writeback: do strict bdi dirty_exceeded
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>
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 422cf4ed..936dc7b 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -1160,7 +1160,7 @@
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;