[PATCH 04/17] writeback: quit throttling when bdi dirty/writeback pages go down

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

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Tests show that bdi_thresh may take minutes to ramp up on a typical
desktop. The time should be improvable but cannot be eliminated totally.
So when (background_thresh + dirty_thresh)/2 is reached and
balance_dirty_pages() starts to throttle the task, it will suddenly find
the (still low and ramping up) bdi_thresh is exceeded _excessively_. Here
we definitely don't want to stall the task for one minute. So introduce
an alternative way to break out of the loop when the bdi dirty/write
pages has dropped by a reasonable amount.

When dirty_background_ratio is set close to dirty_ratio, bdi_thresh may
also be constantly exceeded due to the task_dirty_limit() gap.

It will take at least 200ms before trying to break out.

(pages_dirtied * 8) is used because in this situation pages_dirtied will
typically be small numbers (eg. 3 pages) due to the fast back off logic.

Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
 mm/page-writeback.c |   15 +++++++++++++++
 1 file changed, 15 insertions(+)

--- linux-next.orig/mm/page-writeback.c	2010-09-09 15:51:38.000000000 +0800
+++ linux-next/mm/page-writeback.c	2010-09-12 13:10:02.000000000 +0800
@@ -463,6 +463,7 @@ static void balance_dirty_pages(struct a
 {
 	long nr_reclaimable, bdi_nr_reclaimable;
 	long nr_writeback, bdi_nr_writeback;
+	long bdi_prev_dirty3 = 0;
 	unsigned long background_thresh;
 	unsigned long dirty_thresh;
 	unsigned long bdi_thresh;
@@ -516,6 +517,20 @@ static void balance_dirty_pages(struct a
 			bdi_nr_writeback = bdi_stat(bdi, BDI_WRITEBACK);
 		}
 
+		/*
+		 * bdi_thresh could get exceeded for long time:
+		 * - bdi_thresh takes some time to ramp up from the initial 0
+		 * - users may set dirty_background_ratio close to dirty_ratio
+		 *   (at least 1/8 gap is preferred)
+		 * So offer a complementary way to break out of the loop when
+		 * enough bdi pages have been cleaned during our pause time.
+		 */
+		if (nr_reclaimable + nr_writeback <= dirty_thresh &&
+		    bdi_prev_dirty3 - (bdi_nr_reclaimable + bdi_nr_writeback) >
+							(long)pages_dirtied * 8)
+			break;
+		bdi_prev_dirty3 = bdi_nr_reclaimable + bdi_nr_writeback;
+
 		if (bdi_nr_reclaimable + bdi_nr_writeback <=
 			bdi_thresh - bdi_thresh / DIRTY_SOFT_THROTTLE_RATIO)
 			goto check_exceeded;