selftests/net: packetdrill: import tcp/slow_start

Same import process as previous tests.

Also add CONFIG_NET_SCH_FQ to config, as one test uses that.

Same test process as previous tests. Both with and without debug mode.
Recording the steps once:

make mrproper
vng --build \
        --config tools/testing/selftests/net/packetdrill/config \
        --config kernel/configs/debug.config
vng -v --run . --user root --cpus 4 -- \
	make -C tools/testing/selftests TARGETS=net/packetdrill run_tests

Link: https://github.com/linux-netdev/nipa/wiki/How-to-run-netdev-selftests-CI-style#how-to-build
Signed-off-by: Willem de Bruijn <willemb@google.com>
Acked-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20240912005317.1253001-4-willemdebruijn.kernel@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

11 files changed, 427 insertions, 0 deletions

diff --git a/tools/testing/selftests/net/packetdrill/config b/tools/testing/selftests/net/packetdrill/config
index a7877819081f..0237ed98f3c0 100644
--- a/tools/testing/selftests/net/packetdrill/config
+++ b/tools/testing/selftests/net/packetdrill/config
@@ -3,6 +3,7 @@ CONFIG_HZ_1000=y
 CONFIG_HZ=1000
 CONFIG_NET_NS=y
 CONFIG_NET_SCH_FIFO=y
+CONFIG_NET_SCH_FQ=y
 CONFIG_PROC_SYSCTL=y
 CONFIG_SYN_COOKIES=y
 CONFIG_TCP_CONG_CUBIC=y
diff --git a/tools/testing/selftests/net/packetdrill/tcp_slow_start_slow-start-ack-per-1pkt.pkt b/tools/testing/selftests/net/packetdrill/tcp_slow_start_slow-start-ack-per-1pkt.pkt
new file mode 100644
index 000000000000..795c476d222d
--- /dev/null
+++ b/tools/testing/selftests/net/packetdrill/tcp_slow_start_slow-start-ack-per-1pkt.pkt
@@ -0,0 +1,56 @@
+// SPDX-License-Identifier: GPL-2.0
+// Test of slow start when not application-limited, so that
+// the cwnd continues to grow.
+// In this variant, the receiver ACKs every packet.
+
+// Set up config. To keep things simple, disable the
+// mechanism that defers sending in order to send bigger TSO packets.
+`./defaults.sh
+sysctl -q net.ipv4.tcp_tso_win_divisor=100`
+
+    0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
+   +0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
+   +0 bind(3, ..., ...) = 0
+   +0 listen(3, 1) = 0
+
+  +.1 < S 0:0(0) win 32792 <mss 1000,sackOK,nop,nop,nop,wscale 7>
+   +0 > S. 0:0(0) ack 1 <mss 1460,nop,nop,sackOK,nop,wscale 8>
+  +.1 < . 1:1(0) ack 1 win 257
+   +0 accept(3, ..., ...) = 4
+   +0 setsockopt(4, SOL_SOCKET, SO_SNDBUF, [200000], 4) = 0
+
+   +0 write(4, ..., 30000) = 30000
+   +0 > P. 1:10001(10000) ack 1
+   +0 %{ assert tcpi_snd_cwnd == 10, tcpi_snd_cwnd }%
+
++.105 < . 1:1(0) ack 1001 win 257
+   +0 > P. 10001:12001(2000) ack 1
+
+   +0 < . 1:1(0) ack 2001 win 257
+   +0 > P. 12001:14001(2000) ack 1
+
++.005 < . 1:1(0) ack 3001 win 257
+   +0 > P. 14001:16001(2000) ack 1
+
+   +0 < . 1:1(0) ack 4001 win 257
+   +0 > P. 16001:18001(2000) ack 1
+
++.005 < . 1:1(0) ack 5001 win 257
+   +0 > P. 18001:20001(2000) ack 1
+
+   +0 < . 1:1(0) ack 6001 win 257
+   +0 > P. 20001:22001(2000) ack 1
+
