writeback: trace wakeup event for background writeback

This tracks when balance_dirty_pages() tries to wakeup the flusher thread
for background writeback (if it was not started already).

Suggested-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Jan Engelhardt <jengelh@medozas.de>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 2 insertions, 0 deletions

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 3a07f6d8bc0b..482de0a92ca7 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -169,6 +169,7 @@ void bdi_start_background_writeback(struct backing_dev_info *bdi)
          * We just wake up the flusher thread. It will perform background
          * writeback as soon as there is no other work to do.
          */
+        trace_writeback_wake_background(bdi);
         spin_lock_bh(&bdi->wb_lock);
         bdi_wakeup_flusher(bdi);
         spin_unlock_bh(&bdi->wb_lock);
diff --git a/include/trace/events/writeback.h b/include/trace/events/writeback.h
index 89a2b2db4375..4e249b927eaa 100644
--- a/include/trace/events/writeback.h
+++ b/include/trace/events/writeback.h
@@ -81,6 +81,7 @@ DEFINE_EVENT(writeback_class, name, \
         TP_ARGS(bdi))
 
 DEFINE_WRITEBACK_EVENT(writeback_nowork);
+DEFINE_WRITEBACK_EVENT(writeback_wake_background);
 DEFINE_WRITEBACK_EVENT(writeback_wake_thread);
 DEFINE_WRITEBACK_EVENT(writeback_wake_forker_thread);
 DEFINE_WRITEBACK_EVENT(writeback_bdi_register);