Re: [PATCH net v2] Revert "tcp: change pingpong threshold to 3"

[Jiri Slaby <jirislaby@kernel.org>]

On 06. 08. 22, 13:24, Neal Cardwell wrote:
> On Sat, Aug 6, 2022 at 6:02 AM Jiri Slaby <jirislaby@kernel.org> wrote:
>>
>> On 21. 07. 22, 22:44, Wei Wang wrote:
>>> This reverts commit 4a41f453bedfd5e9cd040bad509d9da49feb3e2c.
>>>
>>> This to-be-reverted commit was meant to apply a stricter rule for the
>>> stack to enter pingpong mode. However, the condition used to check for
>>> interactive session "before(tp->lsndtime, icsk->icsk_ack.lrcvtime)" is
>>> jiffy based and might be too coarse, which delays the stack entering
>>> pingpong mode.
>>> We revert this patch so that we no longer use the above condition to
>>> determine interactive session, and also reduce pingpong threshold to 1.
>>>
>>> Fixes: 4a41f453bedf ("tcp: change pingpong threshold to 3")
>>> Reported-by: LemmyHuang <hlm3280@163.com>
>>> Suggested-by: Neal Cardwell <ncardwell@google.com>
>>> Signed-off-by: Wei Wang <weiwan@google.com>
>>
>>
>> This breaks python-eventlet [1] (and was backported to stable trees):
>> ________________ TestHttpd.test_018b_http_10_keepalive_framing
>> _________________
>>
>> self = <tests.wsgi_test.TestHttpd
>> testMethod=test_018b_http_10_keepalive_framing>
>>
>>       def test_018b_http_10_keepalive_framing(self):
>>           # verify that if an http/1.0 client sends connection: keep-alive
>>           # that we don't mangle the request framing if the app doesn't
>> read the request
>>           def app(environ, start_response):
>>               resp_body = {
>>                   '/1': b'first response',
>>                   '/2': b'second response',
>>                   '/3': b'third response',
>>               }.get(environ['PATH_INFO'])
>>               if resp_body is None:
>>                   resp_body = 'Unexpected path: ' + environ['PATH_INFO']
>>                   if six.PY3:
>>                       resp_body = resp_body.encode('latin1')
>>               # Never look at wsgi.input!
>>               start_response('200 OK', [('Content-type', 'text/plain')])
>>               return [resp_body]
>>
>>           self.site.application = app
>>           sock = eventlet.connect(self.server_addr)
>>           req_body = b'GET /tricksy HTTP/1.1\r\n'
>>           body_len = str(len(req_body)).encode('ascii')
>>
>>           sock.sendall(b'PUT /1 HTTP/1.0\r\nHost:
>> localhost\r\nConnection: keep-alive\r\n'
>>                        b'Content-Length: ' + body_len + b'\r\n\r\n' +
>> req_body)
>>           result1 = read_http(sock)
>>           self.assertEqual(b'first response', result1.body)
>>           self.assertEqual(result1.headers_original.get('Connection'),
>> 'keep-alive')
>>
>>           sock.sendall(b'PUT /2 HTTP/1.0\r\nHost:
>> localhost\r\nConnection: keep-alive\r\n'
>>                        b'Content-Length: ' + body_len + b'\r\nExpect:
>> 100-continue\r\n\r\n')
>>           # Client may have a short timeout waiting on that 100 Continue
>>           # and basically immediately send its body
>>           sock.sendall(req_body)
>>           result2 = read_http(sock)
>>           self.assertEqual(b'second response', result2.body)
>>           self.assertEqual(result2.headers_original.get('Connection'),
>> 'close')
>>
>>   >       sock.sendall(b'PUT /3 HTTP/1.0\r\nHost:
>> localhost\r\nConnection: close\r\n\r\n')
>>
>> tests/wsgi_test.py:648:
>> _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
>> _ _ _ _
>> eventlet/greenio/base.py:407: in sendall
>>       tail = self.send(data, flags)
>> eventlet/greenio/base.py:401: in send
>>       return self._send_loop(self.fd.send, data, flags)
>> _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
>> _ _ _ _
>>
>> self = <eventlet.greenio.base.GreenSocket object at 0x7f5f2f73c9a0>
>> send_method = <built-in method send of socket object at 0x7f5f2f73d520>
>> data = b'PUT /3 HTTP/1.0\r\nHost: localhost\r\nConnection: close\r\n\r\n'
>> args = (0,), _timeout_exc = timeout('timed out'), eno = 32
>>
>>       def _send_loop(self, send_method, data, *args):
>>           if self.act_non_blocking:
>>               return send_method(data, *args)
>>
>>           _timeout_exc = socket_timeout('timed out')
>>           while True:
>>               try:
>>   >               return send_method(data, *args)
>> E               BrokenPipeError: [Errno 32] Broken pipe
>>
>> eventlet/greenio/base.py:388: BrokenPipeError
>> ====================
>>
>> Reverting this revert on the top of 5.19 solves the issue.
>>
>> Any ideas?
> 
> Interesting. This revert should return the kernel back to the delayed
> ACK behavior it had for many years before May 2019 and Linux 5.1,
> which contains the commit it is reverting:
> 
>    4a41f453bedfd tcp: change pingpong threshold to 3
> 
> It sounds like perhaps this test you mention has an implicit
> dependence on the timing of delayed ACKs.
> 
> A few questions:

Dunno. I am only an openSUSE kernel maintainer and this popped out at 
me. Feel free to dig to eventlet's sources on your own :P.

> (1) What are the timeout values in this test? If there is some
> implicit or explicit timeout value less than the typical Linux TCP
> 40ms delayed ACK timer value then this could be the problem. If you
> make sure all timeouts are at least, say, 300ms then this should
> remove dependencies on delayed ACK behavior (and make the test more
> portable).
> 
> (2) Does this test use the TCP_NODELAY socket option to disable
> Nagle's algorithm? Presumably it should, given that it's a network app
> that cares about latency. Omitting the TCP_NODELAY socket option can
> cause request/response traffic to depend on delayed ACK behavior.
> 
> (3) If (1) and (2) do not fix the test, would you be able to provide
> binary .pcap traces of the behavior with the test (a) passing and (b)
> failing? For example:
>     sudo tcpdump -i any -w /tmp/trace.pcap -s 100 port 80 &
>     # run test
>     killall tcpdump
> 
> thanks,
> neal


-- 
js