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

[Breno Leitao <leitao@debian.org>]

On Thu, Jun 26, 2025 at 12:31:10PM -0400, Willem de Bruijn wrote:
> 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@debian.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().

If I use `hits += 1` then yes, but, I've learned that I don't need it if
I use `count()`. So, I will see something like:

kprobe:netpoll_poll_dev { @hits = 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.

Yes. I will remove it.

Thanks for the review,
--breno