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.

As part of this, switch the internals to use the full tracing path
instead of relying upon adding /tracing/ to the path of files used.

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 use debugfs, append "/tracing" to the path
* If we find neither, just use /sys/kernel/debug/tracing

Using the default means we couldn't find tracefs or debugfs mounted
and so is likely to fail later if you are using ftrace, but should have
no impact if you're not tracing.

All of the path selection 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]>

doc/tutorial.txt

@@ -92,9 +92,8 @@ The category names currently supported are:
 
 Default value is "none".
 
-* ftracedir : String. Prefix used to find tracefs. Defaults to
-"/sys/kernel/debug" if not specified. In that case, we expect to find tracing
-controls inside "/sys/kernel/debug/tracing/". Has no effect if ftrace is not
+* ftracedir : String. Mountpoint of tracefs. Defaults to
+"/sys/kernel/debug/tracing" if not specified. Has no effect if ftrace is not
 enabled.
 
 * gnuplot : Boolean. If True, it will create a gnu plot compatible file for

src/rt-app.c

@@ -1542,7 +1542,7 @@ int main(int argc, char* argv[])
 		log_notice("configuring ftrace");
 		// check if tracing is enabled
 		strcpy(tmp, opts.ftracedir);
-		strcat(tmp, "/tracing/tracing_on");
+		strcat(tmp, "/tracing_on");
 		int ftrace_f = open(tmp, O_RDONLY);
 		if (ftrace_f < 0){
 			log_error("Cannot open tracing_on file %s", tmp);
@@ -1557,7 +1557,7 @@ int main(int argc, char* argv[])
 		close(ftrace_f);
 		// set the marker
 		strcpy(tmp, opts.ftracedir);
-		strcat(tmp, "/tracing/trace_marker");
+		strcat(tmp, "/trace_marker");
 		ft_data.marker_fd = open(tmp, O_WRONLY);
 		if (ft_data.marker_fd < 0) {
 			log_error("Cannot open trace_marker file %s", tmp);

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_types.h

@@ -290,7 +290,7 @@ typedef struct _rtapp_options_t {
 
 	int cumulative_slack;
 
-	char *ftracedir;
+	char *ftracedir; /* should end in 'tracing' */
 } rtapp_options_t;
 
 typedef struct _timing_point_t {

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 +268,60 @@ resource_to_string(resource_t resource, char *resource_name)
 	return 0;
 }
 
+/* point directly to the tracing directory */
+#define DEFAULT_FTRACE_DIR "/sys/kernel/debug/tracing"
+
+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);
+				return path;
+			}
+		}
+		endmntent(fd_mntent);
+	}
+	return NULL;
+}
+
+char *ftrace_dir(void)
+{
+	char *path;
+
+	/* tracefs paths point directly to the tracing directory */
+	path = mtab_by_type("tracefs", sizeof("tracefs")-1);
+	if(path)
+		return path;
+
+	/* debugfs paths point one level above the tracing directory */
+	path = mtab_by_type("debugfs", sizeof("debugfs")-1);
+	if(path)
+	{
+		char *tmp;
+		int len = strlen(path) + sizeof("/tracing");
+		tmp = malloc(len);
+		if(!tmp)
+			return NULL;
+		strcpy(tmp, path);
+		free(path);
+		strcat(tmp, "/tracing");
+		return tmp;
+	}
+
+	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 */
+char *
+ftrace_dir(void);
+
 int
 ftrace_setup(char *categories);