Re: [PATCH net-next 4/4] selftests: net: Fix slow configurations in big_tcp_tunnels.sh

On Wed, Aug 19, 2026, at 12:25, Matthieu Baerts wrote:
> Hi Alice,
>
> On 14/08/2026 21:41, Alice Mikityanska wrote:
>> From: Alice Mikityanska <[email protected]>
>> 
>> The combination of checksum offload disabled (that causes software GSO)
>> and CONFIG_DEBUG_NET=y is inherently slow. Depending on the CPU power
>> and load, RTT may increase, limiting sk_pacing_rate, so tcp_tso_autosize
>> caps SKBs at around 40 segments, and zero BIG TCP packets are produced.
>> 
>> Increase sysctl net.ipv4.tcp_min_tso_segs and set a bigger initial value
>> of CWND in these configurations to force BIG TCP.
> Just an innocent question: would it affect the test somehow if the
> sysctl and initial cwnd is always modified, not just for a specific config?

That would guarantee that BIG TCP kicks in, but IMO it reduces the
coverage by not letting BIG TCP kick in naturally. I'd keep the defaults
in fast tests.

> (Note that apart from looking at ${KSFT_MACHINE_SLOW} in patch 2, I
> think the series looks good to me, and can probably be sent to net after
> the merge with net-next.)

Thanks! I'll wait for the selftest to be merged to net and send the follow-
ups there.

> Cheers,
> Matt