[PATCH 3/3] writeback: Add tracing to write_cache_pages

[Dave Chinner <david@fromorbit.com>]

From: Dave Chinner <dchinner@redhat.com>

Add a trace event to the ->writepage loop in write_cache_pages to give
visibility into how the ->writepage call is changing variables within the
writeback control structure. Of most interest is how wbc->nr_to_write changes
from call to call, especially with filesystems that write multiple pages
in ->writepage.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
---
 include/trace/events/writeback.h |    1 +
 mm/page-writeback.c              |    1 +
 2 files changed, 2 insertions(+), 0 deletions(-)

diff --git a/include/trace/events/writeback.h b/include/trace/events/writeback.h
index 4e219e2..9d8460e 100644
--- a/include/trace/events/writeback.h
+++ b/include/trace/events/writeback.h
@@ -155,6 +155,7 @@ DEFINE_WBC_EVENT(wbc_writeback_wait);
 DEFINE_WBC_EVENT(wbc_balance_dirty_start);
 DEFINE_WBC_EVENT(wbc_balance_dirty_written);
 DEFINE_WBC_EVENT(wbc_balance_dirty_wait);
+DEFINE_WBC_EVENT(wbc_writepage);
 
 #endif /* _TRACE_WRITEBACK_H */
 
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index d556cd8..3d2111a 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -933,6 +933,7 @@ continue_unlock:
 			if (!clear_page_dirty_for_io(page))
 				goto continue_unlock;
 
+			trace_wbc_writepage(wbc, mapping->backing_dev_info);
 			ret = (*writepage)(page, wbc, data);
 			if (unlikely(ret)) {
 				if (ret == AOP_WRITEPAGE_ACTIVATE) {
-- 
1.7.1