Re: [PATCH net-next V2] selftests: drv-net: Test queue stall upon reconfig

[Jakub Kicinski <kuba@kernel.org>]

On Fri, 12 Jun 2026 18:48:54 -0700 Mohsin Bashir wrote:
> From: Mohsin Bashir <hmohsin@meta.com>
> 
> Add a reconfig_tx_stall test that detects the possibility of a TX stall
> after ring reconfiguration. The key observation is that drivers using
> netif_tx_start_all_queues() are prone to experiencing a stall when
> reconfiguration completes compared to drivers using
> netif_tx_wake_all_queues(). start_all_queues only clears DRV_XOFF, while
> wake_all_queues also calls __netif_schedule() to kick the qdisc. Without
> the kick, qdisc backlog present at reconfig time can stay stuck until a
> new trigger is issued.
> 
> The test caps the TX ring at 64 entries so it fills quickly, then
> installs FQ on a target TX queue and sends UDP packets with SO_TXTIME
> scheduled in the future. With napi_defer_hard_irqs slowing completions,
> the small ring can fill when FQ releases the burst, leaving requeued
> qdisc backlog with no FQ timer to rescue it. A subsequent ring reconfig
> must wake the queues to drain the backlog. Simply starting the queues can
> leave it stuck.
> 
> On host with problematic driver:
>  Sent 128 SO_TXTIME packets (+100ms)
>  Sent 128 SO_TXTIME packets (+200ms)
>  Backlog before reconfig: 52632 bytes
>  Check| At /root/ksft-net-drv/./drivers/net/ring_reconfig.py, ...
>  Check|     ksft_eq(0, backlog,
>  Check failed 0 != 52632 qdisc backlog stuck on queue 1 after ring reconfig
> not ok 3 ring_reconfig.reconfig_tx_stall
> 
> On host with fixed driver:
>  Sent 128 SO_TXTIME packets (+100ms)
>  Sent 128 SO_TXTIME packets (+200ms)
>  Backlog before reconfig: 76024 bytes
> ok 3 ring_reconfig.reconfig_tx_stall
> 
> Signed-off-by: Mohsin Bashir <hmohsin@meta.com>
> Signed-off-by: Jakub Kicinski <kuba@kernel.org>


pylint is not on board:

+tools/testing/selftests/drivers/net/ring_reconfig.py:169:37: W1514: Using open without explicitly specifying an encoding (unspecified-encoding)
+tools/testing/selftests/drivers/net/ring_reconfig.py:162:17: W0613: Unused argument 'cfg' (unused-argument)
+tools/testing/selftests/drivers/net/ring_reconfig.py:253:0: C0116: Missing function or method docstring (missing-function-docstring)

