[PATCH v3 5/6] mm: khugepaged: skip lazy-free folios at scanning

[Vernon Yang <vernon2gm@gmail.com>]

For example, create three task: hot1 -> cold -> hot2. After all three
task are created, each allocate memory 128MB. the hot1/hot2 task
continuously access 128 MB memory, while the cold task only accesses
its memory briefly andthen call madvise(MADV_FREE). However, khugepaged
still prioritizes scanning the cold task and only scans the hot2 task
after completing the scan of the cold task.

So if the user has explicitly informed us via MADV_FREE that this memory
will be freed, it is appropriate for khugepaged to skip it only, thereby
avoiding unnecessary scan and collapse operations to reducing CPU
wastage.

Here are the performance test results:
(Throughput bigger is better, other smaller is better)

Testing on x86_64 machine:

| task hot2           | without patch | with patch    |  delta  |
|---------------------|---------------|---------------|---------|
| total accesses time |  3.14 sec     |  2.93 sec     | -6.69%  |
| cycles per access   |  4.96         |  2.21         | -55.44% |
| Throughput          |  104.38 M/sec |  111.89 M/sec | +7.19%  |
| dTLB-load-misses    |  284814532    |  69597236     | -75.56% |

Testing on qemu-system-x86_64 -enable-kvm:

| task hot2           | without patch | with patch    |  delta  |
|---------------------|---------------|---------------|---------|
| total accesses time |  3.35 sec     |  2.96 sec     | -11.64% |
| cycles per access   |  7.29         |  2.07         | -71.60% |
| Throughput          |  97.67 M/sec  |  110.77 M/sec | +13.41% |
| dTLB-load-misses    |  241600871    |  3216108      | -98.67% |

Signed-off-by: Vernon Yang <yanglincheng@kylinos.cn>
---
 include/trace/events/huge_memory.h | 1 +
 mm/khugepaged.c                    | 6 ++++++
 2 files changed, 7 insertions(+)

diff --git a/include/trace/events/huge_memory.h b/include/trace/events/huge_memory.h
index 01225dd27ad5..e99d5f71f2a4 100644
--- a/include/trace/events/huge_memory.h
+++ b/include/trace/events/huge_memory.h
@@ -25,6 +25,7 @@
 	EM( SCAN_PAGE_LRU,		"page_not_in_lru")		\
 	EM( SCAN_PAGE_LOCK,		"page_locked")			\
 	EM( SCAN_PAGE_ANON,		"page_not_anon")		\
+	EM( SCAN_PAGE_LAZYFREE,		"page_lazyfree")		\
 	EM( SCAN_PAGE_COMPOUND,		"page_compound")		\
 	EM( SCAN_ANY_PROCESS,		"no_process_for_page")		\
 	EM( SCAN_VMA_NULL,		"vma_null")			\
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 30786c706c4a..1ca034a5f653 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -45,6 +45,7 @@ enum scan_result {
 	SCAN_PAGE_LRU,
 	SCAN_PAGE_LOCK,
 	SCAN_PAGE_ANON,
+	SCAN_PAGE_LAZYFREE,
 	SCAN_PAGE_COMPOUND,
 	SCAN_ANY_PROCESS,
 	SCAN_VMA_NULL,
@@ -1337,6 +1338,11 @@ static int hpage_collapse_scan_pmd(struct mm_struct *mm,
 		}
 		folio = page_folio(page);
 
+		if (folio_is_lazyfree(folio)) {
+			result = SCAN_PAGE_LAZYFREE;
+			goto out_unmap;
+		}
+
 		if (!folio_test_anon(folio)) {
 			result = SCAN_PAGE_ANON;
 			goto out_unmap;
-- 
2.51.0