mm/vmscan: reduce lru_lock contention via vmstat-derived scan-balance cost

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/20260727162550.2032-4-usama.arif@linux.dev
Signed-off-by: Usama Arif <usama.arif@linux.dev>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Baoquan He <baoquan.he@linux.dev>
Cc: Chris Li <chrisl@kernel.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Kairui Song <kasong@tencent.com>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Wei Xu <weixugc@google.com>
Cc: Yuanchu Xie <yuanchu@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

2 files changed, 11 insertions, 3 deletions

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 1cc5ea506b7c..158c1fba2393 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -757,6 +757,12 @@ void lru_gen_reparent_memcg(struct mem_cgroup *memcg, struct mem_cgroup *parent,
 
 #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 */
diff --git a/include/linux/vmstat.h b/include/linux/vmstat.h
index fb8c76289e02..5b31d8e7ae40 100644
--- a/include/linux/vmstat.h
+++ b/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;