trace2: refactor setting process starting time

Create trace2_initialize_clock() and call from main() to capture
process start time in isolation and before other sub-systems are
ready.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/api-trace2.txt

@@ -160,17 +160,23 @@ purposes.
 
 These are concerned with the lifetime of the overall git process.
 
+`void trace2_initialize_clock()`::
+
+	Initialize the Trace2 start clock and nothing else.  This should
+	be called at the very top of main() to capture the process start
+	time and reduce startup order dependencies.
+
 `void trace2_initialize()`::
 
 	Determines if any Trace2 Targets should be enabled and
-	initializes the Trace2 facility.  This includes starting the
-	elapsed time clocks and thread local storage (TLS).
+	initializes the Trace2 facility.  This includes setting up the
+	Trace2 thread local storage (TLS).
 +
 This function emits a "version" message containing the version of git
 and the Trace2 protocol.
 +
 This function should be called from `main()` as early as possible in
-the life of the process.
+the life of the process after essential process initialization.
 
 `int trace2_is_enabled()`::
 

common-main.c

@@ -27,6 +27,8 @@ int main(int argc, const char **argv)
 {
 	int result;
 
+	trace2_initialize_clock();
+
 	/*
 	 * Always open file descriptors 0/1/2 to avoid clobbering files
 	 * in die().  It also avoids messing up when the pipes are dup'ed

compat/mingw.c

@@ -2569,6 +2569,8 @@ void mingw_startup(void)
 	wchar_t **wenv, **wargv;
 	_startupinfo si;
 
+	trace2_initialize_clock();
+
 	maybe_redirect_std_handles();
 
 	/* get wide char arguments and environment */

trace2.c

@@ -142,6 +142,11 @@ static void tr2main_signal_handler(int signo)
 	raise(signo);
 }
 
+void trace2_initialize_clock(void)
+{
+	tr2tls_start_process_clock();
+}
+
 void trace2_initialize_fl(const char *file, int line)
 {
 	struct tr2_tgt *tgt_j;
@@ -428,7 +433,7 @@ void trace2_thread_start_fl(const char *file, int line, const char *thread_name)
 	us_now = getnanotime() / 1000;
 	us_elapsed_absolute = tr2tls_absolute_elapsed(us_now);
 
-	tr2tls_create_self(thread_name);
+	tr2tls_create_self(thread_name, us_now);
 
 	for_each_wanted_builtin (j, tgt_j)
 		if (tgt_j->pfn_thread_start_fl)

trace2.h

@@ -19,6 +19,23 @@ struct json_writer;
  * [] trace2_printf*    -- legacy trace[1] messages.
  */
 
+/*
+ * Initialize the TRACE2 clock and do nothing else, in particular
+ * no mallocs, no system inspection, and no environment inspection.
+ *
+ * This should be called at the very top of main() to capture the
+ * process start time.  This is intended to reduce chicken-n-egg
+ * bootstrap pressure.
+ *
+ * It is safe to call this more than once.  This allows capturing
+ * absolute startup costs on Windows which uses a little trickery
+ * to do setup work before common-main.c:main() is called.
+ *
+ * The main trace2_initialize_fl() may be called a little later
+ * after more infrastructure is established.
+ */
+void trace2_initialize_clock(void);
+
 /*
  * Initialize TRACE2 tracing facility if any of the builtin TRACE2
  * targets are enabled in the environment.  Emits a 'version' event.

trace2/tr2_tls.c

@@ -10,16 +10,30 @@
 #define TR2_REGION_NESTING_INITIAL_SIZE (100)
 
 static struct tr2tls_thread_ctx *tr2tls_thread_main;
-static uint64_t tr2tls_us_start_main;
+static uint64_t tr2tls_us_start_process;
 
 static pthread_mutex_t tr2tls_mutex;
 static pthread_key_t tr2tls_key;
 
 static int tr2_next_thread_id; /* modify under lock */
 
-struct tr2tls_thread_ctx *tr2tls_create_self(const char *thread_name)
+void tr2tls_start_process_clock(void)
+{
+	if (tr2tls_us_start_process)
+		return;
+
+	/*
+	 * Keep the absolute start time of the process (i.e. the main
+	 * process) in a fixed variable since other threads need to
+	 * access it.  This allows them to do that without a lock on
+	 * main thread's array data (because of reallocs).
+	 */
+	tr2tls_us_start_process = getnanotime() / 1000;
+}
+
+struct tr2tls_thread_ctx *tr2tls_create_self(const char *thread_name,
+					     uint64_t us_thread_start)
 {
-	uint64_t us_now = getnanotime() / 1000;
 	struct tr2tls_thread_ctx *ctx = xcalloc(1, sizeof(*ctx));
 
 	/*
@@ -29,7 +43,7 @@ struct tr2tls_thread_ctx *tr2tls_create_self(const char *thread_name)
 	 */
 	ctx->alloc = TR2_REGION_NESTING_INITIAL_SIZE;
 	ctx->array_us_start = (uint64_t *)xcalloc(ctx->alloc, sizeof(uint64_t));
-	ctx->array_us_start[ctx->nr_open_regions++] = us_now;
+	ctx->array_us_start[ctx->nr_open_regions++] = us_thread_start;
 
 	ctx->thread_id = tr2tls_locked_increment(&tr2_next_thread_id);
 
@@ -55,7 +69,7 @@ struct tr2tls_thread_ctx *tr2tls_get_self(void)
 	 * here and silently continue.
 	 */
 	if (!ctx)
-		ctx = tr2tls_create_self("unknown");
+		ctx = tr2tls_create_self("unknown", getnanotime() / 1000);
 
 	return ctx;
 }
@@ -124,22 +138,18 @@ uint64_t tr2tls_absolute_elapsed(uint64_t us)
 	if (!tr2tls_thread_main)
 		return 0;
 
-	return us - tr2tls_us_start_main;
+	return us - tr2tls_us_start_process;
 }
 
 void tr2tls_init(void)
 {
+	tr2tls_start_process_clock();
+
 	pthread_key_create(&tr2tls_key, NULL);
 	init_recursive_mutex(&tr2tls_mutex);
 
-	tr2tls_thread_main = tr2tls_create_self("main");
-	/*
-	 * Keep a copy of the absolute start time of the main thread
-	 * in a fixed variable since other threads need to access it.
-	 * This also eliminates the need to lock accesses to the main
-	 * thread's array (because of reallocs).
-	 */
-	tr2tls_us_start_main = tr2tls_thread_main->array_us_start[0];
+	tr2tls_thread_main =
+		tr2tls_create_self("main", tr2tls_us_start_process);
 }
 
 void tr2tls_release(void)

trace2/tr2_tls.h

@@ -31,7 +31,8 @@ struct tr2tls_thread_ctx {
  * In this and all following functions the term "self" refers to the
  * current thread.
  */
-struct tr2tls_thread_ctx *tr2tls_create_self(const char *thread_name);
+struct tr2tls_thread_ctx *tr2tls_create_self(const char *thread_name,
+					     uint64_t us_thread_start);
 
 /*
  * Get our TLS data.
@@ -94,4 +95,9 @@ void tr2tls_release(void);
  */
 int tr2tls_locked_increment(int *p);
 
+/*
+ * Capture the process start time and do nothing else.
+ */
+void tr2tls_start_process_clock(void);
+
 #endif /* TR2_TLS_H */