> diff --git a/tools/testing/selftests/drivers/net/config b/tools/testing/selftests/drivers/net/config
> index 617de8aaf551..1ef07fae74c1 100644
> --- a/tools/testing/selftests/drivers/net/config
> +++ b/tools/testing/selftests/drivers/net/config
> @@ -4,6 +4,10 @@ CONFIG_DEBUG_INFO_BTF_MODULES=n
>  CONFIG_INET_PSP=y
>  CONFIG_IPV6=y
>  CONFIG_MACSEC=m
> +CONFIG_NET_ACT_SKBEDIT=m
> +CONFIG_NET_CLS_ACT=y
> +CONFIG_NET_CLS_FLOWER=m
> +CONFIG_NET_CLS_MATCHALL=m
>  CONFIG_NETCONSOLE=m
>  CONFIG_NETCONSOLE_DYNAMIC=y
>  CONFIG_NETCONSOLE_EXTENDED_LOG=y
> diff --git a/tools/testing/selftests/drivers/net/ring_reconfig.py b/tools/testing/selftests/drivers/net/ring_reconfig.py
> index f9530a8b0856..11491a0b7013 100755
> --- a/tools/testing/selftests/drivers/net/ring_reconfig.py
> +++ b/tools/testing/selftests/drivers/net/ring_reconfig.py
> @@ -5,10 +5,18 @@
>  Test channel and ring size configuration via ethtool (-L / -G).
>  """
>  
> +import socket
> +import struct
> +import time
> +
>  from lib.py import ksft_run, ksft_exit, ksft_pr
>  from lib.py import ksft_eq
> +from lib.py import KsftSkipEx
>  from lib.py import NetDrvEpEnv, EthtoolFamily, GenerateTraffic
> -from lib.py import defer, NlError
> +from lib.py import cmd, defer, rand_port, tc, NlError
> +
> +# Added in Python 3.13; fallback to 61 for x86/ARM/MIPS
> +SO_TXTIME = getattr(socket, "SO_TXTIME", 61)
>  
>  
>  def channels(cfg) -> None:
> @@ -151,6 +159,169 @@ def ringparam(cfg) -> None:
>          GenerateTraffic(cfg).wait_pkts_and_stop(10000)
>  
>  
> +def _write_sysfs(cfg, path, val):
> +    with open(path, "r", encoding="utf-8") as fp:
> +        orig_val = fp.read().strip()
> +    if str(val) == orig_val:
> +        return
> +    with open(path, "w", encoding="utf-8") as fp:
> +        fp.write(str(val))
> +    defer(lambda p=path, v=orig_val: open(p, "w").write(v))
> +
> +
> +def _get_mq_handle(cfg):
> +    qdiscs = tc(f"qdisc show dev {cfg.ifname}", json=True)
> +    for q in qdiscs:
> +        if q.get("kind") == "mq":
> +            return q["handle"]
> +    raise KsftSkipEx(f"no mq qdisc found on {cfg.ifname}")
> +
> +
> +def _get_qdisc_backlog(cfg, queue, mq_handle):
> +    qdiscs = tc(f"-s qdisc show dev {cfg.ifname}", json=True)
> +    target_parent = f"{mq_handle}{queue + 1:x}"
> +    for q in qdiscs:
> +        if q.get("parent", "") == target_parent:
> +            return q.get("backlog")
> +    return None
> +
> +
> +def _setup_fq_qdisc(cfg, mq_handle, port, target_queue, other_queue):
> +    mq_child_parent = f"{mq_handle}{target_queue + 1:x}"
> +
> +    # Save the original child qdisc to restore after test
> +    qdiscs = tc(f"qdisc show dev {cfg.ifname}", json=True)
> +    default_qdisc = cmd("sysctl -n net.core.default_qdisc").stdout.strip()
> +    orig_kind = default_qdisc
> +    for q in qdiscs:
> +        if q.get("parent", "") == mq_child_parent:
> +            orig_kind = q.get("kind", default_qdisc)
> +            break
> +    try:
> +        tc(f"qdisc replace dev {cfg.ifname} parent {mq_child_parent} fq")
> +    except Exception as exc:
> +        raise KsftSkipEx("fq not available (CONFIG_NET_SCH_FQ)") from exc
> +    defer(tc,
> +          f"qdisc replace dev {cfg.ifname} parent {mq_child_parent} {orig_kind}")
> +
> +    qdisc_j = tc(f"qdisc show dev {cfg.ifname}", json=True)
> +    has_clsact = any(q['kind'] == 'clsact' for q in qdisc_j)
> +    if not has_clsact:
> +        tc(f"qdisc add dev {cfg.ifname} clsact")
> +        defer(tc, f"qdisc del dev {cfg.ifname} clsact")
> +
> +    proto = "ipv6" if int(cfg.addr_ipver) == 6 else "ip"
> +    try:
> +        tc(f"filter add dev {cfg.ifname} egress protocol {proto} "
> +           f"pref 1 flower ip_proto udp dst_port {port} "
> +           f"action skbedit queue_mapping {target_queue}")
> +    except Exception as exc:
> +        raise KsftSkipEx("tc flower/act_skbedit not available") from exc
> +    defer(tc, f"filter del dev {cfg.ifname} egress pref 1")
> +
> +    tc(f"filter add dev {cfg.ifname} egress pref 100 "
> +       f"matchall action skbedit queue_mapping {other_queue}")
