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

[nramaswamy@openai.com]

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