Re: [PATCH net-next 2/3] tcp: implement coalescing on backlog queue

["Bruno Prémont" <bonbons@sysophe.eu>]

Hi Eric,

On Wed, 24 Apr 2019 08:47:27 -0700 Eric Dumazet wrote:
> On Wed, Apr 24, 2019 at 7:51 AM Bruno Prémont wrote:
> >
> > Hi Eric,
> >
> > I'm seeing issues with this patch as well, not as regular as for
> > Richard but still (about up to one in 30-50 TCP sessions).
> >
> > In my case I have a virtual machine (on VMWare) with this patch where
> > NGINX as reverse proxy misses part (end) of payload from its upstream
> > and times out on the upstream connection (while according to tcpdump all
> > packets including upstream's FIN were sent and the upstream did get
> > ACKs from the VM).
> >
> > From when browsers get from NGINX it feels as if at some point reading
> > from the socket or waiting for data using select() never returned data
> > that arrived as more than just EOF is missing.
> >
> > The upstream is a hardware machine in the same subnet.
> >
> > My VM is using VMware VMXNET3 Ethernet Controller [15ad:07b0] (rev 01)
> > as network adapter which lists the following features:
> >  
> 
> Hi Bruno.
> 
> I suspect a EPOLLIN notification being lost by the application.
> 
> Fact that TCP backlog contains 1 instead of 2+ packets should not
> change stack behavior,
> this packet should land into socket receive queue eventually.
> 
> Are you using epoll() in Edge Trigger mode. You mention select() but
> select() is a rather old and inefficient API.

nginx is using epoll (c.f. http://nginx.org/en/docs/events.html)

For source, see here
https://trac.nginx.org/nginx/browser/nginx/src/event/modules/ngx_epoll_module.c?rev=ebf8c9686b8ce7428f975d8a567935ea3722da70

> Could you watch/report the output of " ss -temoi "  for the frozen TCP flow ?

Here it is, three distinct reproducing attempts:
State      Recv-Q  Send-Q  Local Address:Port  Peer Address:Port
ESTAB      0       0       158.64.2.228:44248  158.64.2.217:webcache              uid:83 ino:13245 sk:87 <->
         skmem:(r0,rb131072,t0,tb46080,f0,w0,o0,bl0,d0) ts sack cubic wscale:7,7 rto:210 rtt:0.24/0.118 ato:40 mss:1448 rcvmss:1448 advmss:1448 cwnd:10 bytes_acked:949 bytes_received:28381 segs_out:12 segs_in:12 data_segs_out:1 data_segs_in:10 send 482.7Mbps lastsnd:46810 lastrcv:46790 lastack:46790 pacing_rate 965.3Mbps delivery_rate 74.3Mbps app_limited rcv_rtt:1 rcv_space:14480 minrtt:0.156


ESTAB      0      0                                                                   2001:a18:1:6::228:33572                                                                             2001:a18:1:6::217:webcache              uid:83 ino:16699 sk:e1 <->
         skmem:(r0,rb131072,t0,tb46080,f0,w0,o0,bl0,d0) ts sack cubic wscale:7,7 rto:210 rtt:0.231/0.11 ato:40 mss:1428 rcvmss:1428 advmss:1428 cwnd:10 bytes_acked:948 bytes_received:28474 segs_out:12 segs_in:12 data_segs_out:1 data_segs_in:10 send 494.5Mbps lastsnd:8380 lastrcv:8360 lastack:8360 pacing_rate 989.1Mbps delivery_rate 71.0Mbps app_limited rcv_rtt:1.109 rcv_space:14280 minrtt:0.161


