Message-Id: <20250620-netpoll_test-v1-1-5068832f72fc@debian.org>
Date: Fri, 20 Jun 2025 02:38:31 -0700
From: Breno Leitao <leitao@...ian.org>
To: Andrew Lunn <andrew+netdev@...n.ch>, 
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
 Shuah Khan <shuah@...nel.org>
Cc: linux-kernel@...r.kernel.org, netdev@...r.kernel.org, 
 linux-kselftest@...r.kernel.org, 
 Willem de Bruijn <willemdebruijn.kernel@...il.com>, gustavold@...il.com, 
 Breno Leitao <leitao@...ian.org>
Subject: [PATCH net-next] selftests: net: add netpoll basic functionality
 test

Add a basic selftest for the netpoll polling mechanism, specifically
targeting the netpoll poll() side.

The test creates a scenario where network transmission is running at
maximum speed, and netpoll needs to poll the NIC. This is achieved by:

  1. Configuring a single RX/TX queue to create contention
  2. Generating background traffic to saturate the interface
  3. Sending netconsole messages to trigger netpoll polling
  4. Using dynamic netconsole targets via configfs
  5. Delete and create new netconsole targets after 5 iterations

The test validates a critical netpoll code path by monitoring traffic
flow and ensuring netpoll_poll_dev() is called when the normal TX path
is blocked. Perf probing confirms this test successfully triggers
netpoll_poll_dev() in typical test runs.

This addresses a gap in netpoll test coverage for a path that is
tricky for the network stack.

Signed-off-by: Breno Leitao <leitao@...ian.org>
---
Changes since RFC:
- Toggle the netconsole interfaces up and down after 5 iterations.
- Moved the traffic check under DEBUG (Willem de Bruijn).
- Bumped the iterations to 20 given it runs faster now.
- Link to the RFC: https://lore.kernel.org/r/20250612-netpoll_test-v1-1-4774fd95933f@debian.org
---
 tools/testing/selftests/drivers/net/Makefile       |   1 +
 .../testing/selftests/drivers/net/netpoll_basic.py | 231 +++++++++++++++++++++
 2 files changed, 232 insertions(+)

