+ mm-vmscan-reduce-lru_lock-contention-via-vmstat-derived-scan-balance-cost.patch added to mm-unstable branch

The patch titled
     Subject: mm/vmscan: reduce lru_lock contention via vmstat-derived scan-balance cost
has been added to the -mm mm-unstable branch.  Its filename is
     mm-vmscan-reduce-lru_lock-contention-via-vmstat-derived-scan-balance-cost.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-vmscan-reduce-lru_lock-contention-via-vmstat-derived-scan-balance-cost.patch

This patch will later appear in the mm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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and is updated there most days

------------------------------------------------------
From: Usama Arif <[email protected]>
Subject: mm/vmscan: reduce lru_lock contention via vmstat-derived scan-balance cost
Date: Mon, 27 Jul 2026 09:23:25 -0700

The anon/file scan balance in get_scan_count() is driven by two scalars in
struct lruvec, anon_cost and file_cost, accumulated by every reclaim
producer under lruvec->lru_lock.  The acquisition sites for cost work
specifically are:

  - shrink_inactive_list() re-takes lru_lock at function exit purely
    to call lru_note_cost_unlock_irq() with (nr_pageout, nr_scanned -
    nr_reclaimed). One acquisition per inactive shrink.
  - shrink_active_list() does the same with (0, nr_rotated). One
    acquisition per active shrink.
  - workingset_refault() takes the lock via folio_lruvec_lock_irq()
    purely to record the refault cost. One acquisition per refault.
  - prepare_scan_control() takes lru_lock just to snapshot the two
    scalars into sc->{anon,file}_cost.
  - lru_note_cost_unlock_irq() itself walks parent_lruvec and
    re-acquires lru_lock on each ancestor to propagate the update,
    adding O(memcg-depth) acquisitions per producer call.

This hurts because lru_lock is already a heavy contention point on
memory-heavy workloads: every isolate_lru_folios(), move_folios_to_lru()
and folio_add_lru() takes it.  The cost work itself is trivial (two scalar
bumps and one comparison), but it contends with and causes contention for
actual LRU manipulation.  The parent_lruvec() walk also multiplies
cost-update overhead by memcg hierarchy depth.

The balance formula for anon and file, respectively, is this:

    cost = nr_io * SWAP_CLUSTER_MAX + nr_rotated

Instead of recording cost and running averaging logic directly when these
events occur, snapshot running vmstat counters once per reclaim cycle and
derive the balance from event deltas since the last run.

Use PGROTATE_* from the preceding patch for the rotation input. 
WORKINGSET_RESTORE_* and NR_VMSCAN_WRITE provide the remaining event
counters.  Charge NR_VMSCAN_WRITE through lruvec stats so all inputs can
be sampled per lruvec and aggregated through the memcg hierarchy.  This is
overall cheaper and has fewer lock acquisition sites.

Moving accumulation and decay to the reclaim side also improves the cost
model across reclaim gaps.  With producer-side decay, events that happen
while reclaim is idle still age each other before reclaim ever samples the
costs.  If a workload refaults a large anon set and then a smaller file
set before reclaim runs again, the later file activity can age the earlier
anon activity out of the cost model.  The new scheme observes the whole
between-reclaim delta and decays anon and file proportionally, so the
scan-balance history better represents what happened since the last
reclaim pass.

A dedicated per-lruvec spinlock, cost_lock, serialises the delta
extraction, the cost->count update and the halving loop against concurrent
reclaimers in the same memcg+node.

NR_VMSCAN_WRITE is accounted at writeout(), so reclaim_stat.nr_pageout is
no longer needed and is removed.

memcg-v1's memory.stat anon_cost/file_cost is now sourced from
cost[].count instead of the removed lruvec anon_cost/file_cost fields. 
The reported values only refresh when prepare_scan_control() runs and are
bounded at ~lrusize/4 by the halving loop; the scan-balance signal they
express is unchanged.