ESTAB      0      0                                                                        158.64.2.228:44578                                                                                  158.64.2.217:webcache              uid:83 ino:17628 sk:12c <->
         skmem:(r0,rb131072,t0,tb46080,f0,w0,o0,bl0,d0) ts sack cubic wscale:7,7 rto:210 rtt:0.279/0.136 ato:40 mss:1448 rcvmss:1448 advmss:1448 cwnd:10 bytes_acked:949 bytes_received:28481 segs_out:12 segs_in:12 data_segs_out:1 data_segs_in:10 send 415.2Mbps lastsnd:11360 lastrcv:11330 lastack:11340 pacing_rate 828.2Mbps delivery_rate 61.9Mbps app_limited rcv_rtt:1 rcv_space:14480 minrtt:0.187


From nginx debug logging I don't get a real clue though it seems for working connections
the last event obtained is 2005 (EPOLLMSG | EPOLLWRBAND | EPOLLWRNORM |
EPOLLRDBAND | EPOLLRDNORM | EPOLLHUP | EPOLLIN | EPOLLOUT) - previous ones are 5
while for failing connections it looks like last event seen is 5 (EPOLLIN | EPOLLOUT).

> This migtht give us a clue about packets being dropped, say the the
> accumulated packet became too big.


The following minor patch (might be white-space mangled) does prevent the issue
happening for me:

diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index 4904250a9aac..c102cd367c79 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -1667,7 +1667,7 @@ bool tcp_add_backlog(struct sock *sk, struct sk_buff *skb)
        if (TCP_SKB_CB(tail)->end_seq != TCP_SKB_CB(skb)->seq ||
            TCP_SKB_CB(tail)->ip_dsfield != TCP_SKB_CB(skb)->ip_dsfield ||
            ((TCP_SKB_CB(tail)->tcp_flags |
-             TCP_SKB_CB(skb)->tcp_flags) & TCPHDR_URG) ||
+             TCP_SKB_CB(skb)->tcp_flags) & (TCPHDR_URG | TCPHDR_FIN)) ||
            ((TCP_SKB_CB(tail)->tcp_flags ^
              TCP_SKB_CB(skb)->tcp_flags) & (TCPHDR_ECE | TCPHDR_CWR)) ||
 #ifdef CONFIG_TLS_DEVICE

