writeback: set max_pause to lowest value on zero bdi_dirty

Some trace shows lots of bdi_dirty=0 lines where it's actually some
small value if w/o the accounting errors in the per-cpu bdi stats.

In this case the max pause time should really be set to the smallest
(non-zero) value to avoid IO queue underrun and improve throughput.

Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 17403e3a7c89f03ba484db91c0673565398ce982..50f08241f9815668d73b1adfcd5c9585f11bbc15 100644 (file)
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -989,8 +989,7 @@ static unsigned long bdi_max_pause(struct backing_dev_info *bdi,
         *
         * 8 serves as the safety ratio.
         */
-       if (bdi_dirty)
-               t = min(t, bdi_dirty * HZ / (8 * bw + 1));
+       t = min(t, bdi_dirty * HZ / (8 * bw + 1));
 
        /*
         * The pause time will be settled within range (max_pause/4, max_pause).