writeback: Add tracing to write_cache_pages

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>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

2 files changed, 2 insertions, 0 deletions

diff --git a/include/trace/events/writeback.h b/include/trace/events/writeback.h
index 0be26acae064..bde92e07fa4f 100644
--- a/include/trace/events/writeback.h
+++ b/include/trace/events/writeback.h
@@ -148,6 +148,7 @@ 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 d556cd829af6..3d2111a22236 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) {