Add UMA for the kernel's TCP RTT estimate.

net/socket/tcp_socket_libevent.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "net/socket/tcp_socket.h"
 
 #include <errno.h>
 #include <netinet/tcp.h>
 #include <sys/socket.h>
 
@@ skipping 704 matching lines @@
     return;
   }
 
   bool getsockopt_success = false;
   bool server_acked_data = false;
 #if defined(TCP_INFO)
   // Probe to see the if the socket used TCP FastOpen.
   tcp_info info;
   socklen_t info_len = sizeof(tcp_info);
   getsockopt_success = getsockopt(socket_->socket_fd(), IPPROTO_TCP, TCP_INFO,
                                   &info, &info_len) == 0 &&

[mmenke, 2015/07/07 19:53:49]

Can we just make this into a method?

It's a bit more code, and still has to be TCP_INFO guarded, but the getsocketopt
+ info_len check is sufficiently ugly that it may be a bit of an improvement.

Could just make it a static method in this file, and pass in the fd and the
tcp_info, and have it return false on failure.

[Bryan McQuade, 2015/07/07 21:05:10]

Done.

                        info_len == sizeof(tcp_info);
   server_acked_data = getsockopt_success &&
                       (info.tcpi_options & TCPI_OPT_SYN_DATA);
 #endif
 
   if (getsockopt_success) {
     if (tcp_fastopen_status_ == TCP_FASTOPEN_FAST_CONNECT_RETURN) {
       tcp_fastopen_status_ = (server_acked_data ?
                               TCP_FASTOPEN_SYN_DATA_ACK :
                               TCP_FASTOPEN_SYN_DATA_NACK);
     } else {
       tcp_fastopen_status_ = (server_acked_data ?
                               TCP_FASTOPEN_NO_SYN_DATA_ACK :
                               TCP_FASTOPEN_NO_SYN_DATA_NACK);
     }
   } else {
     tcp_fastopen_status_ =
         (tcp_fastopen_status_ == TCP_FASTOPEN_FAST_CONNECT_RETURN ?
          TCP_FASTOPEN_SYN_DATA_GETSOCKOPT_FAILED :
          TCP_FASTOPEN_NO_SYN_DATA_GETSOCKOPT_FAILED);
   }
 }
 
+bool TCPSocketLibevent::GetEstimatedRoundTripTime(base::TimeDelta* out_rtt) {

[mmenke, 2015/07/07 19:53:50]

How do you intend to use this to inform future code decisions?  Just wondering
what role localhost requests and the like play here, and if we care.

[Bryan McQuade, 2015/07/07 21:03:33]

I'm mostly interested in understanding TCP RTT in UMA, but the team working on
the network quality estimator is also interested in using this to improve their
estimation. I imagine they'd want to ignore connections to localhost.

+  DCHECK(out_rtt);
+  if (!socket_ || !socket_->IsConnected()) {

[mmenke, 2015/07/07 19:53:50]

The IsConnected() check seems concerning - if the server hangs up on us, it will
return false, but I think it may still have stats we can get?

[Bryan McQuade, 2015/07/07 21:03:32]

Yes, good point. I had added that to prevent logging the RTT when the kernel
doesn't have an estimate yet, but I think it's better for me to heck if the
kernel estimate is zero and discard if so. I suppose that technically a
connection could have a zero-microsecond RTT but I'm inclined to interpret zero
values as indicating the kernel doesn't yet have an estimate.

+    return false;
+  }

[mmenke, 2015/07/07 19:53:49]

nit:  Remove braces.

[Bryan McQuade, 2015/07/07 21:03:33]

Done.

+
+  bool success = false;
+#if defined(TCP_INFO)
+  tcp_info info;
+  socklen_t info_len = sizeof(tcp_info);
+  success = getsockopt(socket_->socket_fd(), IPPROTO_TCP, TCP_INFO, &info,
+                       &info_len) == 0 &&
+            info_len == sizeof(tcp_info);
+  if (success) {
+    *out_rtt = base::TimeDelta::FromMicroseconds(info.tcpi_rtt);

[mmenke, 2015/07/07 19:53:49]

Suggest getting rid of success variable, and just returning true here.

[Bryan McQuade, 2015/07/07 21:03:33]

Done.

+  }

[mmenke, 2015/07/07 19:53:50]

nit:  Remove braces.

[Bryan McQuade, 2015/07/07 21:03:32]

Done.

+#endif

[mmenke, 2015/07/07 19:53:49]

nit:  #endif  // defined(TCP_INFO)

(Not done in most places in this file, but it's the preferred style).

[Bryan McQuade, 2015/07/07 21:03:32]

Done.

+
+  return success;
+}
+
 }  // namespace net