(0) ack 5001 win 20000 <nop,nop,TS val 1360 ecr 450>
-- 
2.55.0

Re: [PATCH net v2 1/2] tcp: restore RACK list membership when undoing loss

[Eric Dumazet]

On 10/6/26 07:15, nramaswamy@openai.com wrote:
> From: Neil Ramaswamy <nramaswamy@openai.com>
> 
> Partial undo can clear the TCPCB_LOST flag on segments already removed
> from RACK's list, which prevents subsequent RACK loss detection, leading
> to segments only being retransmitted after the RTO. Restoring them to
> the RACK list as part of partial undo makes sure we can properly
> reconsider them for fast retransmission in the future.
> 
> To do this, we first sort (by transmission time) the segments whose
> TCPCB_LOST flag is being cleared and then linearly insert those back
> into the RACK list, which is sorted by transmission time.
> 
> My investigation started from seeing repeated TCP stalls in prod and the
> mitigation that seemed to prevent these stalls was limiting SO_SNDBUF to
> 96 KiB. It also seems like others have seen similar symptoms before [1].
> 
> [1]
> https://lore.kernel.org/netdev/35A4DDAA-7E8D-43CB-A1F5-D1E46A4ED42E@gmail.com/
> 
> Fixes: 043b87d7599e ("tcp: more efficient RACK loss detection")
> Signed-off-by: Neil Ramaswamy <nramaswamy@openai.com>
> Assisted-by: LLM sparse
> ---
>   net/ipv4/tcp_input.c | 35 +++++++++++++++++++++++++++++++++++
>   1 file changed, 35 insertions(+)
> 
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index 92bc60716f33..51f04c7474dc 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -69,6 +69,7 @@
>   #include <linux/module.h>
>   #include <linux/sysctl.h>
>   #include <linux/kernel.h>
> +#include <linux/list_sort.h>
>   #include <linux/prefetch.h>
>   #include <linux/bitops.h>
>   #include <net/dst.h>
> @@ -2840,16 +2841,50 @@ static void DBGUNDO(struct sock *sk, const char *msg)
>   #endif
>   }
>   
> +static int tcp_rack_skb_cmp(void *priv, const struct list_head *a,
> +			    const struct list_head *b)
> +{
> +	const struct sk_buff *skb_a = list_entry(a, struct sk_buff,
> +					       tcp_tsorted_anchor);
> +	const struct sk_buff *skb_b = list_entry(b, struct sk_buff,
> +					       tcp_tsorted_anchor);
> +
> +	return tcp_skb_sent_after(tcp_skb_timestamp_us(skb_a),
> +				  tcp_skb_timestamp_us(skb_b),
> +				  TCP_SKB_CB(skb_a)->end_seq,
> +				  TCP_SKB_CB(skb_b)->end_seq);

Patch LGTM, but perhaps we could avoid the two div_u64() calls here.


	return tcp_skb_sent_after(skb_a->skb_mstamp_ns,
				  skb_b->skb_mstamp_ns,
				  TCP_SKB_CB(skb_a)->end_seq,
				  TCP_SKB_CB(skb_b)->end_seq);



> +}
> +
>   static void tcp_undo_cwnd_reduction(struct sock *sk, bool unmark_loss)
>   {
>   	struct tcp_sock *tp = tcp_sk(sk);
>   
>   	if (unmark_loss) {
> +		LIST_HEAD(restored);
>   		struct sk_buff *skb;
>   
>   		skb_rbtree_walk(skb, &sk->tcp_rtx_queue) {
> +			if (TCP_SKB_CB(skb)->sacked & TCPCB_LOST)
> +				list_move_tail(&skb->tcp_tsorted_anchor,
> +					       &restored);
>   			TCP_SKB_CB(skb)->sacked &= ~TCPCB_LOST;
>   		}
> +		if (!list_empty(&restored)) {
> +			struct list_head *pos = &tp->tsorted_sent_queue;
> +
> +			/* Ensure lost skbs are added in transmission order */
> +			list_sort(NULL, &restored, tcp_rack_skb_cmp);
> +			while (!list_empty(&restored)) {
> +				struct list_head *entry = restored.next;
> +
> +				while (pos->next != &tp->tsorted_sent_queue &&
> +				       !tcp_rack_skb_cmp(NULL, pos->next,
> +						 entry))
> +					pos = pos->next;
> +				list_move(entry, pos);
> +				pos = entry;
> +			}
> +		}
>   		tp->lost_out = 0;
>   		tcp_clear_all_retrans_hints(tp);
>   	}

Re: [PATCH net v2 2/2] selftests: net: packetdrill: test RACK after partial undo

[Eric Dumazet]

On 10/6/26 07:15, nramaswamy@openai.com wrote:
> From: Neil Ramaswamy <nramaswamy@openai.com>
> 
> Reproduces a partial undo bug where a segment is unmarked as lost but is
> never returned to RACK's timestamp sorted list, which makes it ineligible
> for future fast retransmission.
> 
> Signed-off-by: Neil Ramaswamy <nramaswamy@openai.com>
> Assisted-by: LLM sparse
> ---

Thanks for this test!

Reviewed-by: Eric Dumazet <edumazet@kernel.org>