linux-kernel - Re: [PATCH net-next v2 4/4] selftests: net: add netpoll basic functionality test

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Message-ID: <685d75ceb113c_2e676c294d1@willemb.c.googlers.com.notmuch>
Date: Thu, 26 Jun 2025 12:31:10 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Breno Leitao <leitao@...ian.org>, 
 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>, 
 Simon Horman <horms@...nel.org>
Cc: linux-kernel@...r.kernel.org, 
 netdev@...r.kernel.org, 
 linux-kselftest@...r.kernel.org, 
 Willem de Bruijn <willemdebruijn.kernel@...il.com>, 
 bpf@...r.kernel.org, 
 gustavold@...il.com, 
 Breno Leitao <leitao@...ian.org>
Subject: Re: [PATCH net-next v2 4/4] selftests: net: add netpoll basic
 functionality test

Breno Leitao wrote:
> 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 some messages
>   6. Start a bpftrace in parallel to make sure netpoll_poll_dev() is
>      called
>   7. If bpftrace exists and netpoll_poll_dev() was called, stop.
> 
> 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.
> 
> 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>

> +def bpftrace_call() -> None:
> +    """Call bpftrace to find how many times netpoll_poll_dev() is called.
> +    Output is saved in the global variable `maps`"""
> +
> +    # This is going to update the global variable, that will be seen by the
> +    # main function
> +    global MAPS  # pylint: disable=W0603
> +
> +    # This will be passed to bpftrace as in bpftrace -e "expr"
> +    expr = "BEGIN{ @hits = 0;} kprobe:netpoll_poll_dev { @hits += 1; }"

Is that BEGIN statement needed? I generally just use count().

> +
> +    MAPS = bpftrace(expr, timeout=BPFTRACE_TIMEOUT, json=True)
> +    logging.debug("BPFtrace output: %s", MAPS)
> +
> +
> +def bpftrace_start():
> +    """Start a thread to call `call_bpf` in parallel for 2 seconds."""

Stale comment? BPFTRACE_TIMEOUT is set to 15.

> +    global BPF_THREAD  # pylint: disable=W0603
> +
> +    BPF_THREAD = threading.Thread(target=bpftrace_call)
> +    BPF_THREAD.start()
> +    if not BPF_THREAD.is_alive():
> +        raise KsftSkipEx("BPFtrace thread is not alive. Skipping test")
> +
> +