diff --git a/tools/testing/selftests/drivers/net/Makefile b/tools/testing/selftests/drivers/net/Makefile
index bd309b2d39095..9bd84d6b542e5 100644
--- a/tools/testing/selftests/drivers/net/Makefile
+++ b/tools/testing/selftests/drivers/net/Makefile
@@ -16,6 +16,7 @@ TEST_PROGS := \
 	netcons_fragmented_msg.sh \
 	netcons_overflow.sh \
 	netcons_sysdata.sh \
+	netpoll_basic.py \
 	ping.py \
 	queues.py \
 	stats.py \
diff --git a/tools/testing/selftests/drivers/net/netpoll_basic.py b/tools/testing/selftests/drivers/net/netpoll_basic.py
new file mode 100755
index 0000000000000..2a81926169262
--- /dev/null
+++ b/tools/testing/selftests/drivers/net/netpoll_basic.py
@@ -0,0 +1,231 @@
+#!/usr/bin/env python3
+# SPDX-License-Identifier: GPL-2.0
+
+# This test aims to evaluate the netpoll polling mechanism (as in
+# netpoll_poll_dev()). It presents a complex scenario where the network
+# attempts to send a packet but fails, prompting it to poll the NIC from within
+# the netpoll TX side.
+#
+# This has been a crucial path in netpoll that was previously untested. Jakub
+# suggested using a single RX/TX queue, pushing traffic to the NIC, and then
+# sending netpoll messages (via netconsole) to trigger the poll. `perf` probing
+# of netpoll_poll_dev() showed that this test indeed triggers
+# netpoll_poll_dev() once or twice in 10 iterations.
+
+# Author: Breno Leitao <leitao@...ian.org>
+
+import errno
+import os
+import random
+import string
+import time
+
+from lib.py import (
+    ethtool,
+    GenerateTraffic,
+    ksft_exit,
+    ksft_pr,
+    ksft_run,
+    KsftFailEx,
+    KsftSkipEx,
+    NetdevFamily,
+    NetDrvEpEnv,
+)
+
+NETCONSOLE_CONFIGFS_PATH = "/sys/kernel/config/netconsole"
+REMOTE_PORT = 6666
+LOCAL_PORT = 1514
+# Number of netcons messages to send. I usually see netpoll_poll_dev()
+# being called at least once in 10 iterations. Having 20 to have some buffers
+ITERATIONS = 20
+DEBUG = False
+
+
+def generate_random_netcons_name() -> str:
+    """Generate a random target name starting with 'netcons'"""
+    random_suffix = "".join(random.choices(string.ascii_lowercase + string.digits, k=8))
+    return f"netcons_{random_suffix}"
+
+
+def get_stats(cfg: NetDrvEpEnv, netdevnl: NetdevFamily) -> dict[str, int]:
+    """Get the statistics for the interface"""
+    return netdevnl.qstats_get({"ifindex": cfg.ifindex}, dump=True)[0]
+
+
+def set_single_rx_tx_queue(interface_name: str) -> None:
+    """Set the number of RX and TX queues to 1 using ethtool"""
+    try:
+        # This don't need to be reverted, since interfaces will be deleted after test
+        ethtool(f"-G {interface_name} rx 1 tx 1")
+    except Exception as e:
+        raise KsftSkipEx(
+            f"Failed to configure RX/TX queues: {e}. Ethtool not available?"
+        )
+
+
+def create_netconsole_target(
+    config_data: dict[str, str],
+    target_name: str,
+) -> None:
+    """Create a netconsole dynamic target against the interfaces"""
+    ksft_pr(f"Using netconsole name: {target_name}")
+    try:
+        os.makedirs(f"{NETCONSOLE_CONFIGFS_PATH}/{target_name}", exist_ok=True)
+        ksft_pr(f"Created target directory: {NETCONSOLE_CONFIGFS_PATH}/{target_name}")
+    except OSError as e:
+        if e.errno != errno.EEXIST:
+            raise KsftFailEx(f"Failed to create netconsole target directory: {e}")
+
+    try:
+        for key, value in config_data.items():
+            if DEBUG:
+                ksft_pr(f"Setting {key} to {value}")
+            with open(
+                f"{NETCONSOLE_CONFIGFS_PATH}/{target_name}/{key}",
+                "w",
+                encoding="utf-8",
+            ) as f:
+                # Always convert to string to write to file
+                f.write(str(value))
+                f.close()
+
+        if DEBUG:
+            # Read all configuration values for debugging
+            for debug_key in config_data.keys():
+                with open(
+                    f"{NETCONSOLE_CONFIGFS_PATH}/{target_name}/{debug_key}",
+                    "r",
+                    encoding="utf-8",
+                ) as f:
+                    content = f.read()
+                    ksft_pr(
+                        f"{NETCONSOLE_CONFIGFS_PATH}/{target_name}/{debug_key} {content}"
+                    )
+
