squeeze max-pause area and drop pass-good area

Revert the pass-good area introduced in ffd1f609ab10 ("writeback:
introduce max-pause and pass-good dirty limits") and make the max-pause
area smaller and safe.

This fixes ~30% performance regression in the ext3 data=writeback
fio_mmap_randwrite_64k/fio_mmap_randrw_64k test cases, where there are
12 JBOD disks, on each disk runs 8 concurrent tasks doing reads+writes.

Using deadline scheduler also has a regression, but not that big as CFQ,
so this suggests we have some write starvation.

The test logs show that

- the disks are sometimes under utilized

- global dirty pages sometimes rush high to the pass-good area for
  several hundred seconds, while in the mean time some bdi dirty pages
  drop to very low value (bdi_dirty << bdi_thresh).  Then suddenly the
  global dirty pages dropped under global dirty threshold and bdi_dirty
  rush very high (for example, 2 times higher than bdi_thresh). During
  which time balance_dirty_pages() is not called at all.

So the problems are

1) The random writes progress so slow that they break the assumption of
   the max-pause logic that "8 pages per 200ms is typically more than
   enough to curb heavy dirtiers".

2) The max-pause logic ignored task_bdi_thresh and thus opens the possibility
   for some bdi's to over dirty pages, leading to (bdi_dirty >> bdi_thresh)
   and then (bdi_thresh >> bdi_dirty) for others.

3) The higher max-pause/pass-good thresholds somehow leads to the bad
   swing of dirty pages.

The fix is to allow the task to slightly dirty over task_bdi_thresh, but
no way to exceed bdi_dirty and/or global dirty_thresh.

Tests show that it fixed the JBOD regression completely (both behavior
and performance), while still being able to cut down large pause times
in balance_dirty_pages() for single-disk cases.

Reported-by: Li Shaohua <shaohua.li@intel.com>
Tested-by: Li Shaohua <shaohua.li@intel.com>
Acked-by: Jan Kara <jack@suse.cz>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>

1 files changed, 2 insertions, 13 deletions

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index d1960744f88..0e309cd1b5b 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -754,21 +754,10 @@ static void balance_dirty_pages(struct address_space *mapping,
 		 * 200ms is typically more than enough to curb heavy dirtiers;
 		 * (b) the pause time limit makes the dirtiers more responsive.
 		 */
-		if (nr_dirty < dirty_thresh +
-			       dirty_thresh / DIRTY_MAXPAUSE_AREA &&
+		if (nr_dirty < dirty_thresh &&
+		    bdi_dirty < (task_bdi_thresh + bdi_thresh) / 2 &&
 		    time_after(jiffies, start_time + MAX_PAUSE))
 			break;
-		/*
-		 * pass-good area. When some bdi gets blocked (eg. NFS server
-		 * not responding), or write bandwidth dropped dramatically due
-		 * to concurrent reads, or dirty threshold suddenly dropped and
-		 * the dirty pages cannot be brought down anytime soon (eg. on
-		 * slow USB stick), at least let go of the good bdi's.
-		 */
-		if (nr_dirty < dirty_thresh +
-			       dirty_thresh / DIRTY_PASSGOOD_AREA &&
-		    bdi_dirty < bdi_thresh)
-			break;
 
 		/*
 		 * Increase the delay for each loop, up to our previous