[PATCH v3 3/6] mm: khugepaged: just skip when the memory has been collapsed

[Vernon Yang <vernon2gm@gmail.com>]

The following data is traced by bpftrace on a desktop system. After
the system has been left idle for 10 minutes upon booting, a lot of
SCAN_PMD_MAPPED or SCAN_NO_PTE_TABLE are observed during a full scan
by khugepaged.

@scan_pmd_status[1]: 1           ## SCAN_SUCCEED
@scan_pmd_status[6]: 2           ## SCAN_EXCEED_SHARED_PTE
@scan_pmd_status[3]: 142         ## SCAN_PMD_MAPPED
@scan_pmd_status[2]: 178         ## SCAN_NO_PTE_TABLE
total progress size: 674 MB
Total time         : 419 seconds ## include khugepaged_scan_sleep_millisecs

The khugepaged_scan list save all task that support collapse into hugepage,
as long as the task is not destroyed, khugepaged will not remove it from
the khugepaged_scan list. This exist a phenomenon where task has already
collapsed all memory regions into hugepage, but khugepaged continues to
scan it, which wastes CPU time and invalid, and due to
khugepaged_scan_sleep_millisecs (default 10s) causes a long wait for
scanning a large number of invalid task, so scanning really valid task
is later.

After applying this patch, when the memory is either SCAN_PMD_MAPPED or
SCAN_NO_PTE_TABLE, just skip it, as follow:

@scan_pmd_status[6]: 2
@scan_pmd_status[3]: 147
@scan_pmd_status[2]: 173
total progress size: 45 MB
Total time         : 20 seconds

Signed-off-by: Vernon Yang <yanglincheng@kylinos.cn>
---
 mm/khugepaged.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 4b124e854e2e..30786c706c4a 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -66,7 +66,10 @@ enum scan_result {
 static struct task_struct *khugepaged_thread __read_mostly;
 static DEFINE_MUTEX(khugepaged_mutex);
 
-/* default scan 8*HPAGE_PMD_NR ptes (or vmas) every 10 second */
+/*
+ * default scan 8*HPAGE_PMD_NR ptes, pmd_mapped, no_pte_table or vmas
+ * every 10 second.
+ */
 static unsigned int khugepaged_pages_to_scan __read_mostly;
 static unsigned int khugepaged_pages_collapsed;
 static unsigned int khugepaged_full_scans;
@@ -1264,7 +1267,7 @@ static int hpage_collapse_scan_pmd(struct mm_struct *mm,
 
 	result = find_pmd_or_thp_or_none(mm, start_addr, &pmd);
 	if (result != SCAN_SUCCEED) {
-		_progress = HPAGE_PMD_NR;
+		_progress = 1;
 		goto out;
 	}
 
@@ -1272,7 +1275,7 @@ static int hpage_collapse_scan_pmd(struct mm_struct *mm,
 	nodes_clear(cc->alloc_nmask);
 	pte = pte_offset_map_lock(mm, pmd, start_addr, &ptl);
 	if (!pte) {
-		_progress = HPAGE_PMD_NR;
+		_progress = 1;
 		result = SCAN_NO_PTE_TABLE;
 		goto out;
 	}
@@ -2336,8 +2339,6 @@ static int hpage_collapse_scan_file(struct mm_struct *mm, unsigned long addr,
 			continue;
 		}
 
-		_progress += folio_nr_pages(folio);
-
 		if (folio_order(folio) == HPAGE_PMD_ORDER &&
 		    folio->index == start) {
 			/* Maybe PMD-mapped */
@@ -2349,9 +2350,12 @@ static int hpage_collapse_scan_file(struct mm_struct *mm, unsigned long addr,
 			 * returning.
 			 */
 			folio_put(folio);
+			_progress++;
 			break;
 		}
 
+		_progress += folio_nr_pages(folio);
+
 		node = folio_nid(folio);
 		if (hpage_collapse_scan_abort(node, cc)) {
 			result = SCAN_SCAN_ABORT;
-- 
2.51.0