+    except Exception as e:
+        raise KsftFailEx(f"Failed to configure netconsole target: {e}")
+
+
+def set_netconsole(cfg: NetDrvEpEnv, interface_name: str, target_name: str) -> None:
+    """Configure netconsole on the interface with the given target name"""
+    config_data = {
+        "extended": "1",
+        "dev_name": interface_name,
+        "local_port": LOCAL_PORT,
+        "remote_port": REMOTE_PORT,
+        "local_ip": cfg.addr_v["4"] if cfg.addr_ipver == "4" else cfg.addr_v["6"],
+        "remote_ip": (
+            cfg.remote_addr_v["4"] if cfg.addr_ipver == "4" else cfg.remote_addr_v["6"]
+        ),
+        "remote_mac": "00:00:00:00:00:00",  # Not important for this test
+        "enabled": "1",
+    }
+
+    create_netconsole_target(config_data, target_name)
+    ksft_pr(f"Created netconsole target: {target_name} on interface {interface_name}")
+
+
+def delete_netconsole_target(name: str) -> None:
+    """Delete a netconsole dynamic target"""
+    target_path = f"{NETCONSOLE_CONFIGFS_PATH}/{name}"
+    try:
+        if os.path.exists(target_path):
+            os.rmdir(target_path)
+    except OSError as e:
+        raise KsftFailEx(f"Failed to delete netconsole target: {e}")
+
+
+def check_traffic_flowing(cfg: NetDrvEpEnv, netdevnl: NetdevFamily) -> int:
+    """Check if traffic is flowing on the interface"""
+    stat1 = get_stats(cfg, netdevnl)
+    time.sleep(1)
+    stat2 = get_stats(cfg, netdevnl)
+    pkts_per_sec = stat2["rx-packets"] - stat1["rx-packets"]
+    # Just make sure this will not fail even in slow/debug kernels
+    if pkts_per_sec < 10:
+        raise KsftFailEx(f"Traffic seems low: {pkts_per_sec}")
+    if DEBUG:
+        ksft_pr(f"Traffic per second {pkts_per_sec}")
+
+    return pkts_per_sec
+
+
+def do_netpoll_flush(
+    cfg: NetDrvEpEnv, netdevnl: NetdevFamily, ifname: str, target_name: str
+) -> None:
+    """Print messages to the console, trying to trigger a netpoll poll"""
+
+    set_netconsole(cfg, ifname, target_name)
+    for i in range(int(ITERATIONS)):
+        msg = f"netcons test #{i}."
+
+        if DEBUG:
+            pkts_per_s = check_traffic_flowing(cfg, netdevnl)
+            msg += f" ({pkts_per_s} packets/s)"
+
+        with open("/dev/kmsg", "w", encoding="utf-8") as kmsg:
+            kmsg.write(msg)
+
+        if not i % 5:
+            # Every 5 iterations, toggle netconsole
+            delete_netconsole_target(target_name)
+            set_netconsole(cfg, ifname, target_name)
+
+
+def test_netpoll(cfg: NetDrvEpEnv, netdevnl: NetdevFamily) -> None:
+    """
+    Test netpoll by sending traffic to the interface and then sending
+    netconsole messages to trigger a poll
+    """
+
+    target_name = generate_random_netcons_name()
+    ifname = cfg.dev["ifname"]
+    traffic = None
+
+    try:
+        set_single_rx_tx_queue(ifname)
+        traffic = GenerateTraffic(cfg)
+        check_traffic_flowing(cfg, netdevnl)
+        do_netpoll_flush(cfg, netdevnl, ifname, target_name)
+    finally:
+        if traffic:
+            traffic.stop()
+        delete_netconsole_target(target_name)
+
+
+def check_dependencies() -> None:
+    """Check if the dependencies are met"""
+    if not os.path.exists(NETCONSOLE_CONFIGFS_PATH):
+        raise KsftSkipEx(
+            f"Directory {NETCONSOLE_CONFIGFS_PATH} does not exist. CONFIG_NETCONSOLE_DYNAMIC might not be set."
+        )
+
+
+def load_netconsole_module() -> None:
+    """Try to load the netconsole module"""
+    try:
+        os.system("modprobe netconsole")
+    except Exception:
+        # It is fine if we fail to load the module, it will fail later
+        # at check_dependencies()
+        pass
+
+
+def main() -> None:
+    """Main function to run the test"""
+    load_netconsole_module()
+    check_dependencies()
+    netdevnl = NetdevFamily()
+    with NetDrvEpEnv(__file__, nsim_test=True) as cfg:
+        ksft_run(
+            [test_netpoll],
+            args=(
+                cfg,
+                netdevnl,
+            ),
+        )
+    ksft_exit()
+
+
+if __name__ == "__main__":
+    main()

---
base-commit: 4f4040ea5d3e4bebebbef9379f88085c8b99221c
change-id: 20250612-netpoll_test-a1324d2057c8

Best regards,
--  
Breno Leitao <leitao@...ian.org>

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