Re: [PATCH mptcp-next] selftests: mptcp: more stable simult_flows tests

[Matthieu Baerts <matttbe@kernel.org>]

Hi Paolo,

On 16/02/2026 22:20, Paolo Abeni wrote:
> By default, the netem qdisc can keep up to 1000 packets under its belly
> to deal with the configured rate and delay. The simult flows test-case
> simulates very low speed links, to avoid problems due to slow CPUs and
> the TCP stack tend to transmit at a slightly higher rate than the
> (virtual) link constraints.
> 
> All the above causes a relatively large amount of packets being enqueued
> in the netem qdiscs - the longer the transfer, the longer the queue -
> producing increasingly high TCP RTT samples and consequently increasingly
> larger receive buffer size due to DRS.
> 
> When the receive buffer size becomes considerably larger than the needed
> size, the tests results can flake, i.e. because minimal inaccuracy in the
> pacing rate can lead to a single subflow usage towards the end of the
> connection for a considerable amount of data.
> 
> Address the issue explicitly setting netem limits suitable for the
> configured link speeds and unflake all the affected tests.

Now in our tree:

New patches for t/upstream-net and t/upstream:
- d4a47ee7f8da: selftests: mptcp: more stable simult_flows tests
- Results: cf97a02cb523..656b5899eb47 (export-net)
- Results: 40e2b2255e42..683fca3ca9ae (export)

New patches for t/upstream:
- 86c0019571dc: tg: revert 'mark some simult flows tests as flaky'
- Results: 683fca3ca9ae..eb01783b4b05 (export)


Tests are now in progress:

- export:
https://github.com/multipath-tcp/mptcp_net-next/commit/ea2e74b9a2b44abc3e5e102cb25cf415b3eb3727/checks
- export-net:
https://github.com/multipath-tcp/mptcp_net-next/commit/abb26bf620660fa96804166f9452ad49d6b14c18/checks

Cheers,
Matt
-- 
Sponsored by the NGI0 Core fund.