Under pure MGLRU the scan-balance signal itself is not consumed (both
prepare_scan_control() and get_scan_count() are short-circuited on the
MGLRU paths, and MGLRU's own type/tier selection comes from
read_ctrl_pos() on lrugen->{avg_refaulted,avg_total,refaulted,evicted},
not from anon_cost/file_cost).  NR_VMSCAN_WRITE naturally covers writeout
from either reclaim implementation.  The preceding patch also bumps
PGROTATE_{ANON,FILE} from evict_folios(), so rotation-driven reclaim work
is accounted consistently across both implementations.

Link: https://lore.kernel.org/[email protected]
Signed-off-by: Usama Arif <[email protected]>
Acked-by: Shakeel Butt <[email protected]>
Acked-by: Johannes Weiner <[email protected]>
Acked-by: Vlastimil Babka (SUSE) <[email protected]>
Cc: Axel Rasmussen <[email protected]>
Cc: Baoquan He <[email protected]>
Cc: Chris Li <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Kairui Song <[email protected]>
Cc: Liam R. Howlett <[email protected]>
Cc: Lorenzo Stoakes <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Mike Rapoport <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: Nhat Pham <[email protected]>
Cc: Roman Gushchin <[email protected]>
Cc: Suren Baghdasaryan <[email protected]>
Cc: Wei Xu <[email protected]>
Cc: Yuanchu Xie <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
---

 include/linux/mmzone.h |   13 +++++-
 include/linux/vmstat.h |    1 
 mm/folio.c             |   69 ----------------------------------
 mm/internal.h          |    3 -
 mm/memcontrol-v1.c     |    4 -
 mm/memcontrol.c        |    1 
 mm/mmzone.c            |    1 
 mm/vmscan.c            |   79 ++++++++++++++++++++++++++++++++-------
 mm/workingset.c        |    5 --
 9 files changed, 81 insertions(+), 95 deletions(-)

--- a/include/linux/mmzone.h~mm-vmscan-reduce-lru_lock-contention-via-vmstat-derived-scan-balance-cost
+++ a/include/linux/mmzone.h
@@ -757,6 +757,12 @@ void lru_gen_reparent_memcg(struct mem_c
 
 #endif /* CONFIG_LRU_GEN */
 
+struct lru_cost {
+	unsigned long		count;
+	unsigned long		last_rotated;
+	unsigned long		last_io;
+};
+
 struct lruvec {
 	struct list_head		lists[NR_LRU_LISTS];
 	/* per lruvec lru_lock for memcg */
@@ -765,9 +771,12 @@ struct lruvec {
 	 * These track the cost of reclaiming one LRU - file or anon -
 	 * over the other. As the observed cost of reclaiming one LRU
 	 * increases, the reclaim scan balance tips toward the other.
+	 * Updated and decayed at prepare_scan_control() time; cost_lock
+	 * serialises that update.
 	 */
-	unsigned long			anon_cost;
-	unsigned long			file_cost;
+	struct lru_cost			cost[ANON_AND_FILE];
+	/* Protects cost[]. */
+	spinlock_t			cost_lock;
 	/* Non-resident age, driven by LRU movement */
 	atomic_long_t			nonresident_age;
 	/* Refaults at the time of last reclaim cycle */
--- a/include/linux/vmstat.h~mm-vmscan-reduce-lru_lock-contention-via-vmstat-derived-scan-balance-cost
+++ a/include/linux/vmstat.h
@@ -20,7 +20,6 @@ struct reclaim_stat {
 	unsigned nr_congested;
 	unsigned nr_writeback;
 	unsigned nr_immediate;
-	unsigned nr_pageout;
 	unsigned nr_activate[ANON_AND_FILE];
 	unsigned nr_ref_keep;
 	unsigned nr_unmap_fail;
--- a/mm/folio.c~mm-vmscan-reduce-lru_lock-contention-via-vmstat-derived-scan-balance-cost
+++ a/mm/folio.c
@@ -265,73 +265,6 @@ void folio_rotate_reclaimable(struct fol
 	folio_batch_add_and_move(folio, lru_move_tail);
 }
 
