selftests: net: Fix slow configurations in big_tcp_tunnels.sh

The combination of checksum offload disabled (that causes software GSO)
and a debug kernel 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.

Fixes: 5cb53743e1ff ("selftests: net: Add a test for BIG TCP in UDP tunnels")
Signed-off-by: Alice Mikityanska <alice@isovalent.com>
Acked-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20260822120308.1165200-5-alice.kernel@fastmail.im
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/tools/testing/selftests/net/big_tcp_tunnels.sh b/tools/testing/selftests/net/big_tcp_tunnels.sh
index 003878df7415facbfa49885f5386d04da83976b6..cc0875e52fb967e624018a370cd83ad5caa434a0 100755 (executable)
--- a/tools/testing/selftests/net/big_tcp_tunnels.sh
+++ b/tools/testing/selftests/net/big_tcp_tunnels.sh
@@ -49,6 +49,8 @@ setup() {
 
        ip netns exec "$SERVER_NS" netserver >/dev/null
        wait_local_port_listen "$SERVER_NS" 12865 tcp
+
+       DEFAULT_TCP_MIN_TSO_SEGS=$(ip netns exec "$CLIENT_NS" sysctl -n net.ipv4.tcp_min_tso_segs)
 }
 
 setup_tunnel() {
@@ -135,6 +137,21 @@ do_test() {
                IPTABLES_SACK=ip6tables
        fi
 
+       if [ "$3" != 'on' ] && [ "$KSFT_MACHINE_SLOW" = yes ]; then
+               echo 'Slow configuration; increasing net.ipv4.tcp_min_tso_segs and initcwnd'
+               ip netns exec "$CLIENT_NS" sysctl -w net.ipv4.tcp_min_tso_segs=52
+               if [ "$2" = 4 ]; then
+                       ip -netns "$CLIENT_NS" \
+                           route change 192.168.2.0/24 dev tun0 initcwnd 100
+               else
+                       ip -netns "$CLIENT_NS" -6 \
+                           route change 2001:db8::2:0/112 dev tun0 initcwnd 100
+               fi
+       else
+               ip netns exec "$CLIENT_NS" \
+                   sysctl -w net.ipv4.tcp_min_tso_segs="$DEFAULT_TCP_MIN_TSO_SEGS"
+       fi
+
        ip netns exec "$SERVER_NS" "$IPTABLES" -w -t raw -I PREROUTING -i "${CAPTURE_IFACE}1" -m length ! --length 0:65535 -m comment --comment "bigtcp"
        ip netns exec "$CLIENT_NS" "$IPTABLES" -w -t raw -I OUTPUT -o "${CAPTURE_IFACE}0" -m length ! --length 0:65535 -m comment --comment "bigtcp"
        ip netns exec "$SERVER_NS" "$IPTABLES_SACK" -w -t raw -I OUTPUT -o "tun1" -p tcp -m tcp --tcp-flags ACK ACK --tcp-option 5 -m comment --comment "sack"