ftrace: Autodetect location of tracing folder

If you don't provide a tracing location in the global config key section,
rt-app usually looks in /sys/kernel/debug for the tracing folder.

This patch allows rt-app to examine the content of /etc/mtab, and apply
the following ordering:

* If tracefs is mounted, use that location
* If tracefs is not found anywhere, then look for debugfs
* If we find neither, just use /sys/kernel/debug

All of this is overridden if you provide the config key.

This should allow rt-app to work on any platform, at least from tracing
POV.

Signed-off-by: Chris Redpath <[email protected]>

src/rt-app_parse_config.c

@@ -40,8 +40,6 @@ Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
 #define JSON_FILE_BUF_SIZE 4096
 #define DEFAULT_MEM_BUF_SIZE (4 * 1024 * 1024)
 
-#define DEFAULT_FTRACE_DIR "/sys/kernel/debug"
-
 #ifndef TRUE
 #define TRUE true
 #define FALSE false
@@ -1209,7 +1207,7 @@ parse_global(struct json_object *global, rtapp_options_t *opts)
 		opts->io_device = strdup("/dev/null");
 		opts->mem_buffer_size = DEFAULT_MEM_BUF_SIZE;
 		opts->cumulative_slack = 0;
-		opts->ftracedir = strdup(DEFAULT_FTRACE_DIR);
+		opts->ftracedir = ftrace_dir();
 		return;
 	}
 
@@ -1307,7 +1305,7 @@ parse_global(struct json_object *global, rtapp_options_t *opts)
 	    log_critical(PFX "Invalid ftrace categories");
 	    exit(EXIT_INV_CONFIG);
 	}
-	opts->ftracedir = get_string_value_from(global, "ftracedir", TRUE, DEFAULT_FTRACE_DIR);
+	opts->ftracedir = get_string_value_from(global, "ftracedir", TRUE, ftrace_dir());
 
 	opts->lock_pages = get_bool_value_from(global, "lock_pages", TRUE, 1);
 	opts->pi_enabled = get_bool_value_from(global, "pi_enabled", TRUE, 0);

src/rt-app_utils.c

@@ -25,6 +25,7 @@ Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
 #include <errno.h>
 #include <math.h>
 #include <stdarg.h>
+#include <mntent.h>
 
 #include "rt-app_utils.h"
 
@@ -267,6 +268,48 @@ resource_to_string(resource_t resource, char *resource_name)
 	return 0;
 }
 
+#define DEFAULT_FTRACE_DIR "/sys/kernel/debug"
+
+char *mtab_by_type(const char *type, int typelen)
+{
+	FILE *fd_mntent;
+
+	fd_mntent = setmntent("/etc/mtab", "r");
+	if(fd_mntent)
+	{
+		struct mntent *mntent;
+
+		while(mntent = getmntent(fd_mntent), mntent)
+		{
+			log_debug("mntent->dir=%s type=%s", mntent->mnt_dir, mntent->mnt_type);
+			if(!strncmp(type, mntent->mnt_type, typelen))
+			{
+				char *path = strdup(mntent->mnt_dir);
+				endmntent(fd_mntent);
+				log_info("Found target");
+				return path;
+			}
+		}
+		endmntent(fd_mntent);
+	}
+	return NULL;
+}
+
+const char *ftrace_dir(void)
+{
+	char *path;
+
+	path = mtab_by_type("tracefs", sizeof("tracefs")-1);
+	if(path)
+		return path;
+
+	path = mtab_by_type("debugfs", sizeof("debugfs")-1);
+	if(path)
+		return path;
+
+	return strdup(DEFAULT_FTRACE_DIR);
+}
+
 int ftrace_setup(char *categories)
 {
 	char *cat = strtok(categories, ",");

src/rt-app_utils.h

@@ -141,6 +141,10 @@ string_to_resource(const char *name, resource_t *resource);
 int
 resource_to_string(resource_t resource, char *name);
 
+/* ftrace_dir always returns a string on the heap */
+const char *
+ftrace_dir(void);
+
 int
 ftrace_setup(char *categories);