++.005 < . 1:1(0) ack 7001 win 257
+   +0 > P. 22001:24001(2000) ack 1
+
+   +0 < . 1:1(0) ack 8001 win 257
+   +0 > P. 24001:26001(2000) ack 1
+
++.005 < . 1:1(0) ack 9001 win 257
+   +0 > P. 26001:28001(2000) ack 1
+
+   +0 < . 1:1(0) ack 10001 win 257
+   +0 > P. 28001:30001(2000) ack 1
+
+   +0 %{ assert tcpi_snd_cwnd == 20, tcpi_snd_cwnd }%
diff --git a/tools/testing/selftests/net/packetdrill/tcp_slow_start_slow-start-ack-per-2pkt-send-5pkt.pkt b/tools/testing/selftests/net/packetdrill/tcp_slow_start_slow-start-ack-per-2pkt-send-5pkt.pkt
new file mode 100644
index 000000000000..9212ae1fd0f2
--- /dev/null
+++ b/tools/testing/selftests/net/packetdrill/tcp_slow_start_slow-start-ack-per-2pkt-send-5pkt.pkt
@@ -0,0 +1,33 @@
+// SPDX-License-Identifier: GPL-2.0
+// Test of slow start when an outstanding flight of packets is
+// less than the current cwnd, and not big enough to bump up cwnd.
+//
+// In this variant, the receiver ACKs every other packet,
+// approximating standard delayed ACKs.
+
+// Set up config.
+`./defaults.sh`
+
+    0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
+   +0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
+   +0 bind(3, ..., ...) = 0
+   +0 listen(3, 1) = 0
+
+   +0 < S 0:0(0) win 32792 <mss 1000,sackOK,nop,nop,nop,wscale 7>
+   +0 > S. 0:0(0) ack 1 <mss 1460,nop,nop,sackOK,nop,wscale 8>
+   +0 < . 1:1(0) ack 1 win 257
+   +0 accept(3, ..., ...) = 4
+
+// Only send 5 packets.
+   +0 write(4, ..., 5000) = 5000
+   +0 > P. 1:5001(5000) ack 1
+   +0 %{ assert tcpi_snd_cwnd == 10, tcpi_snd_cwnd }%
+
+   +0 < . 1:1(0) ack 2001 win 257
+   +0 %{ assert tcpi_snd_cwnd == 10, 'cwnd=%d' % tcpi_snd_cwnd }%
+
+   +0 < . 1:1(0) ack 4001 win 257
+   +0 %{ assert tcpi_snd_cwnd == 10, 'cwnd=%d' % tcpi_snd_cwnd }%
+
+   +0 < . 1:1(0) ack 5001 win 257
+   +0 %{ assert tcpi_snd_cwnd == 10, 'cwnd=%d' % tcpi_snd_cwnd }%
diff --git a/tools/testing/selftests/net/packetdrill/tcp_slow_start_slow-start-ack-per-2pkt-send-6pkt.pkt b/tools/testing/selftests/net/packetdrill/tcp_slow_start_slow-start-ack-per-2pkt-send-6pkt.pkt
new file mode 100644
index 000000000000..416c901ddf51
--- /dev/null
+++ b/tools/testing/selftests/net/packetdrill/tcp_slow_start_slow-start-ack-per-2pkt-send-6pkt.pkt
@@ -0,0 +1,34 @@
+// SPDX-License-Identifier: GPL-2.0
+// Test of slow start when an outstanding flight of packets is
+// less than the current cwnd, but still big enough that in slow
+// start we want to increase our cwnd a little.
+//
+// In this variant, the receiver ACKs every other packet,
+// approximating standard delayed ACKs.
+
+// Set up config.
+`./defaults.sh`
+
+    0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
+   +0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
+   +0 bind(3, ..., ...) = 0
+   +0 listen(3, 1) = 0
+
+   +0 < S 0:0(0) win 32792 <mss 1000,sackOK,nop,nop,nop,wscale 7>
+   +0 > S. 0:0(0) ack 1 <mss 1460,nop,nop,sackOK,nop,wscale 8>
+   +0 < . 1:1(0) ack 1 win 257
+   +0 accept(3, ..., ...) = 4
+
+// Only send 6 packets.
