mm/ksm: add pages_skipped metric

This change adds the "pages skipped" metric.  To be able to evaluate how
successful smart page scanning is, the pages skipped metric can be
compared to the pages scanned metric.

The pages skipped metric is a cumulative counter.  The counter is stored
under /sys/kernel/mm/ksm/pages_skipped.

Link: https://lkml.kernel.org/r/20230926040939.516161-3-shr@devkernel.io
Signed-off-by: Stefan Roesch <shr@devkernel.io>
Reviewed-by: David Hildenbrand <david@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Rik van Riel <riel@surriel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/ksm.c b/mm/ksm.c
index 8ad66b1c91b9d0356e067a239bb21fd0c216f8e9..7efcc68ccc6eaeaa910474a745f835c7f7b04d66 100644 (file)
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -294,6 +294,9 @@ static bool ksm_smart_scan = true;
 /* The number of zero pages which is placed by KSM */
 unsigned long ksm_zero_pages;
 
+/* The number of pages that have been skipped due to "smart scanning" */
+static unsigned long ksm_pages_skipped;
+
 #ifdef CONFIG_NUMA
 /* Zeroed when merging across nodes is not allowed */
 static unsigned int ksm_merge_across_nodes = 1;
@@ -2377,6 +2380,7 @@ static bool should_skip_rmap_item(struct page *page,
        }
 
        /* Skip this page */
+       ksm_pages_skipped++;
        rmap_item->remaining_skips--;
        remove_rmap_item_from_tree(rmap_item);
        return true;
@@ -3464,6 +3468,13 @@ static ssize_t pages_volatile_show(struct kobject *kobj,
 }
 KSM_ATTR_RO(pages_volatile);
 
+static ssize_t pages_skipped_show(struct kobject *kobj,
+                                 struct kobj_attribute *attr, char *buf)
+{
+       return sysfs_emit(buf, "%lu\n", ksm_pages_skipped);
+}
+KSM_ATTR_RO(pages_skipped);
+
 static ssize_t ksm_zero_pages_show(struct kobject *kobj,
                                struct kobj_attribute *attr, char *buf)
 {
@@ -3561,6 +3572,7 @@ static struct attribute *ksm_attrs[] = {
        &pages_sharing_attr.attr,
        &pages_unshared_attr.attr,
        &pages_volatile_attr.attr,
+       &pages_skipped_attr.attr,
        &ksm_zero_pages_attr.attr,
        &full_scans_attr.attr,
 #ifdef CONFIG_NUMA