-void lru_note_cost_unlock_irq(struct lruvec *lruvec, bool file,
-		unsigned int nr_io, unsigned int nr_rotated)
-		__releases(lruvec->lru_lock)
-		__releases(rcu)
-{
-	unsigned long cost;
-
-	/*
-	 * Reflect the relative cost of incurring IO and spending CPU
-	 * time on rotations. This doesn't attempt to make a precise
-	 * comparison, it just says: if reloads are about comparable
-	 * between the LRU lists, or rotations are overwhelmingly
-	 * different between them, adjust scan balance for CPU work.
-	 */
-	cost = nr_io * SWAP_CLUSTER_MAX + nr_rotated;
-	if (!cost) {
-		spin_unlock_irq(&lruvec->lru_lock);
-		rcu_read_unlock();
-		return;
-	}
-
-	for (;;) {
-		unsigned long lrusize;
-
-		/* Record cost event */
-		if (file)
-			lruvec->file_cost += cost;
-		else
-			lruvec->anon_cost += cost;
-
-		/*
-		 * Decay previous events
-		 *
-		 * Because workloads change over time (and to avoid
-		 * overflow) we keep these statistics as a floating
-		 * average, which ends up weighing recent refaults
-		 * more than old ones.
-		 */
-		lrusize = lruvec_page_state(lruvec, NR_INACTIVE_ANON) +
-			  lruvec_page_state(lruvec, NR_ACTIVE_ANON) +
-			  lruvec_page_state(lruvec, NR_INACTIVE_FILE) +
-			  lruvec_page_state(lruvec, NR_ACTIVE_FILE);
-
-		if (lruvec->file_cost + lruvec->anon_cost > lrusize / 4) {
-			lruvec->file_cost /= 2;
-			lruvec->anon_cost /= 2;
-		}
-
-		spin_unlock_irq(&lruvec->lru_lock);
-		lruvec = parent_lruvec(lruvec);
-		if (!lruvec) {
-			rcu_read_unlock();
-			break;
-		}
-		spin_lock_irq(&lruvec->lru_lock);
-	}
-}
-
-void lru_note_cost_refault(struct folio *folio)
-{
-	struct lruvec *lruvec;
-
-	lruvec = folio_lruvec_lock_irq(folio);
-	lru_note_cost_unlock_irq(lruvec, folio_is_file_lru(folio),
-				folio_nr_pages(folio), 0);
-}
-
 static void lru_activate(struct lruvec *lruvec, struct folio *folio)
 {
 	long nr_pages = folio_nr_pages(folio);
@@ -1162,8 +1095,6 @@ void lru_reparent_memcg(struct mem_cgrou
 
 	child_lruvec = mem_cgroup_lruvec(memcg, NODE_DATA(nid));
 	parent_lruvec = mem_cgroup_lruvec(parent, NODE_DATA(nid));
-	parent_lruvec->anon_cost += child_lruvec->anon_cost;
-	parent_lruvec->file_cost += child_lruvec->file_cost;
 
 	for_each_lru(lru)
 		lruvec_reparent_lru(child_lruvec, parent_lruvec, lru, nid);
--- a/mm/internal.h~mm-vmscan-reduce-lru_lock-contention-via-vmstat-derived-scan-balance-cost
+++ a/mm/internal.h
@@ -41,9 +41,6 @@ void workingset_refault(struct folio *fo
 void workingset_activation(struct folio *folio);
 
 /* mm/folio.c */
-void lru_note_cost_unlock_irq(struct lruvec *lruvec, bool file,
-		unsigned int nr_io, unsigned int nr_rotated);
-void lru_note_cost_refault(struct folio *folio);
 void folio_add_lru_vma(struct folio *folio, struct vm_area_struct *vma);
 
 static inline bool folio_may_be_lru_cached(struct folio *folio)
--- a/mm/memcontrol.c~mm-vmscan-reduce-lru_lock-contention-via-vmstat-derived-scan-balance-cost
+++ a/mm/memcontrol.c
@@ -399,6 +399,7 @@ static const unsigned int memcg_node_sta
 	NR_SHMEM_THPS,
 	NR_FILE_THPS,
 	NR_ANON_THPS,
+	NR_VMSCAN_WRITE,
 	NR_VMALLOC,
 	NR_KERNEL_STACK_KB,
 	NR_PAGETABLE,
--- a/mm/memcontrol-v1.c~mm-vmscan-reduce-lru_lock-contention-via-vmstat-derived-scan-balance-cost
+++ a/mm/memcontrol-v1.c
@@ -2287,8 +2287,8 @@ void memcg1_stat_format(struct mem_cgrou
 		for_each_online_pgdat(pgdat) {
 			mz = memcg->nodeinfo[pgdat->node_id];
 