> +    defer(tc, f"filter del dev {cfg.ifname} egress pref 100")
> +
> +
> +def _create_sotxtime_socket(cfg):
> +    sock = socket.socket(socket.AF_INET6 if cfg.addr_ipver == "6"
> +                         else socket.AF_INET, socket.SOCK_DGRAM)
> +    try:
> +        sock.setsockopt(socket.SOL_SOCKET, SO_TXTIME, struct.pack("Ii", 1, 0))
> +    except OSError as exc:
> +        sock.close()
> +        raise KsftSkipEx("SO_TXTIME not supported") from exc
> +    sock.setsockopt(socket.SOL_SOCKET, socket.SO_BINDTODEVICE,
> +                    cfg.ifname.encode())
> +    return sock
> +
> +
> +def _send_sotxtime_burst(sock, addr, port, count, delay_ns, ipver):
> +    payload = b'\x00' * 1400
> +    txtime_ns = time.clock_gettime_ns(time.CLOCK_MONOTONIC) + delay_ns
> +
> +    ancdata = [(socket.SOL_SOCKET, SO_TXTIME, struct.pack("Q", txtime_ns))]
> +    if int(ipver) == 6:
> +        dest = (addr, port, 0, 0)
> +    else:
> +        dest = (addr, port)
> +    for _ in range(count):
> +        sock.sendmsg([payload], ancdata, 0, dest)
> +
> +
> +def reconfig_tx_stall(cfg) -> None:
> +    target_queue = 1
> +    other_queue = 0
> +
> +    ehdr = {'header': {'dev-index': cfg.ifindex}}
> +    chans = cfg.eth.channels_get(ehdr)
> +
> +    if 'combined-max' not in chans:
> +        raise KsftSkipEx("device does not support combined channels")
> +    if chans['combined-count'] < 2:
> +        raise KsftSkipEx("need at least 2 combined channels")
> +
> +    rings = cfg.eth.rings_get(ehdr)
> +    if 'rx' not in rings or 'tx' not in rings:
> +        raise KsftSkipEx("device does not expose rx/tx ring params")
> +    tx_cur = rings['tx']
> +    if tx_cur <= 64:
> +        raise KsftSkipEx("tx ring size already at minimum")
> +    defer(cfg.eth.rings_set, ehdr | {'tx': tx_cur})
> +
> +    tx_min = 64
> +    cfg.eth.rings_set(ehdr | {'tx': tx_min})
> +
> +    # Slow completions so the ring stays full after FQ releases packets
> +    napi_defer = f"/sys/class/net/{cfg.ifname}/napi_defer_hard_irqs"
> +    gro_timeout = f"/sys/class/net/{cfg.ifname}/gro_flush_timeout"
> +    _write_sysfs(cfg, napi_defer, 100)
> +    _write_sysfs(cfg, gro_timeout, 1000000000)
> +
> +    mq_handle = _get_mq_handle(cfg)
> +    port = rand_port()
> +    _setup_fq_qdisc(cfg, mq_handle, port, target_queue, other_queue)
> +
> +    sock = _create_sotxtime_socket(cfg)
> +    defer(sock.close)
> +
> +    pkt_count = tx_min * 2
> +
> +    for delay_ms in [100, 200, 500]:
> +        delay_ns = delay_ms * 1_000_000
> +        _send_sotxtime_burst(sock, cfg.remote_addr, port, pkt_count,
> +                             delay_ns, cfg.addr_ipver)
> +        ksft_pr(f"Sent {pkt_count} SO_TXTIME packets (+{delay_ms}ms)")
> +        time.sleep(delay_ms / 1000 + 0.3)
> +
> +        backlog = _get_qdisc_backlog(cfg, target_queue, mq_handle)
> +        if backlog:
> +            break
> +    else:
> +        raise KsftSkipEx("failed to build qdisc backlog")
> +
> +    ksft_pr(f"Backlog before reconfig: {backlog} bytes")
> +
> +    # Trigger ring reconfig — driver should call wake, not just start
> +    cfg.eth.rings_set(ehdr | {'tx': tx_cur})
> +
> +    # Let completions proceed normally
> +    _write_sysfs(cfg, napi_defer, 0)
> +    _write_sysfs(cfg, gro_timeout, 0)
> +
> +    # Poll for backlog to drain
> +    for _ in range(100):
> +        backlog = _get_qdisc_backlog(cfg, target_queue, mq_handle)
> +        if not backlog:
> +            break
> +        time.sleep(0.1)
> +
> +    ksft_eq(0, backlog,
> +            comment=f"qdisc backlog stuck on queue {target_queue} "
> +                    f"after ring reconfig")
> +
> +
>  def main() -> None:
>      """ Ksft boiler plate main """
>  
> @@ -158,7 +329,8 @@ def main() -> None:

the NetDrvEpEnv() setup needs to ask for 2+ queues, otherwise this
fails in netdevsim mode:

# ok 3 ring_reconfig.reconfig_tx_stall # SKIP need at least 2 combined channels

>          cfg.eth = EthtoolFamily()
>  
>          ksft_run([channels,
> -                  ringparam],
> +                  ringparam,
> +                  reconfig_tx_stall],
>                   args=(cfg, ))
>      ksft_exit()
>