+   +0 write(4, ..., 6000) = 6000
+   +0 > P. 1:6001(6000) ack 1
+   +0 %{ assert tcpi_snd_cwnd == 10, tcpi_snd_cwnd }%
+
+   +0 < . 1:1(0) ack 2001 win 257
+   +0 %{ assert tcpi_snd_cwnd == 12, 'cwnd=%d' % tcpi_snd_cwnd }%
+
+   +0 < . 1:1(0) ack 4001 win 257
+   +0 %{ assert tcpi_snd_cwnd == 12, 'cwnd=%d' % tcpi_snd_cwnd }%
+
+   +0 < . 1:1(0) ack 6001 win 257
+   +0 %{ assert tcpi_snd_cwnd == 12, 'cwnd=%d' % tcpi_snd_cwnd }%
diff --git a/tools/testing/selftests/net/packetdrill/tcp_slow_start_slow-start-ack-per-2pkt.pkt b/tools/testing/selftests/net/packetdrill/tcp_slow_start_slow-start-ack-per-2pkt.pkt
new file mode 100644
index 000000000000..a894b7d4559c
--- /dev/null
+++ b/tools/testing/selftests/net/packetdrill/tcp_slow_start_slow-start-ack-per-2pkt.pkt
@@ -0,0 +1,42 @@
+// SPDX-License-Identifier: GPL-2.0
+// Test of slow start when not application-limited, so that
+// the cwnd continues to grow.
+// In this variant, the receiver ACKs every other packet,
+// approximating standard delayed ACKs.
+
+// Set up config. To keep things simple, disable the
+// mechanism that defers sending in order to send bigger TSO packets.
+`./defaults.sh
+sysctl -q net.ipv4.tcp_tso_win_divisor=100`
+
+    0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
+   +0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
+   +0 bind(3, ..., ...) = 0
+   +0 listen(3, 1) = 0
+
+  +.1 < S 0:0(0) win 32792 <mss 1000,sackOK,nop,nop,nop,wscale 7>
+   +0 > S. 0:0(0) ack 1 <mss 1460,nop,nop,sackOK,nop,wscale 8>
+  +.1 < . 1:1(0) ack 1 win 257
+   +0 accept(3, ..., ...) = 4
+   +0 setsockopt(4, SOL_SOCKET, SO_SNDBUF, [200000], 4) = 0
+
+   +0 write(4, ..., 30000) = 30000
+   +0 > P. 1:10001(10000) ack 1
+   +0 %{ assert tcpi_snd_cwnd == 10, tcpi_snd_cwnd }%
+
++.105 < . 1:1(0) ack 2001 win 257
+   +0 > P. 10001:14001(4000) ack 1
+
++.005 < . 1:1(0) ack 4001 win 257
+   +0 > P. 14001:18001(4000) ack 1
+
++.005 < . 1:1(0) ack 6001 win 257
+   +0 > P. 18001:22001(4000) ack 1
+
++.005 < . 1:1(0) ack 8001 win 257
+   +0 > P. 22001:26001(4000) ack 1
+
++.005 < . 1:1(0) ack 10001 win 257
+   +0 > P. 26001:30001(4000) ack 1
+
+   +0 %{ assert tcpi_snd_cwnd == 20, tcpi_snd_cwnd }%
diff --git a/tools/testing/selftests/net/packetdrill/tcp_slow_start_slow-start-ack-per-4pkt.pkt b/tools/testing/selftests/net/packetdrill/tcp_slow_start_slow-start-ack-per-4pkt.pkt
new file mode 100644
index 000000000000..065fae9e9abd
--- /dev/null
+++ b/tools/testing/selftests/net/packetdrill/tcp_slow_start_slow-start-ack-per-4pkt.pkt
@@ -0,0 +1,35 @@
+// SPDX-License-Identifier: GPL-2.0
+// Test of slow start when not application-limited, so that
+// the cwnd continues to grow.
