unix: Implement `-X realtime` command-line option on macOS.

This adds a new command line option to the unix port `-X realtime` to
enable realtime priority on threads.  This enables high precision timers
for applications that need more accurate timers.

Related docs:
https://developer.apple.com/library/archive/technotes/tn2169/_index.html

Fixes issue #8621.

Signed-off-by: David Lechner <david@pybricks.com>

docs/unix/quickref.rst

@@ -73,6 +73,8 @@ General options:
     - ``-X heapsize=<n>[w][K|M]`` sets the heap size for the garbage collector.
       The suffix ``w`` means words instead of bytes. ``K`` means x1024 and ``M``
       means x1024x1024.
+    - ``-X realtime`` sets thread priority to realtime. This can be used to
+      improve timer precision. Only available on macOS.
 
 
 

ports/unix/main.c

@@ -328,6 +328,10 @@ STATIC void print_help(char **argv) {
         , heap_size);
     impl_opts_cnt++;
     #endif
+    #if defined(__APPLE__)
+    printf("  realtime -- set thread priority to realtime\n");
+    impl_opts_cnt++;
+    #endif
 
     if (impl_opts_cnt == 0) {
         printf("  (none)\n");
@@ -399,6 +403,15 @@ STATIC void pre_process_options(int argc, char **argv) {
                         goto invalid_arg;
                     }
                 #endif
+                #if defined(__APPLE__)
+                } else if (strcmp(argv[a + 1], "realtime") == 0) {
+                    #if MICROPY_PY_THREAD
+                    mp_thread_is_realtime_enabled = true;
+                    #endif
+                    // main thread was already intialized before the option
+                    // was parsed, so we have to enable realtime here.
+                    mp_thread_set_realtime();
+                #endif
                 } else {
                 invalid_arg:
                     exit(invalid_args());

ports/unix/mpthreadport.c

@@ -192,6 +192,13 @@ void mp_thread_set_state(mp_state_thread_t *state) {
 }
 
 void mp_thread_start(void) {
+    // enable realtime priority if `-X realtime` command line parameter was set
+    #if defined(__APPLE__)
+    if (mp_thread_is_realtime_enabled) {
+        mp_thread_set_realtime();
+    }
+    #endif
+
     pthread_setcanceltype(PTHREAD_CANCEL_ASYNCHRONOUS, NULL);
     mp_thread_unix_begin_atomic_section();
     for (mp_thread_t *th = thread; th != NULL; th = th->next) {
@@ -310,3 +317,39 @@ void mp_thread_mutex_unlock(mp_thread_mutex_t *mutex) {
 }
 
 #endif // MICROPY_PY_THREAD
+
+// this is used even when MICROPY_PY_THREAD is disabled
+
+#if defined(__APPLE__)
+#include <mach/mach_error.h>
+#include <mach/mach_time.h>
+#include <mach/thread_act.h>
+#include <mach/thread_policy.h>
+
+bool mp_thread_is_realtime_enabled;
+
+// based on https://developer.apple.com/library/archive/technotes/tn2169/_index.html
+void mp_thread_set_realtime(void) {
+    mach_timebase_info_data_t timebase_info;
+
+    mach_timebase_info(&timebase_info);
+
+    const uint64_t NANOS_PER_MSEC = 1000000ULL;
+    double clock2abs = ((double)timebase_info.denom / (double)timebase_info.numer) * NANOS_PER_MSEC;
+
+    thread_time_constraint_policy_data_t policy;
+    policy.period = 0;
+    policy.computation = (uint32_t)(5 * clock2abs); // 5 ms of work
+    policy.constraint = (uint32_t)(10 * clock2abs);
+    policy.preemptible = FALSE;
+
+    int 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) {
+        mach_error("thread_policy_set:", kr);
+    }
+}
+#endif

ports/unix/mpthreadport.h

@@ -25,6 +25,7 @@
  */
 
 #include <pthread.h>
+#include <stdbool.h>
 
 typedef pthread_mutex_t mp_thread_mutex_t;
 
@@ -36,3 +37,9 @@ void mp_thread_gc_others(void);
 // Functions as a port-global lock for any code that must be serialised.
 void mp_thread_unix_begin_atomic_section(void);
 void mp_thread_unix_end_atomic_section(void);
+
+// for `-X realtime` command line option
+#if defined(__APPLE__)
+extern bool mp_thread_is_realtime_enabled;
+void mp_thread_set_realtime(void);
+#endif