Steven Rostedt [off-list ref] writes:

On Fri, 10 Feb 2023 13:46:45 -0800
Stefan Roesch [off-list ref] wrote:

Sorry for the late reply, I just noticed this (I had the flu when this was
originally sent).

quoted

+/**
+ * ksm_remove_ksm_page - called after a ksm page has been removed
+ *
+ * @pfn:		page frame number of ksm page
+ *
+ * Allows to trace the removing of stable ksm pages.
+ */
+TRACE_EVENT(ksm_remove_ksm_page,
+
+	TP_PROTO(unsigned long pfn),
+
+	TP_ARGS(pfn),
+
+	TP_STRUCT__entry(
+		__field(unsigned long, pfn)
+	),
+
+	TP_fast_assign(
+		__entry->pfn = pfn;
+	),
+
+	TP_printk("pfn %lu", __entry->pfn)
+);
+
+/**
+ * ksm_remove_rmap_item - called after a rmap_item has been removed from the
+ *                        stable tree
+ *
+ * @pfn:		page frame number of ksm page
+ * @rmap_item:		address of rmap_item  object
+ * @mm:			address of the process mm struct
+ *
+ * Allows to trace the removal of pages from the stable tree list.
+ */
+TRACE_EVENT(ksm_remove_rmap_item,
+
+	TP_PROTO(unsigned long pfn, void *rmap_item, void *mm),
+
+	TP_ARGS(pfn, rmap_item, mm),
+
+	TP_STRUCT__entry(
+		__field(unsigned long,	pfn)
+		__field(void *,		rmap_item)
+		__field(void *,		mm)
+	),
+
+	TP_fast_assign(
+		__entry->pfn		= pfn;
+		__entry->rmap_item	= rmap_item;
+		__entry->mm		= mm;
+	),
+
+	TP_printk("pfn %lu rmap_item %p mm %p",
+			__entry->pfn, __entry->rmap_item, __entry->mm)
+);
+
+#endif /* _TRACE_KSM_H */
+
+/* This part must be outside protection */
+#include <trace/define_trace.h>

diff --git a/mm/ksm.c b/mm/ksm.c
index 56808e3bfd19..4356af760735 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c

@@ -45,6 +45,9 @@
 #include "internal.h"
 #include "mm_slot.h"

+#define CREATE_TRACE_POINTS
+#include <trace/events/ksm.h>
+
 #ifdef CONFIG_NUMA
 #define NUMA(x)		(x)
 #define DO_NUMA(x)	do { (x); } while (0)

@@ -655,10 +658,12 @@ static void remove_node_from_stable_tree(struct ksm_stable_node *stable_node)
 	BUG_ON(stable_node->rmap_hlist_len < 0);

 	hlist_for_each_entry(rmap_item, &stable_node->hlist, hlist) {
-		if (rmap_item->hlist.next)
+		if (rmap_item->hlist.next) {
 			ksm_pages_sharing--;
-		else
+			trace_ksm_remove_rmap_item(stable_node->kpfn, rmap_item, rmap_item->mm);

Instead of dereferencing the stable_node here, where the work could
possibly happen outside the trace event and in the hot path, could you pass
in the stable_node instead, and then in the TP_fast_assign() do:

		__entry->pfn = stable_node->kpfn;

I'll make the change in the next version.

quoted

+		} else {
 			ksm_pages_shared--;
+		}

 		rmap_item->mm->ksm_merging_pages--;

@@ -679,6 +684,7 @@ static void remove_node_from_stable_tree(struct ksm_stable_node *stable_node)
 	BUILD_BUG_ON(STABLE_NODE_DUP_HEAD <= &migrate_nodes);
 	BUILD_BUG_ON(STABLE_NODE_DUP_HEAD >= &migrate_nodes + 1);

+	trace_ksm_remove_ksm_page(stable_node->kpfn);

Here too?

-- Steve

I'll make the change in the next version.

quoted

 	if (stable_node->head == &migrate_nodes)
 		list_del(&stable_node->list);
 	else

@@ -1367,6 +1373,8 @@ static int try_to_merge_with_ksm_page(struct ksm_rmap_item *rmap_item,
 	get_anon_vma(vma->anon_vma);
 out:
 	mmap_read_unlock(mm);
+	trace_ksm_merge_with_ksm_page(kpage, page_to_pfn(kpage ? kpage : page),
+				rmap_item, mm, err);
 	return err;
 }

@@ -2114,6 +2122,9 @@ static int try_to_merge_with_kernel_zero_page(struct ksm_rmap_item *rmap_item,
 		if (vma) {
 			err = try_to_merge_one_page(vma, page,
 						ZERO_PAGE(rmap_item->address));
+			trace_ksm_merge_one_page(
+				page_to_pfn(ZERO_PAGE(rmap_item->address)),
+				rmap_item, mm, err);
 			if (!err) {
 				rmap_item->address |= ZERO_PAGE_FLAG;
 				ksm_zero_pages_sharing++;

@@ -2344,6 +2355,8 @@ static struct ksm_rmap_item *scan_get_next_rmap_item(struct page **page)

 	mm_slot = ksm_scan.mm_slot;
 	if (mm_slot == &ksm_mm_head) {
+		trace_ksm_start_scan(ksm_scan.seqnr, ksm_rmap_items);
+
 		/*
 		 * A number of pages can hang around indefinitely on per-cpu
 		 * pagevecs, raised page count preventing write_protect_page

@@ -2510,6 +2523,7 @@ static struct ksm_rmap_item *scan_get_next_rmap_item(struct page **page)
 	if (mm_slot != &ksm_mm_head)
 		goto next_mm;

+	trace_ksm_stop_scan(ksm_scan.seqnr, ksm_rmap_items);
 	ksm_scan.seqnr++;
 	return NULL;
 }

@@ -2661,6 +2675,7 @@ int __ksm_enter(struct mm_struct *mm)
 	if (needs_wakeup)
 		wake_up_interruptible(&ksm_thread_wait);

+	trace_ksm_enter(mm);
 	return 0;
 }

@@ -2702,6 +2717,8 @@ void __ksm_exit(struct mm_struct *mm)
 		mmap_write_lock(mm);
 		mmap_write_unlock(mm);
 	}
+
+	trace_ksm_exit(mm);
 }

 struct page *ksm_might_need_to_copy(struct page *page,

base-commit: 234a68e24b120b98875a8b6e17a9dead277be16a