[Devel] [PATCH VZ9 2/4] mm/ksm: add pages_skipped metric

[Pavel Tikhomirov]

From: Stefan Roesch <shr at devkernel.io>

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 at devkernel.io>
Reviewed-by: David Hildenbrand <david at redhat.com>
Cc: Johannes Weiner <hannes at cmpxchg.org>
Cc: Rik van Riel <riel at surriel.com>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>

https://virtuozzo.atlassian.net/browse/PSBM-157809
(cherry picked from commit e5a68991268906a6989f1bf273580c2218662ad6)
Signed-off-by: Pavel Tikhomirov <ptikhomirov at virtuozzo.com>
---
 mm/ksm.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/mm/ksm.c b/mm/ksm.c
index 1994db2b50145..767831a013061 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -284,6 +284,9 @@ static bool ksm_use_zero_pages __read_mostly;
 /* Default to true at least temporarily, for testing */
 static bool ksm_smart_scan = true;
 
+/* 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;
@@ -2283,6 +2286,7 @@ 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;
@@ -3177,6 +3181,13 @@ 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 stable_node_dups_show(struct kobject *kobj,
 				     struct kobj_attribute *attr, char *buf)
 {
@@ -3254,6 +3265,7 @@ static struct attribute *ksm_attrs[] = {
 	&pages_sharing_attr.attr,
 	&pages_unshared_attr.attr,
 	&pages_volatile_attr.attr,
+	&pages_skipped_attr.attr,
 	&full_scans_attr.attr,
 #ifdef CONFIG_NUMA
 	&merge_across_nodes_attr.attr,
-- 
2.46.0