-			anon_cost += mz->lruvec.anon_cost;
-			file_cost += mz->lruvec.file_cost;
+			anon_cost += mz->lruvec.cost[WORKINGSET_ANON].count;
+			file_cost += mz->lruvec.cost[WORKINGSET_FILE].count;
 		}
 		seq_buf_printf(s, "anon_cost %lu\n", anon_cost);
 		seq_buf_printf(s, "file_cost %lu\n", file_cost);
--- a/mm/mmzone.c~mm-vmscan-reduce-lru_lock-contention-via-vmstat-derived-scan-balance-cost
+++ a/mm/mmzone.c
@@ -79,6 +79,7 @@ void lruvec_init(struct lruvec *lruvec)
 
 	memset(lruvec, 0, sizeof(struct lruvec));
 	spin_lock_init(&lruvec->lru_lock);
+	spin_lock_init(&lruvec->cost_lock);
 	zswap_lruvec_state_init(lruvec);
 
 	for_each_lru(lru)
--- a/mm/vmscan.c~mm-vmscan-reduce-lru_lock-contention-via-vmstat-derived-scan-balance-cost
+++ a/mm/vmscan.c
@@ -670,7 +670,7 @@ static pageout_t pageout(struct swap_io_
 		folio_clear_reclaim(folio);
 
 	trace_mm_vmscan_write_folio(folio);
-	node_stat_add_folio(folio, NR_VMSCAN_WRITE);
+	lruvec_stat_mod_folio(folio, NR_VMSCAN_WRITE, folio_nr_pages(folio));
 	return PAGE_SUCCESS;
 }
 
@@ -1412,8 +1412,6 @@ retry:
 					sc->nr_scanned -= (nr_pages - 1);
 					nr_pages = 1;
 				}
-				stat->nr_pageout += nr_pages;
-
 				if (folio_test_writeback(folio))
 					goto keep;
 				if (folio_test_dirty(folio))
@@ -2040,9 +2038,6 @@ static unsigned long shrink_inactive_lis
 		mod_lruvec_state(lruvec, PGROTATE_ANON + file,
 				 nr_scanned - nr_reclaimed);
 
-	lruvec_lock_irq(lruvec);
-	lru_note_cost_unlock_irq(lruvec, file, stat.nr_pageout,
-					nr_scanned - nr_reclaimed);
 	handle_reclaim_writeback(nr_taken, pgdat, sc, &stat);
 	trace_mm_vmscan_lru_shrink_inactive(pgdat->node_id,
 			nr_scanned, nr_reclaimed, &stat, sc->priority, file);
@@ -2151,8 +2146,6 @@ static void shrink_active_list(unsigned
 	if (nr_rotated)
 		mod_lruvec_state(lruvec, PGROTATE_ANON + file, nr_rotated);
 
-	lruvec_lock_irq(lruvec);
-	lru_note_cost_unlock_irq(lruvec, file, 0, nr_rotated);
 	trace_mm_vmscan_lru_shrink_active(pgdat->node_id, nr_taken, nr_activate,
 			nr_deactivate, nr_rotated, sc->priority, file);
 }
@@ -2285,8 +2278,10 @@ enum scan_balance {
 
 static void prepare_scan_control(pg_data_t *pgdat, struct scan_control *sc)
 {
-	unsigned long file;
+	struct lru_cost *anon_cost, *file_cost;
 	struct lruvec *target_lruvec;
+	unsigned long lrusize;
+	unsigned long file;
 
 	if (lru_gen_enabled() && !lru_gen_switching())
 		return;
@@ -2302,11 +2297,69 @@ static void prepare_scan_control(pg_data
 
