mm: export fragmentation index via debugfs

The fragmentation fragmentation index, is only meaningful if an allocation
would fail and indicates what the failure is due to.  A value of -1 such
as in many of the examples above states that the allocation would succeed.
 If it would fail, the value is between 0 and 1.  A value tending towards
0 implies the allocation failed due to a lack of memory.  A value tending
towards 1 implies it failed due to external fragmentation.

For the most part, the huge page size will be the size of interest but not
necessarily so it is exported on a per-order and per-zo basis via
/sys/kernel/debug/extfrag/extfrag_index

> cat /sys/kernel/debug/extfrag/extfrag_index
Node 0, zone      DMA -1.000 -1.000 -1.000 -1.000 -1.000 -1.000 -1.000 -1.00
Node 0, zone   Normal -1.000 -1.000 -1.000 -1.000 -1.000 -1.000 -1.000 0.954

Signed-off-by: Mel Gorman <mel@csn.ul.ie>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
Acked-by: Rik van Riel <riel@redhat.com>
Reviewed-by: Christoph Lameter <cl@linux-foundation.org>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
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, 84 insertions, 0 deletions

diff --git a/mm/vmstat.c b/mm/vmstat.c
index d3e0fa169f05..23a5899c7461 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -16,6 +16,7 @@
 #include <linux/cpu.h>
 #include <linux/vmstat.h>
 #include <linux/sched.h>
+#include <linux/math64.h>
 
 #ifdef CONFIG_VM_EVENT_COUNTERS
 DEFINE_PER_CPU(struct vm_event_state, vm_event_states) = {{0}};
@@ -420,6 +421,33 @@ static void fill_contig_page_info(struct zone *zone,
                                                 (order - suitable_order);
         }
 }
+
+/*
+ * A fragmentation index only makes sense if an allocation of a requested
+ * size would fail. If that is true, the fragmentation index indicates
+ * whether external fragmentation or a lack of memory was the problem.
+ * The value can be used to determine if page reclaim or compaction
+ * should be used
+ */
+int fragmentation_index(unsigned int order, struct contig_page_info *info)
+{
+        unsigned long requested = 1UL << order;
+
+        if (!info->free_blocks_total)
+                return 0;
+
+        /* Fragmentation index only makes sense when a request would fail */
+        if (info->free_blocks_suitable)
+                return -1000;
+
+        /*
+         * Index is between 0 and 1 so return within 3 decimal places
+         *
+         * 0 => allocation would fail due to lack of memory
+         * 1 => allocation would fail due to fragmentation
+         */
+        return 1000 - div_u64( (1000+(div_u64(info->free_pages * 1000ULL, requested))), info->free_blocks_total);
+}
 #endif
 
 #if defined(CONFIG_PROC_FS) || defined(CONFIG_COMPACTION)
@@ -1087,6 +1115,58 @@ static const struct file_operations unusable_file_ops = {
         .release        = seq_release,
 };
 
+static void extfrag_show_print(struct seq_file *m,
+                                        pg_data_t *pgdat, struct zone *zone)
+{
+        unsigned int order;
+        int index;
+
+        /* Alloc on stack as interrupts are disabled for zone walk */
+        struct contig_page_info info;
+
+        seq_printf(m, "Node %d, zone %8s ",
+                                pgdat->node_id,
+                                zone->name);
+        for (order = 0; order < MAX_ORDER; ++order) {
+                fill_contig_page_info(zone, order, &info);
+                index = fragmentation_index(order, &info);
+                seq_printf(m, "%d.%03d ", index / 1000, index % 1000);
+        }
+
+        seq_putc(m, '\n');
+}
+
+/*
+ * Display fragmentation index for orders that allocations would fail for
+ */
+static int extfrag_show(struct seq_file *m, void *arg)
+{
+        pg_data_t *pgdat = (pg_data_t *)arg;
+
+        walk_zones_in_node(m, pgdat, extfrag_show_print);
+
+        return 0;
+}
+
+static const struct seq_operations extfrag_op = {
+        .start  = frag_start,
+        .next   = frag_next,
+        .stop   = frag_stop,
+        .show   = extfrag_show,
+};
+
+static int extfrag_open(struct inode *inode, struct file *file)
+{
+        return seq_open(file, &extfrag_op);
+}
+
+static const struct file_operations extfrag_file_ops = {
+        .open           = extfrag_open,
+        .read           = seq_read,
+        .llseek         = seq_lseek,
+        .release        = seq_release,
+};
+
 static int __init extfrag_debug_init(void)
 {
         extfrag_debug_root = debugfs_create_dir("extfrag", NULL);
@@ -1097,6 +1177,10 @@ static int __init extfrag_debug_init(void)
                         extfrag_debug_root, NULL, &unusable_file_ops))
                 return -ENOMEM;
 
+        if (!debugfs_create_file("extfrag_index", 0444,
+                        extfrag_debug_root, NULL, &extfrag_file_ops))
+                return -ENOMEM;
+
         return 0;
 }