util: refine load calculating

Signed-off-by: He Xian <hexian000@outlook.com>

Author: hexian000

src/util.c

@@ -384,6 +384,10 @@ void daemonize(
 	slog_setoutput(SLOG_OUTPUT_SYSLOG, "kcptun-libev");
 }
 
+#define READ_TIMESPEC(ts)                                                      \
+	((int_fast64_t)(ts).tv_sec * INT64_C(1000000000) +                     \
+	 (int_fast64_t)(ts).tv_nsec)
+
 double thread_load(void)
 {
 	static _Thread_local struct {
@@ -399,17 +403,18 @@ double thread_load(void)
 		return load;
 	}
 	if (last.set) {
-		const double total =
-			(double)(monotime.tv_sec - last.monotime.tv_sec) +
-			(double)(monotime.tv_nsec - last.monotime.tv_nsec) *
-				1e-9;
-		const double busy =
-			(double)(cputime.tv_sec - last.cputime.tv_sec) +
-			(double)(cputime.tv_nsec - last.cputime.tv_nsec) * 1e-9;
-		load = busy / total;
+		const int_fast64_t total =
+			READ_TIMESPEC(monotime) - READ_TIMESPEC(last.monotime);
+		const int_fast64_t busy =
+			READ_TIMESPEC(cputime) - READ_TIMESPEC(last.cputime);
+		if (busy > 0 && total > 0 && busy <= total) {
+			load = (double)busy / (double)total;
+		}
 	}
 	last.monotime = monotime;
 	last.cputime = cputime;
 	last.set = true;
 	return load;
 }
+
+#undef READ_TIMESPEC