+// In this variant, the receiver sends one ACK per 4 packets.
+
+// Set up config. To keep things simple, disable the
+// mechanism that defers sending in order to send bigger TSO packets.
+`./defaults.sh
+sysctl -q net.ipv4.tcp_tso_win_divisor=100`
+
+    0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
+   +0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
+   +0 bind(3, ..., ...) = 0
+   +0 listen(3, 1) = 0
+
+  +.1 < S 0:0(0) win 32792 <mss 1000,sackOK,nop,nop,nop,wscale 7>
+   +0 > S. 0:0(0) ack 1 <mss 1460,nop,nop,sackOK,nop,wscale 8>
+  +.1 < . 1:1(0) ack 1 win 257
+   +0 accept(3, ..., ...) = 4
+   +0 setsockopt(4, SOL_SOCKET, SO_SNDBUF, [200000], 4) = 0
+
+   +0 write(4, ..., 30000) = 30000
+   +0 > P. 1:10001(10000) ack 1
+   +0 %{ assert tcpi_snd_cwnd == 10, tcpi_snd_cwnd }%
+
+ +.11 < . 1:1(0) ack 4001 win 257
+   +0 > P. 10001:18001(8000) ack 1
+
+ +.01 < . 1:1(0) ack 8001 win 257
+   +0 > P. 18001:26001(8000) ack 1
+
++.005 < . 1:1(0) ack 10001 win 257
+   +0 > P. 26001:30001(4000) ack 1
+
+   +0 %{ assert tcpi_snd_cwnd == 20, tcpi_snd_cwnd }%
diff --git a/tools/testing/selftests/net/packetdrill/tcp_slow_start_slow-start-after-idle.pkt b/tools/testing/selftests/net/packetdrill/tcp_slow_start_slow-start-after-idle.pkt
new file mode 100644
index 000000000000..11b213be1138
--- /dev/null
+++ b/tools/testing/selftests/net/packetdrill/tcp_slow_start_slow-start-after-idle.pkt
@@ -0,0 +1,39 @@
+// SPDX-License-Identifier: GPL-2.0
+// Test of slow start after idle
+// This test expects tso size to be at least initial cwnd * mss
+
+`./defaults.sh
+./set_sysctls.py /proc/sys/net/ipv4/tcp_slow_start_after_idle=1 \
+		 /proc/sys/net/ipv4/tcp_min_tso_segs=10`
+
+    0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
+   +0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
+   +0 bind(3, ..., ...) = 0
+   +0 listen(3, 1) = 0
+
+   +0 < S 0:0(0) win 65535 <mss 1000,sackOK,nop,nop,nop,wscale 7>
+   +0 > S. 0:0(0) ack 1 <mss 1460,nop,nop,sackOK,nop,wscale 8>
+  +.1 < . 1:1(0) ack 1 win 511
+   +0 accept(3, ..., ...) = 4
+   +0 setsockopt(4, SOL_SOCKET, SO_SNDBUF, [200000], 4) = 0
+
+   +0 write(4, ..., 26000) = 26000
+   +0 > P. 1:5001(5000) ack 1
+   +0 > P. 5001:10001(5000) ack 1
+   +0 %{ assert tcpi_snd_cwnd == 10, tcpi_snd_cwnd }%
+
+  +.1 < . 1:1(0) ack 10001 win 511
+   +0 %{ assert tcpi_snd_cwnd == 20, tcpi_snd_cwnd }%
+   +0 > P. 10001:20001(10000) ack 1
+   +0 > P. 20001:26001(6000) ack 1
+
+  +.1 < . 1:1(0) ack 26001 win 511
+   +0 %{ assert tcpi_snd_cwnd == 36, tcpi_snd_cwnd }%
+
+   +2 write(4, ..., 20000) = 20000
+// If slow start after idle works properly, we should send 5 MSS here (cwnd/2)
+   +0 > P. 26001:31001(5000) ack 1
