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Date: Mon, 15 Aug 2022 09:30:09 -0400
From: Neal Cardwell <ncardwell@...gle.com>
To: Jiri Slaby <jirislaby@...nel.org>
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 Mon, Aug 15, 2022 at 3:48 AM Jiri Slaby <jirislaby@...nel.org> wrote:
>
> 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
It sounds like this test has a very specific dependence on the buggy
delayed ACK timing behavior from the buggy commit
4a41f453bedfd5e9cd040bad509d9da49feb3e2c.
IMHO I don't think we can revert a kernel bug fix based on a test that
decided to depend on the exact timing of delayed ACKs during a time
when that delayed ACK behavior was buggy. :-)
best regards,
neal
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