Cheers,
Bruno


> > rx-checksumming: on
> > tx-checksumming: on
> >         tx-checksum-ipv4: off [fixed]
> >         tx-checksum-ip-generic: on
> >         tx-checksum-ipv6: off [fixed]
> >         tx-checksum-fcoe-crc: off [fixed]
> >         tx-checksum-sctp: off [fixed]
> > scatter-gather: on
> >         tx-scatter-gather: on
> >         tx-scatter-gather-fraglist: off [fixed]
> > tcp-segmentation-offload: on
> >         tx-tcp-segmentation: on
> >         tx-tcp-ecn-segmentation: off [fixed]
> >         tx-tcp-mangleid-segmentation: off
> >         tx-tcp6-segmentation: on
> > udp-fragmentation-offload: off
> > generic-segmentation-offload: on
> > generic-receive-offload: on
> > large-receive-offload: on
> > rx-vlan-offload: on
> > tx-vlan-offload: on
> > ntuple-filters: off [fixed]
> > receive-hashing: off [fixed]
> > highdma: on
> > rx-vlan-filter: on [fixed]
> > vlan-challenged: off [fixed]
> > tx-lockless: off [fixed]
> > netns-local: off [fixed]
> > tx-gso-robust: off [fixed]
> > tx-fcoe-segmentation: off [fixed]
> > tx-gre-segmentation: off [fixed]
> > tx-gre-csum-segmentation: off [fixed]
> > tx-ipxip4-segmentation: off [fixed]
> > tx-ipxip6-segmentation: off [fixed]
> > tx-udp_tnl-segmentation: off [fixed]
> > tx-udp_tnl-csum-segmentation: off [fixed]
> > tx-gso-partial: off [fixed]
> > tx-sctp-segmentation: off [fixed]
> > tx-esp-segmentation: off [fixed]
> > tx-udp-segmentation: off [fixed]
> > fcoe-mtu: off [fixed]
> > tx-nocache-copy: off
> > loopback: off [fixed]
> > rx-fcs: off [fixed]
> > rx-all: off [fixed]
> > tx-vlan-stag-hw-insert: off [fixed]
> > rx-vlan-stag-hw-parse: off [fixed]
> > rx-vlan-stag-filter: off [fixed]
> > l2-fwd-offload: off [fixed]
> > hw-tc-offload: off [fixed]
> > esp-hw-offload: off [fixed]
> > esp-tx-csum-hw-offload: off [fixed]
> > rx-udp_tunnel-port-offload: off [fixed]
> > tls-hw-tx-offload: off [fixed]
> > tls-hw-rx-offload: off [fixed]
> > rx-gro-hw: off [fixed]
> > tls-hw-record: off [fixed]
> >
> >
> > I can reproduce the issue with kernels 5.0.x and as recent as 5.1-rc6.
> >
> > Cheers,
> > Bruno
> >
> > On Sunday, April 7, 2019 11:28:30 PM CEST, richard.purdie@linuxfoundation.org wrote:  
> > > Hi,
> > >
> > > I've been chasing down why a python test from the python3 testsuite
> > > started failing and it seems to point to this kernel change in the
> > > networking stack.
> > >
> > > In kernels beyond commit 4f693b55c3d2d2239b8a0094b518a1e533cf75d5 the
> > > test hangs about 90% of the time (I've reproduced with 5.1-rc3, 5.0.7,
> > > 5.0-rc1 but not 4.18, 4.19 or 4.20). The reproducer is:
> > >
> > > $ python3 -m test test_httplib -v
> > > == CPython 3.7.2 (default, Apr 5 2019, 15:17:15) [GCC 8.3.0]
> > > == Linux-5.0.0-yocto-standard-x86_64-with-glibc2.2.5 little-endian
> > > == cwd: /var/volatile/tmp/test_python_288
> > > == CPU count: 1
> > > == encodings: locale=UTF-8, FS=utf-8
> > > [...]
> > > test_response_fileno (test.test_httplib.BasicTest) ...
> > >
> > > and it hangs in test_response_fileno.
> > >
> > > The test in question comes from Lib/test/test_httplib.py in the python
> > > source tree and the code is:
> > >
> > >     def test_response_fileno(self):
> > >         # Make sure fd returned by fileno is valid.
> > >         serv = socket.socket(
> > >             socket.AF_INET, socket.SOCK_STREAM, socket.IPPROTO_TCP)
> > >         self.addCleanup(serv.close)
> > >         serv.bind((HOST, 0))
> > >         serv.listen()
> > >
> > >         result = None
> > >         def run_server():
> > >             [conn, address] = serv.accept()
> > >             with conn, conn.makefile("rb") as reader:
> > >                 # Read the request header until a blank line
> > >                 while True:
> > >                     line = reader.readline()
> > >                     if not line.rstrip(b"\r\n"):
> > >                         break
> > >                 conn.sendall(b"HTTP/1.1 200 Connection established\r\n\r\n")
> > >                 nonlocal result
> > >                 result = reader.read()
> > >
> > >         thread = threading.Thread(target=run_server)
> > >         thread.start()
> > >         self.addCleanup(thread.join, float(1))
> > >         conn = client.HTTPConnection(*serv.getsockname())
> > >         conn.request("CONNECT", "dummy:1234")
> > >         response = conn.getresponse()
> > >         try:
> > >             self.assertEqual(response.status, client.OK)
> > >             s = socket.socket(fileno=response.fileno())
> > >             try:
> > >                 s.sendall(b"proxied data\n")
> > >             finally:
> > >                 s.detach()
> > >         finally:
> > >             response.close()
> > >             conn.close()
> > >         thread.join()
> > >         self.assertEqual(result, b"proxied data\n")
> > >
> > > I was hoping someone with more understanding of the networking stack
> > > could look at this and tell whether its a bug in the python test, the
> > > kernel change or otherwise give a pointer to where the problem might
> > > be? I'll freely admit this is not an area I know much about.
> > >
> > > Cheers,
> > >
> > > Richard
> > >
> > >
> > >