[PATCH v2 2/4] 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 | 23 ++++++++++++++++++-----
 1 file changed, 18 insertions(+), 5 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 9f99f61689f8..2b3685b195f5 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;
@@ -2487,12 +2490,22 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result,
 					khugepaged_scan.address, &mmap_locked, cc);
 			}
 
-			if (*result == SCAN_SUCCEED)
-				++khugepaged_pages_collapsed;
-
 			/* move to next address */
 			khugepaged_scan.address += HPAGE_PMD_SIZE;
-			progress += HPAGE_PMD_NR;
+
+			switch (*result) {
+			case SCAN_NO_PTE_TABLE:
+			case SCAN_PMD_MAPPED:
+			case SCAN_PTE_MAPPED_HUGEPAGE:
+				progress++;
+				break;
+			case SCAN_SUCCEED:
+				++khugepaged_pages_collapsed;
+				fallthrough;
+			default:
+				progress += HPAGE_PMD_NR;
+			}
+
 			if (!mmap_locked)
 				/*
 				 * We released mmap_lock so break loop.  Note
-- 
2.51.0