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Date: Mon, 15 Aug 2022 09:48:20 +0200 From: Jiri Slaby <jirislaby@...nel.org> To: Neal Cardwell <ncardwell@...gle.com> Cc: Wei Wang <weiwan@...gle.com>, David Miller <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org, Soheil Hassas Yeganeh <soheil@...gle.com>, Yuchung Cheng <ycheng@...gle.com>, LemmyHuang <hlm3280@....com>, stable <stable@...r.kernel.org> Subject: Re: [PATCH net v2] Revert "tcp: change pingpong threshold to 3" On 06. 08. 22, 16:41, Jiri Slaby wrote: > On 06. 08. 22, 13:24, Neal Cardwell wrote: >> On Sat, Aug 6, 2022 at 6:02 AM Jiri Slaby <jirislaby@...nel.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@....com> >>>> Suggested-by: Neal Cardwell <ncardwell@...gle.com> >>>> Signed-off-by: Wei Wang <weiwan@...gle.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. Any updates on this or should I send a revert directly? The "before() &&" part of the patch makes the difference. That is this diff: --- a/net/ipv4/tcp_output.c +++ b/net/ipv4/tcp_output.c @@ -172,9 +172,17 @@ static void tcp_event_data_sent(struct tcp_sock *tp, * and it is a reply for ato after last received packet, * increase pingpong count. */ - if (before(tp->lsndtime, icsk->icsk_ack.lrcvtime) && - (u32)(now - icsk->icsk_ack.lrcvtime) < icsk->icsk_ack.ato) + pr_info("%s: sk=%p (%llx:%x) now=%u lsndtime=%u lrcvtime=%u ping=%u\n", + __func__, sk, sk->sk_addrpair, sk->sk_portpair, now, + tp->lsndtime, icsk->icsk_ack.lrcvtime, + inet_csk(sk)->icsk_ack.pingpong); + if (//before(tp->lsndtime, icsk->icsk_ack.lrcvtime) && + (u32)(now - icsk->icsk_ack.lrcvtime) < icsk->icsk_ack.ato) { inet_csk_inc_pingpong_cnt(sk); + pr_info("\tINC ping=%u before=%u\n", + inet_csk(sk)->icsk_ack.pingpong, + before(tp->lsndtime, icsk->icsk_ack.lrcvtime)); + } tp->lsndtime = now; } makes it work again, and outputs this: > TCP: tcp_event_data_sent: sk=00000000fd67cf8d (100007f0100007f:e858b18b) now=4294902140 lsndtime=4294902140 lrcvtime=4294902140 ping=0 > TCP: tcp_event_data_sent: sk=00000000a4becf82 (100007f0100007f:8bb158e8) now=4294902143 lsndtime=4294902140 lrcvtime=4294902142 ping=0 > TCP: INC ping=1 before=1 > TCP: tcp_event_data_sent: sk=00000000fd67cf8d (100007f0100007f:e858b18b) now=4294902145 lsndtime=4294902140 lrcvtime=4294902144 ping=0 > TCP: INC ping=1 before=1 > TCP: tcp_event_data_sent: sk=00000000fd67cf8d (100007f0100007f:e858b18b) now=4294902147 lsndtime=4294902145 lrcvtime=4294902144 ping=1 > TCP: INC ping=2 before=0 IMO, this "before=0" is the "source" of the problem. But I have no idea what this means at all... > TCP: tcp_event_data_sent: sk=00000000a4becf82 (100007f0100007f:8bb158e8) now=4294902149 lsndtime=4294902143 lrcvtime=4294902148 ping=1 > TCP: INC ping=2 before=1 > TCP: tcp_event_data_sent: sk=00000000fd67cf8d (100007f0100007f:e858b18b) now=4294902151 lsndtime=4294902147 lrcvtime=4294902150 ping=3 > TCP: INC ping=4 before=1 > TCP: tcp_event_data_sent: sk=00000000c7a417e9 (100007f0100007f:e85ab18b) now=4294902153 lsndtime=4294902153 lrcvtime=4294902153 ping=0 > TCP: tcp_event_data_sent: sk=000000008681183e (100007f0100007f:8bb15ae8) now=4294902155 lsndtime=4294902153 lrcvtime=4294902154 ping=0 > TCP: INC ping=1 before=1 >> (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 > > thanks, -- js suse labs
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