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>

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);
 }
 
 /**