[RFC PATCH v4 6/6] mm/swap.c: Enable promotion of unmapped MGLRU page cache pages

[Gregory Price]

From: Donet Tom <donettom@xxxxxxxxxxxxx>

Extend MGLRU to support promotion of page cache pages.

An MGLRU page cache page is eligible for promotion when:

1. Memory Tiering and pagecache_promotion_enabled are enabled
2. It resides in a lower memory tier.
3. It is referenced.
4. It is part of the working set.
5. folio reference count is maximun (LRU_REFS_MASK).

When a page is accessed through a file descriptor, folio_inc_refs()
is invoked. The first access will set the folio’s referenced flag,
and subsequent accesses will increment the reference count in the
folio flag (reference counter size in folio flags is 2 bits). Once
the referenced flag is set, and the folio’s reference count reaches
the maximum value (LRU_REFS_MASK), the working set flag will be set
as well.

If a folio has both the referenced and working set flags set, and its
reference count equals LRU_REFS_MASK, it becomes a good candidate for
promotion. These pages will be added to the promotion list. The
per-process task task_numa_promotion_work() takes the pages from the
promotion list and promotes them to a higher memory tier.

In the MGLRU, for folios accessed through a file descriptor, if the
folio’s referenced and working set flags are set, and the folio's
reference count is equal to LRU_REFS_MASK, the folio is lazily
promoted to the second oldest generation in the eviction path. When
folio_inc_gen() does this, it clears the LRU_REFS_FLAGS so that
lru_gen_inc_refs() can start over.

Test process:
We measured the read time in below scenarios for both LRU and MGLRU.
Scenario 1: Pages are on Lower tier + promotion off
Scenario 2: Pages are on Lower tier + promotion on
Scenario 3: Pages are on higher tier

Test Results MGLRU
----------------------------------------------------------------
Pages on higher   | Pages Lower tier |  Pages on Lower Tier    |
   Tier           |  promotion off   |   Promotion On          |
----------------------------------------------------------------
  0.48s           |    1.6s          |During Promotion - 3.3s  |
                  |                  |After Promotion  - 0.48s |
                  |                  |                         |
----------------------------------------------------------------

Test Results LRU
----------------------------------------------------------------
Pages on higher   | Pages Lower tier |  Pages on Lower Tier    |
   Tier           |  promotion off   |   Promotion On          |
----------------------------------------------------------------
   0.48s          |    1.6s          |During Promotion - 3.3s  |
                  |                  |After Promotion  - 0.48s |
                  |                  |                         |
----------------------------------------------------------------

MGLRU and LRU are showing similar performance benefit.

Signed-off-by: Donet Tom <donettom@xxxxxxxxxxxxx>
---
 mm/swap.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/mm/swap.c b/mm/swap.c
index 382828fde505..3af2377515ad 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -399,8 +399,13 @@ static void lru_gen_inc_refs(struct folio *folio)
 
 	do {
 		if ((old_flags & LRU_REFS_MASK) == LRU_REFS_MASK) {
-			if (!folio_test_workingset(folio))
+			if (!folio_test_workingset(folio)) {
 				folio_set_workingset(folio);
+			} else if (!folio_test_isolated(folio) &&
+				  (sysctl_numa_balancing_mode & NUMA_BALANCING_MEMORY_TIERING) &&
+				   numa_pagecache_promotion_enabled) {
+				promotion_candidate(folio);
+			}
 			return;
 		}
 
-- 
2.49.0