Created attachment 870621 [details]
BAD strace -rT -e trace=network,/epoll

Currently at:
https://build.opensuse.org/package/live_build_log/openSUSE:Factory:Staging:H/python-eventlet/standard/x86_64

> =================================== FAILURES ===================================
> _______________________ TestGreenSocket.test_full_duplex _______________________
>
> self = <tests.greenio_test.TestGreenSocket testMethod=test_full_duplex>
>
>     def test_full_duplex(self):
> ...
> >       large_evt.wait()
>
> tests/greenio_test.py:424:
> _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
> eventlet/greenthread.py:181: in wait
>     return self._exit_event.wait()
> eventlet/event.py:125: in wait
>     result = hub.switch()
...
> E       tests.TestIsTakingTooLong: 1
>
> eventlet/hubs/hub.py:313: TestIsTakingTooLong

to this commit. With the commit, the test takes > 1.5 s. Without the commit it takes only < 300 ms. And they set timeout to 1 s.

The reduced self-stadning test case:
#!/usr/bin/python3
import eventlet
from eventlet.green import select, socket, time, ssl

def bufsized(sock, size=1):
    """ Resize both send and receive buffers on a socket.
    Useful for testing trampoline.  Returns the socket.

    >>> import socket
    >>> sock = bufsized(socket.socket(socket.AF_INET, socket.SOCK_STREAM))
    """
    sock.setsockopt(socket.SOL_SOCKET, socket.SO_SNDBUF, size)
    sock.setsockopt(socket.SOL_SOCKET, socket.SO_RCVBUF, size)
    return sock

def min_buf_size():
    """Return the minimum buffer size that the platform supports."""
    test_sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    test_sock.setsockopt(socket.SOL_SOCKET, socket.SO_SNDBUF, 1)
    return test_sock.getsockopt(socket.SOL_SOCKET, socket.SO_SNDBUF)

def test_full_duplex():
    large_data = b'*' * 10 * min_buf_size()
    listener = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    listener.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
    listener.bind(('127.0.0.1', 0))
    listener.listen(50)
    bufsized(listener)

    def send_large(sock):
        sock.sendall(large_data)

    def read_large(sock):
        result = sock.recv(len(large_data))
        while len(result) < len(large_data):
            result += sock.recv(len(large_data))
        assert result == large_data

    def server():
        (sock, addr) = listener.accept()
        sock = bufsized(sock)
        send_large_coro = eventlet.spawn(send_large, sock)
        eventlet.sleep(0)
        result = sock.recv(10)
        expected = b'hello world'
        while len(result) < len(expected):
            result += sock.recv(10)
        assert result == expected
        send_large_coro.wait()

    server_evt = eventlet.spawn(server)
    client = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    client.connect(('127.0.0.1', listener.getsockname()[1]))
    bufsized(client)
    large_evt = eventlet.spawn(read_large, client)
    eventlet.sleep(0)
    client.sendall(b'hello world')
    server_evt.wait()
    large_evt.wait()
    client.close()

test_full_duplex()

=====================================

I bisected it to:
commit dfa2f0483360d4d6f2324405464c9f281156bd87
Author: Eric Dumazet <edumazet@google.com>
Date:   Mon Jul 17 15:29:17 2023 +0000

    tcp: get rid of sysctl_tcp_adv_win_scale