mm: compaction: add a tunable that decides when memory should be compacted and when it should be reclaimed

The kernel applies some heuristics when deciding if memory should be
compacted or reclaimed to satisfy a high-order allocation.  One of these
is based on the fragmentation.  If the index is below 500, memory will not
be compacted.  This choice is arbitrary and not based on data.  To help
optimise the system and set a sensible default for this value, this patch
adds a sysctl extfrag_threshold.  The kernel will only compact memory if
the fragmentation index is above the extfrag_threshold.

[randy.dunlap@oracle.com: Fix build errors when proc fs is not configured]
Signed-off-by: Mel Gorman <mel@csn.ul.ie>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Minchan Kim <minchan.kim@gmail.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Christoph Lameter <cl@linux-foundation.org>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 15 insertions, 0 deletions

diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 284f330d6a01..84ff5e75c084 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -263,6 +263,11 @@ static int min_sched_shares_ratelimit = 100000; /* 100 usec */
 static int max_sched_shares_ratelimit = NSEC_PER_SEC; /* 1 second */
 #endif
 
+#ifdef CONFIG_COMPACTION
+static int min_extfrag_threshold;
+static int max_extfrag_threshold = 1000;
+#endif
+
 static struct ctl_table kern_table[] = {
         {
                 .procname       = "sched_child_runs_first",
@@ -1130,6 +1135,16 @@ static struct ctl_table vm_table[] = {
                 .mode           = 0200,
                 .proc_handler   = sysctl_compaction_handler,
         },
+        {
+                .procname       = "extfrag_threshold",
+                .data           = &sysctl_extfrag_threshold,
+                .maxlen         = sizeof(int),
+                .mode           = 0644,
+                .proc_handler   = sysctl_extfrag_handler,
+                .extra1         = &min_extfrag_threshold,
+                .extra2         = &max_extfrag_threshold,
+        },
+
 #endif /* CONFIG_COMPACTION */
         {
                 .procname       = "min_free_kbytes",