[RFC] mm/vmstat: Add events for THP migration without split

[Anshuman Khandual <anshuman.khandual@arm.com>]

Add the following new trace events which will help in validating migration
events involving PMD based THP pages.

1. THP_PMD_MIGRATION_ENTRY_SET
2. THP_PMD_MIGRATION_ENTRY_REMOVE

There are no clear method to confirm whether a THP migration happened with
out involving it's split. These trace events along with PGMIGRATE_SUCCESS
and PGMIGRATE_FAILURE will provide additional insights. After this change,

A single 2M THP (2K base page) when migrated

1. Without split

................
pgmigrate_success 1
pgmigrate_fail 0
................
thp_pmd_migration_entry_set 1
thp_pmd_migration_entry_remove 1
................

2. With split

................
pgmigrate_success 512
pgmigrate_fail 0
................
thp_pmd_migration_entry_set 0
thp_pmd_migration_entry_remove 0
................

pgmigrate_success as 1 instead of 512, provides a hint for possible THP
migration event. But then it gets mixed with normal page migrations over
time. These additional trace events provide required co-relation.

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: linux-mm@kvack.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
---
This is an indirect way for counting PMD migrations without split. Is there
a better method possible ? Just request for comments at the moment.

 include/linux/vm_event_item.h | 4 ++++
 mm/migrate.c                  | 1 +
 mm/rmap.c                     | 1 +
 mm/vmstat.c                   | 4 ++++
 4 files changed, 10 insertions(+)

diff --git a/include/linux/vm_event_item.h b/include/linux/vm_event_item.h
index ffef0f279747..4b25102cf3ad 100644
--- a/include/linux/vm_event_item.h
+++ b/include/linux/vm_event_item.h
@@ -91,6 +91,10 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT,
 		THP_ZERO_PAGE_ALLOC_FAILED,
 		THP_SWPOUT,
 		THP_SWPOUT_FALLBACK,
+#ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
+		THP_PMD_MIGRATION_ENTRY_SET,
+		THP_PMD_MIGRATION_ENTRY_REMOVE,
+#endif
 #endif
 #ifdef CONFIG_MEMORY_BALLOON
 		BALLOON_INFLATE,
diff --git a/mm/migrate.c b/mm/migrate.c
index 7160c1556f79..8d50d55cbe97 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -228,6 +228,7 @@ static bool remove_migration_pte(struct page *page, struct vm_area_struct *vma,
 		if (!pvmw.pte) {
 			VM_BUG_ON_PAGE(PageHuge(page) || !PageTransCompound(page), page);
 			remove_migration_pmd(&pvmw, new);
+			count_vm_event(THP_PMD_MIGRATION_ENTRY_REMOVE);
 			continue;
 		}
 #endif
diff --git a/mm/rmap.c b/mm/rmap.c
index f79a206b271a..3c1fe3f45cb5 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1418,6 +1418,7 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
 			VM_BUG_ON_PAGE(PageHuge(page) || !PageTransCompound(page), page);
 
 			set_pmd_migration_entry(&pvmw, page);
+			count_vm_event(THP_PMD_MIGRATION_ENTRY_SET);
 			continue;
 		}
 #endif
diff --git a/mm/vmstat.c b/mm/vmstat.c
index 96d21a792b57..a5254b7ee531 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -1274,6 +1274,10 @@ const char * const vmstat_text[] = {
 	"thp_zero_page_alloc_failed",
 	"thp_swpout",
 	"thp_swpout_fallback",
+#ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
+	"thp_pmd_migration_entry_set",
+	"thp_pmd_migration_entry_remove",
+#endif
 #endif
 #ifdef CONFIG_MEMORY_BALLOON
 	"balloon_inflate",
-- 
2.20.1