ext4: add tracepoint for ext4_journal_start

This will help debug who is responsible for starting a jbd2 transaction.
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>

fs/ext4/super.c

@@ -269,6 +269,7 @@ handle_t *ext4_journal_start_sb(struct super_block *sb, int nblocks)
 	journal_t *journal;
 	handle_t  *handle;
 
+	trace_ext4_journal_start(sb, nblocks, _RET_IP_);
 	if (sb->s_flags & MS_RDONLY)
 		return ERR_PTR(-EROFS);
 

include/trace/events/ext4.h

@@ -1520,6 +1520,28 @@ TRACE_EVENT(ext4_load_inode,
 		  (unsigned long) __entry->ino)
 );
 
+TRACE_EVENT(ext4_journal_start,
+	TP_PROTO(struct super_block *sb, int nblocks, unsigned long IP),
+
+	TP_ARGS(sb, nblocks, IP),
+
+	TP_STRUCT__entry(
+		__field(	dev_t,	dev			)
+		__field(	  int, 	nblocks			)
+		__field(unsigned long,	ip			)
+	),
+
+	TP_fast_assign(
+		__entry->dev	 = sb->s_dev;
+		__entry->nblocks = nblocks;
+		__entry->ip	 = IP;
+	),
+
+	TP_printk("dev %d,%d nblocks %d caller %pF",
+		  MAJOR(__entry->dev), MINOR(__entry->dev),
+		  __entry->nblocks, (void *)__entry->ip)
+);
+
 #endif /* _TRACE_EXT4_H */
 
 /* This part must be outside protection */