[PATCH] swap: scan_swap_map latency breaks

The get_swap_page/scan_swap_map latency can be so bad that even those without
preemption configured deserve relief: periodically cond_resched.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 12 insertions, 2 deletions

diff --git a/mm/swapfile.c b/mm/swapfile.c
index fdee145afc6f..e675ae55f87d 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -56,8 +56,6 @@ static DECLARE_MUTEX(swapon_sem);
  */
 static DECLARE_RWSEM(swap_unplug_sem);
 
-#define SWAPFILE_CLUSTER 256
-
 void swap_unplug_io_fn(struct backing_dev_info *unused_bdi, struct page *page)
 {
         swp_entry_t entry;
@@ -84,9 +82,13 @@ void swap_unplug_io_fn(struct backing_dev_info *unused_bdi, struct page *page)
         up_read(&swap_unplug_sem);
 }
 
+#define SWAPFILE_CLUSTER        256
+#define LATENCY_LIMIT           256
+
 static inline unsigned long scan_swap_map(struct swap_info_struct *si)
 {
         unsigned long offset, last_in_cluster;
+        int latency_ration = LATENCY_LIMIT;
 
         /* 
          * We try to cluster swap pages by allocating them sequentially
@@ -117,6 +119,10 @@ static inline unsigned long scan_swap_map(struct swap_info_struct *si)
                                 si->cluster_next = offset-SWAPFILE_CLUSTER-1;
                                 goto cluster;
                         }
+                        if (unlikely(--latency_ration < 0)) {
+                                cond_resched();
+                                latency_ration = LATENCY_LIMIT;
+                        }
                 }
                 swap_device_lock(si);
                 goto lowest;
@@ -153,6 +159,10 @@ checks:	if (!(si->flags & SWP_WRITEOK))
                         swap_device_lock(si);
                         goto checks;
                 }
+                if (unlikely(--latency_ration < 0)) {
+                        cond_resched();
+                        latency_ration = LATENCY_LIMIT;
+                }
         }
         swap_device_lock(si);
         goto lowest;