[PATCH] mm/page-writeback: introduce tracepoint for wait_on_page_writeback

[Yafang Shao <laoar.shao@gmail.com>]

Recently there're some hungtasks on our server due to
wait_on_page_writeback, and we want to know the details of this
PG_writeback, i.e. this page is writing back to which device.
But it is not so convenient to get the details.

I think it would be better to introduce a tracepoint for diagnosing
the writeback details.

Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
---
 include/linux/pagemap.h          | 10 +---------
 include/trace/events/writeback.h | 16 +++++++++++++++-
 mm/page-writeback.c              | 12 ++++++++++++
 3 files changed, 28 insertions(+), 10 deletions(-)

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index f939e00..0f26c38 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -537,15 +537,7 @@ static inline int wait_on_page_locked_killable(struct page *page)
 
 extern void put_and_wait_on_page_locked(struct page *page);
 
-/* 
- * Wait for a page to complete writeback
- */
-static inline void wait_on_page_writeback(struct page *page)
-{
-	if (PageWriteback(page))
-		wait_on_page_bit(page, PG_writeback);
-}
-
+void wait_on_page_writeback(struct page *page);
 extern void end_page_writeback(struct page *page);
 void wait_for_stable_page(struct page *page);
 
diff --git a/include/trace/events/writeback.h b/include/trace/events/writeback.h
index 32db72c..aa7f3ae 100644
--- a/include/trace/events/writeback.h
+++ b/include/trace/events/writeback.h
@@ -53,7 +53,7 @@
 
 struct wb_writeback_work;
 
-TRACE_EVENT(writeback_dirty_page,
+DECLARE_EVENT_CLASS(writeback_page_template,
 
 	TP_PROTO(struct page *page, struct address_space *mapping),
 
@@ -79,6 +79,20 @@
 	)
 );
 
+DEFINE_EVENT(writeback_page_template, writeback_dirty_page,
+
+	TP_PROTO(struct page *page, struct address_space *mapping),
+
+	TP_ARGS(page, mapping)
+);
+
+DEFINE_EVENT(writeback_page_template, wait_on_page_writeback,
+
+	TP_PROTO(struct page *page, struct address_space *mapping),
+
+	TP_ARGS(page, mapping)
+);
+
 DECLARE_EVENT_CLASS(writeback_dirty_inode_template,
 
 	TP_PROTO(struct inode *inode, int flags),
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 9f61dfe..0765648 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -2808,6 +2808,18 @@ int __test_set_page_writeback(struct page *page, bool keep_write)
 }
 EXPORT_SYMBOL(__test_set_page_writeback);
 
+/*
+ * Wait for a page to complete writeback
+ */
+void wait_on_page_writeback(struct page *page)
+{
+	if (PageWriteback(page)) {
+		trace_wait_on_page_writeback(page, page_mapping(page));
+		wait_on_page_bit(page, PG_writeback);
+	}
+}
+EXPORT_SYMBOL_GPL(wait_on_page_writeback);
+
 /**
  * wait_for_stable_page() - wait for writeback to finish, if necessary.
  * @page:	The page to wait on.
-- 
1.8.3.1