mm: speed up writeback ramp-up on clean systems

We allow violation of bdi limits if there is a lot of room on the system.
Once we hit half the total limit we start enforcing bdi limits and bdi
ramp-up should happen.  Doing it this way avoids many small writeouts on an
otherwise idle system and should also speed up the ramp-up.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Reviewed-by: Fengguang Wu <wfg@mail.ustc.edu.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 17 insertions, 2 deletions

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 838a5e31394c..81a91e6f1f99 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -355,8 +355,8 @@ get_dirty_limits(long *pbackground, long *pdirty, long *pbdi_dirty,
  */
 static void balance_dirty_pages(struct address_space *mapping)
 {
-        long bdi_nr_reclaimable;
-        long bdi_nr_writeback;
+        long nr_reclaimable, bdi_nr_reclaimable;
+        long nr_writeback, bdi_nr_writeback;
         long background_thresh;
         long dirty_thresh;
         long bdi_thresh;
@@ -376,11 +376,26 @@ static void balance_dirty_pages(struct address_space *mapping)
 
                 get_dirty_limits(&background_thresh, &dirty_thresh,
                                 &bdi_thresh, bdi);
+
+                nr_reclaimable = global_page_state(NR_FILE_DIRTY) +
+                                        global_page_state(NR_UNSTABLE_NFS);
+                nr_writeback = global_page_state(NR_WRITEBACK);
+
                 bdi_nr_reclaimable = bdi_stat(bdi, BDI_RECLAIMABLE);
                 bdi_nr_writeback = bdi_stat(bdi, BDI_WRITEBACK);
+
                 if (bdi_nr_reclaimable + bdi_nr_writeback <= bdi_thresh)
                         break;
 
+                /*
+                 * Throttle it only when the background writeback cannot
+                 * catch-up. This avoids (excessively) small writeouts
+                 * when the bdi limits are ramping up.
+                 */
+                if (nr_reclaimable + nr_writeback <
+                                (background_thresh + dirty_thresh) / 2)
+                        break;
+
                 if (!bdi->dirty_exceeded)
                         bdi->dirty_exceeded = 1;