load_mon: get mem_usage by reading /proc/meminfo on Linux

src/modules/load_mon/LoadMon.cpp

@@ -86,6 +86,17 @@ void LoadMon::start()
 
 void LoadMon::Run()
 {
+#if defined (__PX4_LINUX)
+
+	if (_proc_fd == nullptr) {	// init fd
+		_proc_fd = fopen("/proc/meminfo", "r");
+
+		if (_proc_fd == nullptr) {
+			PX4_ERR("Failed to open /proc/meminfo");
+		}
+	}
+
+#endif
 	perf_begin(_cycle_perf);
 
 	cpuload();
@@ -100,6 +111,9 @@ void LoadMon::Run()
 
 	if (should_exit()) {
 		ScheduleClear();
+#if defined (__PX4_LINUX)
+		fclose(_proc_fd);
+#endif
 		exit_and_cleanup();
 	}
 
@@ -142,17 +156,74 @@ void LoadMon::cpuload()
 	const float interval_idletime = total_runtime - _last_idle_time;
 #endif
 
-	// get ram usage
-	struct mallinfo mem = mallinfo();
-	float ram_usage = (float)mem.uordblks / mem.arena;
-
 	cpuload_s cpuload{};
 #if defined(__PX4_LINUX)
+	/* following calculation is based on free(1)
+	 * https://gitlab.com/procps-ng/procps/-/blob/master/proc/sysinfo.c */
+	char line[256];
+	int32_t kb_main_total = -1;
+	int32_t kb_main_free = -1;
+	int32_t kb_page_cache = -1;
+	int32_t kb_slab_reclaimable = -1;
+	int32_t kb_main_buffers = -1;
+	int parsedCount = 0;
+
+	if (_proc_fd != nullptr) {
+		while (fgets(line, sizeof(line), _proc_fd)) {
+			if (sscanf(line, "MemTotal: %d kB", &kb_main_total) == 1) {
+				++parsedCount;
+				continue;
+			}
+
+			if (sscanf(line, "MemFree: %d kB", &kb_main_free) == 1) {
+				++parsedCount;
+				continue;
+			}
+
+			if (sscanf(line, "Cached: %d kB", &kb_page_cache) == 1) {
+				++parsedCount;
+				continue;
+			}
+
+			if (sscanf(line, "SReclaimable: %d kB", &kb_slab_reclaimable) == 1) {
+				++parsedCount;
+				continue;
+			}
+
+			if (sscanf(line, "Buffers: %d kB", &kb_main_buffers) == 1) {
+				++parsedCount;
+				continue;
+			}
+		}
+
+		fseek(_proc_fd, 0, SEEK_END);
+
+		if (parsedCount == 5) {
+			int32_t kb_main_cached = kb_page_cache + kb_slab_reclaimable;
+			int32_t mem_used = kb_main_total - kb_main_free - kb_main_cached - kb_main_buffers;
+
+			if (mem_used < 0) {
+				mem_used = kb_main_total - kb_main_free;
+			}
+
+			cpuload.ram_usage = (float)mem_used / kb_main_total;
+
+		} else {
+			PX4_ERR("Could not parse /proc/meminfo");
+			cpuload.ram_usage = -1;
+		}
+
+	} else {
+		cpuload.ram_usage = -1;
+	}
+
 	cpuload.load = interval_spent_time / interval;
 #elif defined(__PX4_NUTTX)
+	// get ram usage
+	struct mallinfo mem = mallinfo();
+	cpuload.ram_usage = (float)mem.uordblks / mem.arena;
 	cpuload.load = 1.f - interval_idletime / interval;
 #endif
-	cpuload.ram_usage = ram_usage;
 	cpuload.timestamp = hrt_absolute_time();
 
 	_cpuload_pub.publish(cpuload);