+   +0 %{ assert tcpi_snd_cwnd == 10, tcpi_snd_cwnd }%
+
+// Reset sysctls
+`/tmp/sysctl_restore_${PPID}.sh`
diff --git a/tools/testing/selftests/net/packetdrill/tcp_slow_start_slow-start-after-win-update.pkt b/tools/testing/selftests/net/packetdrill/tcp_slow_start_slow-start-after-win-update.pkt
new file mode 100644
index 000000000000..577ed8c8852c
--- /dev/null
+++ b/tools/testing/selftests/net/packetdrill/tcp_slow_start_slow-start-after-win-update.pkt
@@ -0,0 +1,50 @@
+// SPDX-License-Identifier: GPL-2.0
+// Test of slow start after window update
+// This test expects tso size to be at least initial cwnd * mss
+
+`./defaults.sh
+./set_sysctls.py /proc/sys/net/ipv4/tcp_slow_start_after_idle=1 \
+		 /proc/sys/net/ipv4/tcp_min_tso_segs=10`
+
+    0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
+   +0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
+   +0 bind(3, ..., ...) = 0
+   +0 listen(3, 1) = 0
+
+   +0 < S 0:0(0) win 65535 <mss 1000,sackOK,nop,nop,nop,wscale 7>
+   +0 > S. 0:0(0) ack 1 <mss 1460,nop,nop,sackOK,nop,wscale 8>
+  +.1 < . 1:1(0) ack 1 win 511
+   +0 accept(3, ..., ...) = 4
+   +0 setsockopt(4, SOL_SOCKET, SO_SNDBUF, [200000], 4) = 0
+
+   +0 write(4, ..., 26000) = 26000
+   +0 > P. 1:5001(5000) ack 1
+   +0 > P. 5001:10001(5000) ack 1
+   +0 %{ assert tcpi_snd_cwnd == 10, tcpi_snd_cwnd }%
+
+  +.1 < . 1:1(0) ack 10001 win 511
+   +0 %{ assert tcpi_snd_cwnd == 20, tcpi_snd_cwnd }%
+   +0 > P. 10001:20001(10000) ack 1
+   +0 > P. 20001:26001(6000) ack 1
+
+  +.1 < . 1:1(0) ack 26001 win 0
+   +0 %{ assert tcpi_snd_cwnd == 36, tcpi_snd_cwnd }%
+
+   +0 write(4, ..., 20000) = 20000
+// 1st win0 probe
++.3~+.310 > . 26000:26000(0) ack 1
+   +0 %{ assert tcpi_snd_cwnd == 36, tcpi_snd_cwnd }%
+
+// 2nd win0 probe
++.6~+.620 > . 26000:26000(0) ack 1
+   +0 %{ assert tcpi_snd_cwnd == 36, tcpi_snd_cwnd }%
+
+// 3rd win0 probe
++1.2~+1.240 > . 26000:26000(0) ack 1
+   +0 %{ assert tcpi_snd_cwnd == 36, tcpi_snd_cwnd }%
+
+  +.9 < . 1:1(0) ack 26001 win 511
+   +0 > P. 26001:31001(5000) ack 1
+
+// Reset sysctls
+`/tmp/sysctl_restore_${PPID}.sh`
diff --git a/tools/testing/selftests/net/packetdrill/tcp_slow_start_slow-start-app-limited-9-packets-out.pkt b/tools/testing/selftests/net/packetdrill/tcp_slow_start_slow-start-app-limited-9-packets-out.pkt
new file mode 100644
index 000000000000..869f32c35a2a
--- /dev/null
+++ b/tools/testing/selftests/net/packetdrill/tcp_slow_start_slow-start-app-limited-9-packets-out.pkt
@@ -0,0 +1,38 @@
+// SPDX-License-Identifier: GPL-2.0
+// Test of slow start when application-limited: in this case,
+// with IW10, if we don't fully use our cwnd but instead
+// send just 9 packets, then cwnd should grow to twice that
+// value, or 18 packets.
