writeback: add new tracepoints

Add 2 new trace points to the periodic write-back wake up case, just like we do
in the 'bdi_queue_work()' function. Namely, introduce:

1. trace_writeback_wake_thread(bdi)
2. trace_writeback_wake_forker_thread(bdi)

The first event is triggered every time we wake up a bdi thread to start
periodic background write-out. The second event is triggered only when the bdi
thread does not exist and should be created by the forker thread.

This patch was suggested by Dave Chinner and Christoph Hellwig.
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

include/trace/events/writeback.h

@@ -81,6 +81,8 @@ DEFINE_EVENT(writeback_class, name, \
 	TP_ARGS(bdi))
 
 DEFINE_WRITEBACK_EVENT(writeback_nowork);
+DEFINE_WRITEBACK_EVENT(writeback_wake_thread);
+DEFINE_WRITEBACK_EVENT(writeback_wake_forker_thread);
 DEFINE_WRITEBACK_EVENT(writeback_bdi_register);
 DEFINE_WRITEBACK_EVENT(writeback_bdi_unregister);
 DEFINE_WRITEBACK_EVENT(writeback_thread_start);

mm/backing-dev.c

@@ -310,6 +310,7 @@ static void wakeup_timer_fn(unsigned long data)
 
 	spin_lock_bh(&bdi->wb_lock);
 	if (bdi->wb.task) {
+		trace_writeback_wake_thread(bdi);
 		wake_up_process(bdi->wb.task);
 	} else {
 		/*
@@ -317,6 +318,7 @@ static void wakeup_timer_fn(unsigned long data)
 		 * In this case we have to wake-up the forker thread which
 		 * should create and run the bdi thread.
 		 */
+		trace_writeback_wake_forker_thread(bdi);
 		wake_up_process(default_backing_dev_info.wb.task);
 	}
 	spin_unlock_bh(&bdi->wb_lock);