blktrace: add trace/ to /sys/block/sda

Impact: allow ftrace-plugin blktrace to trace device-mapper devices

To trace a single partition:
  # echo 1 > /sys/block/sda/sda1/enable

To trace the whole sda instead:
  # echo 1 > /sys/block/sda/enable

Thus we also fix an issue reported by Ted, that ftrace-plugin blktrace
can't be used to trace device-mapper devices.

Now:

  # echo 1 > /sys/block/dm-0/trace/enable
  echo: write error: No such device or address
  # mount -t ext4 /dev/dm-0 /mnt
  # echo 1 > /sys/block/dm-0/trace/enable
  # echo blk > /debug/tracing/current_tracer

Reported-by: Theodore Tso <tytso@mit.edu>
Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Acked-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Shawn Du <duyuyang@gmail.com>
Cc: Jens Axboe <jens.axboe@oracle.com>
LKML-Reference: <49E42665.6020506@cn.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

block/blk-sysfs.c

@@ -387,16 +387,21 @@ struct kobj_type blk_queue_ktype = {
 int blk_register_queue(struct gendisk *disk)
 {
 	int ret;
+	struct device *dev = disk_to_dev(disk);
 
 	struct request_queue *q = disk->queue;
 
 	if (WARN_ON(!q))
 		return -ENXIO;
 
+	ret = blk_trace_init_sysfs(dev);
+	if (ret)
+		return ret;
+
 	if (!q->request_fn)
 		return 0;
 
-	ret = kobject_add(&q->kobj, kobject_get(&disk_to_dev(disk)->kobj),
+	ret = kobject_add(&q->kobj, kobject_get(&dev->kobj),
 			  "%s", "queue");
 	if (ret < 0)
 		return ret;

include/linux/blktrace_api.h

@@ -198,6 +198,7 @@ extern int blk_trace_setup(struct request_queue *q, char *name, dev_t dev,
 			   char __user *arg);
 extern int blk_trace_startstop(struct request_queue *q, int start);
 extern int blk_trace_remove(struct request_queue *q);
+extern int blk_trace_init_sysfs(struct device *dev);
 
 extern struct attribute_group blk_trace_attr_group;
 
@@ -210,6 +211,11 @@ extern struct attribute_group blk_trace_attr_group;
 # define blk_trace_startstop(q, start)			(-ENOTTY)
 # define blk_trace_remove(q)				(-ENOTTY)
 # define blk_add_trace_msg(q, fmt, ...)			do { } while (0)
+static inline int blk_trace_init_sysfs(struct device *dev)
+{
+	return 0;
+}
+
 #endif /* CONFIG_BLK_DEV_IO_TRACE */
 
 #endif /* __KERNEL__ */

kernel/trace/blktrace.c

@@ -1644,3 +1644,8 @@ out:
 	return ret ? ret : count;
 }
 
+int blk_trace_init_sysfs(struct device *dev)
+{
+	return sysfs_create_group(&dev->kobj, &blk_trace_attr_group);
+}
+