+
+// Set up config.
+`./defaults.sh`
+
+    0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
+   +0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
+   +0 bind(3, ..., ...) = 0
+   +0 listen(3, 1) = 0
+
+  +.1 < S 0:0(0) win 32792 <mss 1000,sackOK,nop,nop,nop,wscale 7>
+   +0 > S. 0:0(0) ack 1 <mss 1460,nop,nop,sackOK,nop,wscale 8>
+  +.1 < . 1:1(0) ack 1 win 257
+   +0 accept(3, ..., ...) = 4
+   +0 setsockopt(4, SOL_SOCKET, SO_SNDBUF, [200000], 4) = 0
+
+   +0 write(4, ..., 9000) = 9000
+   +0 > P. 1:9001(9000) ack 1
+   +0 %{ assert tcpi_snd_cwnd == 10, tcpi_snd_cwnd }%
+
++.105 < . 1:1(0) ack 2001 win 257
+   +0 %{ assert tcpi_snd_cwnd == 12, tcpi_snd_cwnd }%
+
++.005 < . 1:1(0) ack 4001 win 257
+   +0 %{ assert tcpi_snd_cwnd == 14, tcpi_snd_cwnd }%
+
++.005 < . 1:1(0) ack 6001 win 257
+   +0 %{ assert tcpi_snd_cwnd == 16, tcpi_snd_cwnd }%
+
++.005 < . 1:1(0) ack 8001 win 257
+   +0 %{ assert tcpi_snd_cwnd == 18, tcpi_snd_cwnd }%
+
++.005 < . 1:1(0) ack 9001 win 257
+   +0 %{ assert tcpi_snd_cwnd == 18, tcpi_snd_cwnd }%
diff --git a/tools/testing/selftests/net/packetdrill/tcp_slow_start_slow-start-app-limited.pkt b/tools/testing/selftests/net/packetdrill/tcp_slow_start_slow-start-app-limited.pkt
new file mode 100644
index 000000000000..0f77b7955db6
--- /dev/null
+++ b/tools/testing/selftests/net/packetdrill/tcp_slow_start_slow-start-app-limited.pkt
@@ -0,0 +1,36 @@
+// SPDX-License-Identifier: GPL-2.0
+// Test of slow start when application-limited: in this case,
+// with IW10, if we send exactly 10 packets then cwnd should grow to 20.
+
+// Set up config.
+`./defaults.sh`
+
+    0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
+   +0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
+   +0 bind(3, ..., ...) = 0
+   +0 listen(3, 1) = 0
+
+  +.1 < S 0:0(0) win 32792 <mss 1000,sackOK,nop,nop,nop,wscale 7>
+   +0 > S. 0:0(0) ack 1 <mss 1460,nop,nop,sackOK,nop,wscale 8>
+  +.1 < . 1:1(0) ack 1 win 257
+   +0 accept(3, ..., ...) = 4
+   +0 setsockopt(4, SOL_SOCKET, SO_SNDBUF, [200000], 4) = 0
+
+   +0 write(4, ..., 10000) = 10000
+   +0 > P. 1:10001(10000) ack 1
+   +0 %{ assert tcpi_snd_cwnd == 10, tcpi_snd_cwnd }%
+
++.105 < . 1:1(0) ack 2001 win 257
+   +0 %{ assert tcpi_snd_cwnd == 12, tcpi_snd_cwnd }%
+
++.005 < . 1:1(0) ack 4001 win 257
+   +0 %{ assert tcpi_snd_cwnd == 14, tcpi_snd_cwnd }%
+
++.005 < . 1:1(0) ack 6001 win 257
+   +0 %{ assert tcpi_snd_cwnd == 16, tcpi_snd_cwnd }%
+
++.005 < . 1:1(0) ack 8001 win 257
+   +0 %{ assert tcpi_snd_cwnd == 18, tcpi_snd_cwnd }%
+
++.005 < . 1:1(0) ack 10001 win 257
+   +0 %{ assert tcpi_snd_cwnd == 20, tcpi_snd_cwnd }%
diff --git a/tools/testing/selftests/net/packetdrill/tcp_slow_start_slow-start-fq-ack-per-2pkt.pkt b/tools/testing/selftests/net/packetdrill/tcp_slow_start_slow-start-fq-ack-per-2pkt.pkt
new file mode 100644
index 000000000000..7e9c83d617c2
--- /dev/null
+++ b/tools/testing/selftests/net/packetdrill/tcp_slow_start_slow-start-fq-ack-per-2pkt.pkt
@@ -0,0 +1,63 @@
+// SPDX-License-Identifier: GPL-2.0
+// Test of slow start when not application-limited, so that
+// the cwnd continues to grow, even if TSQ triggers.
