fs/fs-writeback.c: inode writeback list tracking tracepoints

The per-sb inode writeback list tracks inodes currently under writeback
to facilitate efficient sync processing.  In particular, it ensures that
sync only needs to walk through a list of inodes that were cleaned by
the sync.

Add a couple tracepoints to help identify when inodes are added/removed
to and from the writeback lists.  Piggyback off of the writeback
lazytime tracepoint template as it already tracks the relevant inode
information.

Link: http://lkml.kernel.org/r/1466594593-6757-3-git-send-email-bfoster@redhat.com
Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: Dave Chinner <dchinner@redhat.com>
cc: Josef Bacik <jbacik@fb.com>
Cc: Holger Hoffstätte <holger.hoffstaette@applied-asynchrony.com>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/fs-writeback.c

@@ -990,8 +990,10 @@ void sb_mark_inode_writeback(struct inode *inode)
 
 	if (list_empty(&inode->i_wb_list)) {
 		spin_lock_irqsave(&sb->s_inode_wblist_lock, flags);
-		if (list_empty(&inode->i_wb_list))
+		if (list_empty(&inode->i_wb_list)) {
 			list_add_tail(&inode->i_wb_list, &sb->s_inodes_wb);
+			trace_sb_mark_inode_writeback(inode);
+		}
 		spin_unlock_irqrestore(&sb->s_inode_wblist_lock, flags);
 	}
 }
@@ -1006,7 +1008,10 @@ void sb_clear_inode_writeback(struct inode *inode)
 
 	if (!list_empty(&inode->i_wb_list)) {
 		spin_lock_irqsave(&sb->s_inode_wblist_lock, flags);
-		list_del_init(&inode->i_wb_list);
+		if (!list_empty(&inode->i_wb_list)) {
+			list_del_init(&inode->i_wb_list);
+			trace_sb_clear_inode_writeback(inode);
+		}
 		spin_unlock_irqrestore(&sb->s_inode_wblist_lock, flags);
 	}
 }

include/trace/events/writeback.h

@@ -696,7 +696,7 @@ DEFINE_EVENT(writeback_single_inode_template, writeback_single_inode,
 	TP_ARGS(inode, wbc, nr_to_write)
 );
 
-DECLARE_EVENT_CLASS(writeback_lazytime_template,
+DECLARE_EVENT_CLASS(writeback_inode_template,
 	TP_PROTO(struct inode *inode),
 
 	TP_ARGS(inode),
@@ -723,25 +723,39 @@ DECLARE_EVENT_CLASS(writeback_lazytime_template,
 		  show_inode_state(__entry->state), __entry->mode)
 );
 
-DEFINE_EVENT(writeback_lazytime_template, writeback_lazytime,
+DEFINE_EVENT(writeback_inode_template, writeback_lazytime,
 	TP_PROTO(struct inode *inode),
 
 	TP_ARGS(inode)
 );
 
-DEFINE_EVENT(writeback_lazytime_template, writeback_lazytime_iput,
+DEFINE_EVENT(writeback_inode_template, writeback_lazytime_iput,
 	TP_PROTO(struct inode *inode),
 
 	TP_ARGS(inode)
 );
 
-DEFINE_EVENT(writeback_lazytime_template, writeback_dirty_inode_enqueue,
+DEFINE_EVENT(writeback_inode_template, writeback_dirty_inode_enqueue,
 
 	TP_PROTO(struct inode *inode),
 
 	TP_ARGS(inode)
 );
 
+/*
+ * Inode writeback list tracking.
+ */
+
+DEFINE_EVENT(writeback_inode_template, sb_mark_inode_writeback,
+	TP_PROTO(struct inode *inode),
+	TP_ARGS(inode)
+);
+
+DEFINE_EVENT(writeback_inode_template, sb_clear_inode_writeback,
+	TP_PROTO(struct inode *inode),
+	TP_ARGS(inode)
+);
+
 #endif /* _TRACE_WRITEBACK_H */
 
 /* This part must be outside protection */