[chrony-dev] [PATCH] MacOS X - Run chronyd as real-time process

[Bryan Christianson <bryan@xxxxxxxxxxxxx>]

Adds option -P to chronyd on MacOS X which can be used to enable the thread time constraint scheduling policy. This near real-time scheduling policy removes a 1usec bias from the 'System time' offset.
---
 chrony.texi.in |  7 ++++---
 chronyd.8.in   |  7 ++++---
 sys.c          |  2 ++
 sys_macosx.c   | 54 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 sys_macosx.h   |  2 +-
 5 files changed, 65 insertions(+), 7 deletions(-)

diff --git a/chrony.texi.in b/chrony.texi.in
index 0b31bb6..89fe5e1 100644
--- a/chrony.texi.in
+++ b/chrony.texi.in
@@ -1039,9 +1039,10 @@ not correct the clock.
 This option displays @code{chronyd's} version number to the terminal and
 exits.
 @item -P <priority>
-This option will select the SCHED_FIFO real-time scheduler at the
-specified priority (which must be between 0 and 100).  This mode is
-supported only on Linux.
+On Linux, this option will select the SCHED_FIFO real-time scheduler at the specified
+priority (which must be between 0 and 100).  On MacOSX, this option must have either
+a value of 0 (the default) to disable the thread time constraint policy or 1 for the
+policy to be enabled. Other systems do not support this option.
 @item -m     
 This option will lock chronyd into RAM so that it will never be paged
 out.  This mode is only supported on Linux.
diff --git a/chronyd.8.in b/chronyd.8.in
index 387ea21..b18d87c 100644
--- a/chronyd.8.in
+++ b/chronyd.8.in
@@ -40,9 +40,10 @@ A summary of the options supported by \fBchronyd\fR is included below.
 
 .TP
 \fB\-P\fR \fIpriority\fR
-This option will select the SCHED_FIFO real-time scheduler at the specified
-priority (which must be between 0 and 100).  This mode is supported only on
-Linux.
+On Linux, this option will select the SCHED_FIFO real-time scheduler at the specified
+priority (which must be between 0 and 100).  On MacOSX, this option must have either
+a value of 0 (the default) to disable the thread time constraint policy or 1 for the
+policy to be enabled. Other systems do not support this option.
 .TP
 .B \-m
 This option will lock chronyd into RAM so that it will never be paged out.
diff --git a/sys.c b/sys.c
index 9de9ee1..5e04c35 100644
--- a/sys.c
+++ b/sys.c
@@ -124,6 +124,8 @@ void SYS_SetScheduler(int SchedPriority)
 {
 #if defined(LINUX) && defined(HAVE_SCHED_SETSCHEDULER)
   SYS_Linux_SetScheduler(SchedPriority);
+#elif defined(MACOSX)
+  SYS_MacOSX_SetScheduler(SchedPriority);
 #else
   LOG_FATAL(LOGF_Sys, "scheduler priority setting not supported");
 #endif
diff --git a/sys_macosx.c b/sys_macosx.c
index 33da60d..bd405f1 100644
--- a/sys_macosx.c
+++ b/sys_macosx.c
@@ -42,7 +42,12 @@
 #include <stdio.h>
 #include <signal.h>
 
+#include <mach/mach.h>
+#include <mach/mach_time.h>
+#include <pthread.h>
+
 #include "sys_macosx.h"
+#include "conf.h"
 #include "localp.h"
 #include "sched.h"
 #include "logging.h"
@@ -85,6 +90,8 @@ static struct timeval Tdrift;
 /* minimum resolution of current_frequency */
 #define FREQUENCY_RES (1.0e-9)
 
+#define NANOS_PER_MSEC (1000000ULL)
+
 /* ================================================== */
 
 static void
@@ -335,6 +342,53 @@ drift_removal_timeout(SCH_ArbitraryArgument not_used)
 }
 
 /* ================================================== */
+/*
+  Give chronyd real time priority so that time critical calculations
+  are not pre-empted by the kernel.
+*/
+
+static int
+set_realtime(void)
+{
+  /* https://developer.apple.com/library/ios/technotes/tn2169/_index.html */
+
+  mach_timebase_info_data_t timebase_info;
+  double clock2abs;
+  thread_time_constraint_policy_data_t policy;
+  int kr;
+
+  mach_timebase_info(&timebase_info);
+  clock2abs = ((double)timebase_info.denom / (double)timebase_info.numer) * NANOS_PER_MSEC;
+
+  policy.period = 0;
+  policy.computation = (uint32_t)(5 * clock2abs); /* 5 ms of work */
+  policy.constraint = (uint32_t)(10 * clock2abs);
+  policy.preemptible = 0;
+
+  kr = thread_policy_set(
+          pthread_mach_thread_np(pthread_self()),
+          THREAD_TIME_CONSTRAINT_POLICY,
+          (thread_policy_t)&policy,
+          THREAD_TIME_CONSTRAINT_POLICY_COUNT);
+
+  if (kr != KERN_SUCCESS) {
+    LOG(LOGS_WARN, LOGF_SysMacOSX, "Cannot set real-time priority: %d", kr);
+    return -1;
+  }
+  return 0;
+}
+
+/* ================================================== */
+
+void
+SYS_MacOSX_SetScheduler(int SchedPriority)
+{
+  if (SchedPriority) {
+    set_realtime();
+  }
+}
+
+/* ================================================== */
 
 void
 SYS_MacOSX_Initialise(void)
diff --git a/sys_macosx.h b/sys_macosx.h
index b579415..707700d 100644
--- a/sys_macosx.h
+++ b/sys_macosx.h
@@ -30,8 +30,8 @@
 #ifndef GOT_SYS_MACOSX_H
 #define GOT_SYS_MACOSX_H
 
+void SYS_MacOSX_SetScheduler(int SchedPriority);
 void SYS_MacOSX_Initialise(void);
-
 void SYS_MacOSX_Finalise(void);
 
 #endif
-- 
2.3.2 (Apple Git-55)


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