writeback: trace global_dirty_state

Add trace event balance_dirty_state for showing the global dirty page
counts and thresholds at each global_dirty_limits() invocation.  This
will cover the callers throttle_vm_writeout(), over_bground_thresh()
and each balance_dirty_pages() loop.
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>

include/trace/events/writeback.h

@@ -204,6 +204,52 @@ TRACE_EVENT(writeback_queue_io,
 		__entry->moved)
 );
 
+TRACE_EVENT(global_dirty_state,
+
+	TP_PROTO(unsigned long background_thresh,
+		 unsigned long dirty_thresh
+	),
+
+	TP_ARGS(background_thresh,
+		dirty_thresh
+	),
+
+	TP_STRUCT__entry(
+		__field(unsigned long,	nr_dirty)
+		__field(unsigned long,	nr_writeback)
+		__field(unsigned long,	nr_unstable)
+		__field(unsigned long,	background_thresh)
+		__field(unsigned long,	dirty_thresh)
+		__field(unsigned long,	dirty_limit)
+		__field(unsigned long,	nr_dirtied)
+		__field(unsigned long,	nr_written)
+	),
+
+	TP_fast_assign(
+		__entry->nr_dirty	= global_page_state(NR_FILE_DIRTY);
+		__entry->nr_writeback	= global_page_state(NR_WRITEBACK);
+		__entry->nr_unstable	= global_page_state(NR_UNSTABLE_NFS);
+		__entry->nr_dirtied	= global_page_state(NR_DIRTIED);
+		__entry->nr_written	= global_page_state(NR_WRITTEN);
+		__entry->background_thresh = background_thresh;
+		__entry->dirty_thresh	= dirty_thresh;
+		__entry->dirty_limit = global_dirty_limit;
+	),
+
+	TP_printk("dirty=%lu writeback=%lu unstable=%lu "
+		  "bg_thresh=%lu thresh=%lu limit=%lu "
+		  "dirtied=%lu written=%lu",
+		  __entry->nr_dirty,
+		  __entry->nr_writeback,
+		  __entry->nr_unstable,
+		  __entry->background_thresh,
+		  __entry->dirty_thresh,
+		  __entry->dirty_limit,
+		  __entry->nr_dirtied,
+		  __entry->nr_written
+	)
+);
+
 DECLARE_EVENT_CLASS(writeback_congest_waited_template,
 
 	TP_PROTO(unsigned int usec_timeout, unsigned int usec_delayed),

mm/page-writeback.c

@@ -447,6 +447,7 @@ void global_dirty_limits(unsigned long *pbackground, unsigned long *pdirty)
 	}
 	*pbackground = background;
 	*pdirty = dirty;
+	trace_global_dirty_state(background, dirty);
 }
 
 /**