[PATCH 3/3] writeback: do strict bdi dirty_exceeded

[Wu Fengguang <fengguang.wu@intel.com>]

This helps to reduce dirty throttling polls and hence CPU overheads.

bdi->dirty_exceeded typically only helps when suddenly starting 100+
dd's on a disk, in which case the dd's may need to poll
balance_dirty_pages() earlier than tsk->nr_dirtied_pause.

CC: Jan Kara <jack@suse.cz>
CC: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
 mm/page-writeback.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- linux-next.orig/mm/page-writeback.c	2011-12-04 12:03:06.000000000 +0800
+++ linux-next/mm/page-writeback.c	2011-12-04 12:03:07.000000000 +0800
@@ -1097,7 +1097,7 @@ static void balance_dirty_pages(struct a
 				    bdi_stat(bdi, BDI_WRITEBACK);
 		}
 
-		dirty_exceeded = (bdi_dirty > bdi_thresh) ||
+		dirty_exceeded = (bdi_dirty > bdi_thresh) &&
 				  (nr_dirty > dirty_thresh);
 		if (dirty_exceeded && !bdi->dirty_exceeded)
 			bdi->dirty_exceeded = 1;