+// In this variant, the receiver ACKs every other packet,
+// approximating standard delayed ACKs.
+
+// Note we use FQ/pacing to check if TCP Small Queues is not hurting
+
+`./defaults.sh
+tc qdisc replace dev tun0 root fq
+sysctl -q net/ipv4/tcp_pacing_ss_ratio=200
+sysctl -e -q net.ipv4.tcp_min_tso_segs=2`
+
+    0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
+   +0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
+   +0 bind(3, ..., ...) = 0
+   +0 listen(3, 1) = 0
+
+  +.1 < S 0:0(0) win 32792 <mss 1460,sackOK,nop,nop,nop,wscale 7>
+   +0 > S. 0:0(0) ack 1 <mss 1460,nop,nop,sackOK,nop,wscale 8>
+  +.1 < . 1:1(0) ack 1 win 500
+   +0 accept(3, ..., ...) = 4
+   +0 setsockopt(4, SOL_SOCKET, SO_SNDBUF, [200000], 4) = 0
+
+   +0 write(4, ..., 40000) = 40000
+// This might change if we cook the initial packet with 10 MSS.
+   +0 > P. 1:2921(2920) ack 1
+   +0 > P. 2921:5841(2920) ack 1
+   +0 > P. 5841:8761(2920) ack 1
+   +0 > P. 8761:11681(2920) ack 1
+   +0 > P. 11681:14601(2920) ack 1
+   +0 %{ assert tcpi_snd_cwnd == 10, tcpi_snd_cwnd }%
+
++.105 < . 1:1(0) ack 2921 win 500
+   +0 %{ assert tcpi_snd_cwnd == 12, tcpi_snd_cwnd }%
+
+// Note: after this commit : "net_sched: sch_fq: account for schedule/timers drifts"
+// FQ notices that this packet missed the 'time to send next packet' computed
+// when prior packet (11681:14601(2920)) was sent.
+// So FQ will allow following packet to be sent a bit earlier (quantum/2)
+// (FQ commit allows an application/cwnd limited flow to get at most quantum/2 extra credit)
+   +0 > P. 14601:17521(2920) ack 1
+
++.003 < . 1:1(0) ack 5841 win 500
+   +0 %{ assert tcpi_snd_cwnd == 14, tcpi_snd_cwnd }%
+
++.001 > P. 17521:20441(2920) ack 1
+
++.001 < . 1:1(0) ack 8761 win 500
+   +0 %{ assert tcpi_snd_cwnd == 16, tcpi_snd_cwnd }%
+
+// remaining packets are delivered at a constant rate.
++.007 > P. 20441:23361(2920) ack 1
+
++.002 < . 1:1(0) ack 11681 win 500
+   +0 %{ assert tcpi_snd_cwnd == 18, tcpi_snd_cwnd }%
++.001 < . 1:1(0) ack 14601 win 500
+
++.004 > P. 23361:26281(2920) ack 1
+
++.007 > P. 26281:29201(2920) ack 1
+
+   +0 %{ assert tcpi_snd_cwnd == 20, 'cwnd=%d' % tcpi_snd_cwnd }%