From: Miroslav Lichvar <mlichvar@redhat.com>
Date: Fri, 2 Mar 2018 10:49:53 +0000 (+0100)
Subject: ntp: delay enabling permanent kernel RX timestamping on Linux
X-Git-Tag: 3.3-pre1~23
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=8b1f68b1b4044a941d4f2ffc579aea1430ce432c;p=thirdparty%2Fchrony.git

ntp: delay enabling permanent kernel RX timestamping on Linux

Wait until a kernel RX timestamp is actually missing before opening the
dummy socket in order to avoid a small performance impact in case the
servers are so slow/distant that the kernel can constantly win the race.
---

diff --git a/ntp_io_linux.c b/ntp_io_linux.c
index eb437e37..c44a13be 100644
--- a/ntp_io_linux.c
+++ b/ntp_io_linux.c
@@ -397,9 +397,7 @@ NIO_Linux_Initialise(void)
 
   monitored_socket = INVALID_SOCK_FD;
   suspended_socket = INVALID_SOCK_FD;
-
-  /* Open a socket to keep the kernel RX timestamping permanently enabled */
-  dummy_rxts_socket = open_dummy_socket();
+  dummy_rxts_socket = INVALID_SOCK_FD;
 }
 
 /* ================================================== */
@@ -763,6 +761,14 @@ NIO_Linux_ProcessMessage(NTP_Remote_Address *remote_addr, NTP_Local_Address *loc
     }
   }
 
+  /* If the kernel is slow with enabling RX timestamping, open a dummy
+     socket to keep the kernel RX timestamping permanently enabled */
+  if (!is_tx && local_ts->source == NTP_TS_DAEMON && ts_flags) {
+    DEBUG_LOG("Missing kernel RX timestamp");
+    if (dummy_rxts_socket == INVALID_SOCK_FD)
+      dummy_rxts_socket = open_dummy_socket();
+  }
+
   /* Return the message if it's not received from the error queue */
   if (!is_tx)
     return 0;