[RFC PATCH v5 2/2] wb: inode writeback list tracking tracepoints

[Brian Foster <bfoster@redhat.com>]

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 affected 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.

Signed-off-by: Brian Foster <bfoster@redhat.com>
---
 fs/fs-writeback.c                | 17 +++++++++++++----
 include/trace/events/writeback.h | 22 ++++++++++++++++++----
 2 files changed, 31 insertions(+), 8 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 6821347..672664b 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -954,8 +954,10 @@ 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);
 	}
 }
@@ -970,7 +972,10 @@ 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);
 	}
 }
@@ -2202,8 +2207,10 @@ static void wait_sb_inodes(struct super_block *sb)
 		 * The mapping can be cleaned before we wait on the inode since
 		 * we do not have the mapping lock.
 		 */
-		if (!mapping_tagged(mapping, PAGECACHE_TAG_WRITEBACK))
+		if (!mapping_tagged(mapping, PAGECACHE_TAG_WRITEBACK)) {
+			trace_sb_clear_inode_writeback(inode);
 			continue;
+		}
 
 		spin_unlock_irq(&sb->s_inode_wblist_lock);
 
@@ -2239,8 +2246,10 @@ static void wait_sb_inodes(struct super_block *sb)
 			if (list_empty(&inode->i_wb_list))
 				list_add_tail(&inode->i_wb_list, &sb->s_inodes_wb);
 			spin_unlock(&sb->s_inode_wblist_lock);
-		} else
+		} else {
 			WARN_ON(!list_empty(&inode->i_wb_list));
+			trace_sb_clear_inode_writeback(inode);
+		}
 		spin_unlock_irq(&mapping->tree_lock);
 
 		iput(inode);
diff --git a/include/trace/events/writeback.h b/include/trace/events/writeback.h
index fff846b..962a7f1 100644
--- a/include/trace/events/writeback.h
+++ b/include/trace/events/writeback.h
@@ -727,7 +727,7 @@ 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),
@@ -754,25 +754,39 @@ 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 */
-- 
2.4.3