 	/*
 	 * Determine the scan balance between anon and file LRUs.
+	 *
+	 * The cost model is based on rotations, refaults and
+	 * reclaim-driven writes (anon only) on each side.
+	 *
+	 * These event counters are monotonic, so each reclaim cycle
+	 * the delta since the last scan is extracted and incorporated
+	 * into a decaying average. This ensures currency, as workloads
+	 * change over time, and avoids overflow in the calculations.
+	 *
+	 * Use lruvec_page_state_monotonic() so unsigned subtraction
+	 * yields the correct delta across a signed-long wraparound of
+	 * the underlying counter (a real hazard on 32-bit that the
+	 * clamp in lruvec_page_state() would otherwise turn into a huge
+	 * spurious delta).
 	 */
-	spin_lock_irq(&target_lruvec->lru_lock);
-	sc->anon_cost = target_lruvec->anon_cost;
-	sc->file_cost = target_lruvec->file_cost;
-	spin_unlock_irq(&target_lruvec->lru_lock);
+	spin_lock(&target_lruvec->cost_lock);
+
+	for (int f = 0; f <= 1; f++) {
+		struct lru_cost *cost = &target_lruvec->cost[f];
+		unsigned long rotated, io, nr_rotated, nr_io;
+
+		rotated = lruvec_page_state_monotonic(target_lruvec,
+						      PGROTATE_ANON + f);
+		io = lruvec_page_state_monotonic(target_lruvec,
+						 WORKINGSET_RESTORE_BASE + f);
+		if (f == WORKINGSET_ANON)
+			io += lruvec_page_state_monotonic(target_lruvec,
+							  NR_VMSCAN_WRITE);
+
+		nr_rotated = rotated - cost->last_rotated;
+		nr_io = io - cost->last_io;
+
+		/*
+		 * Reflect the relative cost of incurring IO and spending
+		 * CPU time on rotations. This doesn't attempt to make a
+		 * precise comparison, it just says: if reloads are about
+		 * comparable between the LRU lists, or rotations are
+		 * overwhelmingly different between them, adjust scan
+		 * balance for CPU work.
+		 */
+		cost->count += nr_io * SWAP_CLUSTER_MAX + nr_rotated;
+
+		cost->last_rotated = rotated;
+		cost->last_io = io;
+	}
+
+	anon_cost = &target_lruvec->cost[WORKINGSET_ANON];
+	file_cost = &target_lruvec->cost[WORKINGSET_FILE];
+
+	lrusize = lruvec_page_state(target_lruvec, NR_INACTIVE_ANON) +
+		  lruvec_page_state(target_lruvec, NR_ACTIVE_ANON) +
+		  lruvec_page_state(target_lruvec, NR_INACTIVE_FILE) +
+		  lruvec_page_state(target_lruvec, NR_ACTIVE_FILE);
+
+	while (anon_cost->count + file_cost->count > lrusize / 4) {
+		anon_cost->count /= 2;
+		file_cost->count /= 2;
+	}
+
+	sc->anon_cost = anon_cost->count;
+	sc->file_cost = file_cost->count;
+
+	spin_unlock(&target_lruvec->cost_lock);
 
 	/*
 	 * Target desirable inactive:active list ratios for the anon
--- a/mm/workingset.c~mm-vmscan-reduce-lru_lock-contention-via-vmstat-derived-scan-balance-cost
+++ a/mm/workingset.c
@@ -584,11 +584,6 @@ void workingset_refault(struct folio *fo
 	/* Folio was active prior to eviction */
 	if (workingset) {
 		folio_set_workingset(folio);
-		/*
-		 * XXX: Move to folio_add_lru() when it supports new vs
-		 * putback
-		 */
-		lru_note_cost_refault(folio);
 		mod_lruvec_state(lruvec, WORKINGSET_RESTORE_BASE + file, nr);
 	}
 out:
_

Patches currently in -mm which might be from [email protected] are

mm-mempolicy-skip-non-present-pmds-when-queueing-folios.patch
mm-madvise-skip-device-private-pmds-in-cold-and-pageout-walks.patch
mm-huge_memory-skip-device-private-pmds-in-madvise_free_huge_pmd.patch
mm-vmstat-mm-memcontrol-add-_monotonic-vmstat-readers.patch
mm-vmscan-add-pgrotate_anon-and-pgrotate_file-vmstat-counters.patch
mm-vmscan-reduce-lru_lock-contention-via-vmstat-derived-scan-balance-cost.patch