linux-kernel - [RFC PATCH v3 1/8] mm: multi-gen LRU: ignore non-leaf pmd_young for force

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Date: Wed, 27 Mar 2024 14:31:00 -0700
From: Yuanchu Xie <yuanchu@...gle.com>
To: David Hildenbrand <david@...hat.com>, "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>, 
	Khalid Aziz <khalid.aziz@...cle.com>, Henry Huang <henry.hj@...group.com>, 
	Yu Zhao <yuzhao@...gle.com>, Dan Williams <dan.j.williams@...el.com>, 
	Gregory Price <gregory.price@...verge.com>, Huang Ying <ying.huang@...el.com>
Cc: Wei Xu <weixugc@...gle.com>, David Rientjes <rientjes@...gle.com>, 
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, "Rafael J. Wysocki" <rafael@...nel.org>, 
	Andrew Morton <akpm@...ux-foundation.org>, Johannes Weiner <hannes@...xchg.org>, 
	Michal Hocko <mhocko@...nel.org>, Roman Gushchin <roman.gushchin@...ux.dev>, 
	Muchun Song <muchun.song@...ux.dev>, Shuah Khan <shuah@...nel.org>, 
	Yosry Ahmed <yosryahmed@...gle.com>, Matthew Wilcox <willy@...radead.org>, 
	Sudarshan Rajagopalan <quic_sudaraja@...cinc.com>, Kairui Song <kasong@...cent.com>, 
	"Michael S. Tsirkin" <mst@...hat.com>, Vasily Averin <vasily.averin@...ux.dev>, Nhat Pham <nphamcs@...il.com>, 
	Miaohe Lin <linmiaohe@...wei.com>, Qi Zheng <zhengqi.arch@...edance.com>, 
	Abel Wu <wuyun.abel@...edance.com>, "Vishal Moola (Oracle)" <vishal.moola@...il.com>, 
	Kefeng Wang <wangkefeng.wang@...wei.com>, Yuanchu Xie <yuanchu@...gle.com>, 
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, cgroups@...r.kernel.org, 
	linux-kselftest@...r.kernel.org
Subject: [RFC PATCH v3 1/8] mm: multi-gen LRU: ignore non-leaf pmd_young for force_scan=true

When non-leaf pmd accessed bits are available, MGLRU page table walks
can clear the accessed bit and promptly ignore the accessed bit on the
pte because it's on a different node, so the walk does not update the
generation of said page. When the next scan comes around on the right
node, the non-leaf pmd accessed bit might remain cleared and the pte
accessed bits won't be checked. While this is sufficient for
reclaim-driven aging, where the goal is to select a reasonably cold
page, the access can be missed when aging proactively for measuring the
working set size of a node/memcg.

Since force_scan disables various other optimizations, we check
force_scan to ignore the non-leaf pmd accessed bit.

Signed-off-by: Yuanchu Xie <yuanchu@...gle.com>
---
 mm/vmscan.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 4f9c854ce6cc..1a7c7d537db6 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3522,7 +3522,7 @@ static void walk_pmd_range(pud_t *pud, unsigned long start, unsigned long end,
 
 		walk->mm_stats[MM_NONLEAF_TOTAL]++;
 
-		if (should_clear_pmd_young()) {
+		if (!walk->force_scan && should_clear_pmd_young()) {
 			if (!pmd_young(val))
 				continue;
 
-- 
2.44.0.396.g6e790dbe36-goog