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From: Mel Gorman on 12 Mar 2010 11:50
The fragmentation index may indicate that a failure it due to external
fragmentation, a compaction run complete and an allocation failure still
fail. There are two obvious reasons as to why

o Page migration cannot move all pages so fragmentation remains
o A suitable page may exist but watermarks are not met

In the event of compaction and allocation failure, this patch prevents
compaction happening for a short interval. It's only recorded on the
preferred zone but that should be enough coverage. This could have been
implemented similar to the zonelist_cache but the increased size of the
zonelist did not appear to be justified.

Signed-off-by: Mel Gorman <mel(a)csn.ul.ie>
Acked-by: Rik van Riel <riel(a)redhat.com>
---
include/linux/compaction.h | 35 +++++++++++++++++++++++++++++++++++
include/linux/mmzone.h | 7 +++++++
mm/page_alloc.c | 5 ++++-
3 files changed, 46 insertions(+), 1 deletions(-)

diff --git a/include/linux/compaction.h b/include/linux/compaction.h
index b851428..bc7059d 100644
--- a/include/linux/compaction.h
+++ b/include/linux/compaction.h
@@ -14,6 +14,32 @@ extern int sysctl_compaction_handler(struct ctl_table *table, int write,
extern int fragmentation_index(struct zone *zone, unsigned int order);
extern unsigned long try_to_compact_pages(struct zonelist *zonelist,
int order, gfp_t gfp_mask, nodemask_t *mask);
+
+/* defer_compaction - Do not compact within a zone until a given time */
+static inline void defer_compaction(struct zone *zone, unsigned long resume)
+{
+ /*
+ * This function is called when compaction fails to result in a page
+ * allocation success. This is somewhat unsatisfactory as the failure
+ * to compact has nothing to do with time and everything to do with
+ * the requested order, the number of free pages and watermarks. How
+ * to wait on that is more unclear, but the answer would apply to
+ * other areas where the VM waits based on time.
+ */
+ zone->compact_resume = resume;
+}
+
+static inline int compaction_deferred(struct zone *zone)
+{
+ /* init once if necessary */
+ if (unlikely(!zone->compact_resume)) {
+ zone->compact_resume = jiffies;
+ return 0;
+ }
+
+ return time_before(jiffies, zone->compact_resume);
+}
+
#else
static inline unsigned long try_to_compact_pages(struct zonelist *zonelist,
int order, gfp_t gfp_mask, nodemask_t *nodemask)
@@ -21,6 +47,15 @@ static inline unsigned long try_to_compact_pages(struct zonelist *zonelist,
return COMPACT_INCOMPLETE;
}

+static inline void defer_compaction(struct zone *zone, unsigned long resume)
+{
+}
+
+static inline int compaction_deferred(struct zone *zone)
+{
+ return 1;
+}
+
#endif /* CONFIG_COMPACTION */

#if defined(CONFIG_COMPACTION) && defined(CONFIG_SYSFS) && defined(CONFIG_NUMA)
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 37df0b3..99b7ecc 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -321,6 +321,13 @@ struct zone {
unsigned long *pageblock_flags;
#endif /* CONFIG_SPARSEMEM */

+#ifdef CONFIG_COMPACTION
+ /*
+ * If a compaction fails, do not try compaction again until
+ * jiffies is after the value of compact_resume
+ */
+ unsigned long compact_resume;
+#endif

ZONE_PADDING(_pad1_)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index e301108..f481df2 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1767,7 +1767,7 @@ __alloc_pages_direct_reclaim(gfp_t gfp_mask, unsigned int order,
cond_resched();

/* Try memory compaction for high-order allocations before reclaim */
- if (order) {
+ if (order && !compaction_deferred(preferred_zone)) {
*did_some_progress = try_to_compact_pages(zonelist,
order, gfp_mask, nodemask);
if (*did_some_progress != COMPACT_INCOMPLETE) {
@@ -1787,6 +1787,9 @@ __alloc_pages_direct_reclaim(gfp_t gfp_mask, unsigned int order,
*/
count_vm_event(COMPACTFAIL);

+ /* On failure, avoid compaction for a short time. */
+ defer_compaction(preferred_zone, jiffies + HZ/50);
+
cond_resched();
}
}
--
1.6.5

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