Neal Cardwell found that recent kernels were having RWIN limited
issues, even when net.ipv4.tcp_rmem[2] was set to a very big value like
512MB.

He suspected that tcp_stream default buffer size (64KB) was triggering
heuristic added in ea33537d8292 ("tcp: add receive queue awareness
in tcp_rcv_space_adjust()").

After more testing, it turns out the bug was added earlier
with commit 65c5287892e9 ("tcp: fix sk_rcvbuf overshoot").

I forgot once again that DRS has one RTT latency.

MPTCP also got the same issue.

This series :

- Prevent calling tcp_rcvbuf_grow() on some MPTCP subflows.
- adds rcv_ssthresh, window_clamp and rcv_wnd to trace_tcp_rcvbuf_grow().
- Refactors code in a patch with no functional changes.
- Fixes the issue in the final patch.

Signed-off-by: Eric Dumazet <edumazet@google.com>
[ Added patch 1/4. ]
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
Changes in v3:
- Fix warnings at build time by moving 'oldval' declaration (Matthieu)
- Prevent possible divide by zero issue in mptcp_rcv_space_adjust() (Paolo)
- Note: this v3 is not being sent by Eric because he is unavailable.
- Link to v2: https://patch.msgid.link/20251027073809.2112498-1-edumazet@google.com
Changes in v2:
- Rebased to net tree
- Changed mptcp_rcvbuf_grow() to read/write msk->rcvq_space.space (Paolo)
- Link to v1: https://patch.msgid.link/20251024075027.3178786-1-edumazet@google.com

---
Eric Dumazet (3):
      trace: tcp: add three metrics to trace_tcp_rcvbuf_grow()
      tcp: add newval parameter to tcp_rcvbuf_grow()
      tcp: fix too slow tcp_rcvbuf_grow() action

Paolo Abeni (1):
      mptcp: fix subflow rcvbuf adjust

 include/net/tcp.h          |  2 +-
 include/trace/events/tcp.h |  9 +++++++++
 net/ipv4/tcp_input.c       | 21 ++++++++++++++-------
 net/mptcp/protocol.c       | 26 +++++++++++++++++---------
 4 files changed, 41 insertions(+), 17 deletions(-)
---
base-commit: 210b35d6a7ea415494ce75490c4b43b4e717d935
change-id: 20251028-net-tcp-recv-autotune-5876d6d85d8a

Best regards,
-- 
Matthieu Baerts (NGI0) <matttbe@kernel.org>

Hello:

This series was applied to netdev/net.git (main)
by Jakub Kicinski <kuba@kernel.org>:

On Tue, 28 Oct 2025 12:57:58 +0100 you wrote:
> Neal Cardwell found that recent kernels were having RWIN limited
> issues, even when net.ipv4.tcp_rmem[2] was set to a very big value like
> 512MB.
> 
> He suspected that tcp_stream default buffer size (64KB) was triggering
> heuristic added in ea33537d8292 ("tcp: add receive queue awareness
> in tcp_rcv_space_adjust()").
> 
> [...]

Here is the summary with links:
  - [net,v3,1/4] mptcp: fix subflow rcvbuf adjust
    https://git.kernel.org/netdev/net/c/a6f0459aadf1
  - [net,v3,2/4] trace: tcp: add three metrics to trace_tcp_rcvbuf_grow()
    https://git.kernel.org/netdev/net/c/24990d89c23d
  - [net,v3,3/4] tcp: add newval parameter to tcp_rcvbuf_grow()
    https://git.kernel.org/netdev/net/c/b1e014a1f327
  - [net,v3,4/4] tcp: fix too slow tcp_rcvbuf_grow() action
    https://git.kernel.org/netdev/net/c/aa251c84636c

You are awesome, thank you!
-- 
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