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a69797fb36
Patch series "mm/vmscan: reduce lru_lock contention via vmstat-derived
scan-balance cost", v5.
The anon/file scan balance heuristic in get_scan_count() is fed by two
scalars in struct lruvec (anon_cost, file_cost) that every reclaim
producer updates under lruvec->lru_lock. The cost-recording work itself
is trivial, but it both contends for and contributes to contention on
lru_lock - which is often a contention point on memory-pressured
workloads. Specifically:
- shrink_inactive_list() re-acquires lru_lock at function exit just
to call lru_note_cost_unlock_irq().
- shrink_active_list() does the same after rotation accounting.
- workingset_refault() takes folio_lruvec_lock_irq() purely to
record the refault cost.
- prepare_scan_control() snapshots anon_cost/file_cost under
lru_lock.
- lru_note_cost_unlock_irq() itself walks parent_lruvec() and
re-acquires lru_lock on every ancestor, multiplying the cost
of every update by memcg-hierarchy depth.
This series removes those producer-side acquisitions entirely. The
rotation inputs become per-LRU PGROTATE_{ANON,FILE} vmstat counters.
NR_VMSCAN_WRITE already captures reclaim-driven pageout at writeout();
charge it through lruvec_stat_mod_folio() so it is available per lruvec
and aggregated through the memcg hierarchy. Reclaim does not submit
filesystem folios for writeback from this path, so pageout contributes
only to anon cost. WORKINGSET_RESTORE_* already captures the refault
input.
PGROTATE_* are also useful independently of scan balancing. They are
cumulative base-page events, not unique-page counts. Classic inactive
reclaim records scan work that does not produce immediate reclaim or
demotion, while active reclaim records referenced executable file folios
retained on the active list. MGLRU records initially isolated pages that
remain unreclaimed after its retry passes. Read alongside pgscan_* and
pgsteal_*, their deltas identify which LRU type is consuming reclaim CPU
without producing immediate yield. Unlike the existing pgrotated event,
they do not imply a move to the inactive-list tail.
prepare_scan_control() reads the raw cost signals without lru_lock:
anon = PGROTATE_ANON +
(NR_VMSCAN_WRITE + WORKINGSET_RESTORE_ANON) * SWAP_CLUSTER_MAX
file = PGROTATE_FILE +
WORKINGSET_RESTORE_FILE * SWAP_CLUSTER_MAX
It folds the deltas into a per-lruvec accumulator. A dedicated per-lruvec
cost_lock, not touched by isolate_lru_folios(), move_folios_to_lru(), or
folio_add_lru(), serialises the accumulator RMW and the lrusize/4 halving
check. Hierarchy aggregation is implicit in rstat propagation, so the
parent_lruvec() walk and the lru_reparent_memcg() cost-splice both
disappear.
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.
Trade-offs:
- Cost reads see rstat-aggregated values that can lag until periodic /
reader-triggered flushing.
- Per-lruvec footprint grows by 4 unsigned longs + a spinlock (a
struct lru_cost { count, last_rotated, last_io } per side), which
is a small cost.
- NR_VMSCAN_WRITE now also updates the folio's lruvec/memcg stat,
adding memcg stat accounting to the reclaim writeout path while
preserving the existing node-level total.
== Numbers ==
Tested on a 176-core, 256 GB host. The benchmark drives sustained
swap-out/refault inside a tight memcg using vm-scalability/usemem:
usemem -n 16 --prealloc --prefault --random $((256*1024*1024))
run inside a two-level memcg with memory.max=512M on the leaf (4 GB anon
working set has to fit in 512 MB -> continuous shrink_inactive_list +
workingset_refault). A 16 GB swap file is used. Measurement is a 30 s
`perf lock record -a` window over otherwise-idle hardware.
Workload rates are identical on both kernels (the bench drives the same
memory pressure):
baseline patched delta
pgscan_direct / s 172,662 171,817 ~0%
pgsteal_direct / s 67,162 66,306 ~0%
workingset_refault_anon / s
40,696 39,830 ~0%
perf lock contention (total wait per 30 s window):
Lock Name Before After % change
shrink_lruvec+0x770 722.84 ms 0 -100% (eliminated)
(= lru_note_cost_unlock_irq)
workingset_refault+0x167 385.26 ms 0 -100% (eliminated)
(= lru_note_cost_refault)
shrink_node+0x4ad 689.43 ms 26.95 ms -96%
shrink_active_list 208.34 ms 15.97 ms -92%
lru_add_drain_cpu+0x34 1.96 s 917.71 ms -53%
Total LRU lock wait ~4.23 s ~1.66 s -61%
The two specific contention sites the patch removes (shrink_lruvec+0x770 =
lru_note_cost_unlock_irq; workingset_refault+0x167 =
lru_note_cost_refault) are completely absent from the patched
perf-lock-contention output. Secondary reductions in shrink_node,
shrink_active_list, lru_add_drain_cpu and pgrefill/pgactivate look like
knock-on effects from removing the cost-recording overhead and the
parent_lruvec walk.
The remaining ~1.66 s of LRU lock wait on the patched kernel is dominated
by the per-CPU pagevec drain (lru_add_drain_cpu) and the main reclaim path
in shrink_lruvec.
The numbers above can be reproduced using the script in [1].
This patch (of 3):
lruvec_page_state(), node_page_state(), and global_node_page_state() all
clamp negative reads to zero on CONFIG_SMP so that a transient per-CPU
delta skew presents as zero pages rather than as a garbage unsigned value.
This is the right behaviour for non-monotonic page-count readers.
It is however incorrect for callers that snapshot a monotonically-
incremented event counter and compute a delta from two samples. Once the
underlying signed long wraps past LONG_MAX, the clamped read drops to zero
while the previously-recorded snapshot still holds the pre-wrap value; the
unsigned subtraction then underflows into a ~2^31 spurious delta for
32-bit architecture and corrupts the caller's accumulator.
Add non-clamping siblings that return the underlying state value cast to
unsigned long:
global_node_page_state_monotonic()
node_page_state_monotonic()
lruvec_page_state_monotonic()
With both samples read via the _monotonic variant, unsigned modular
subtraction stays correct across a signed-long wraparound as long as the
true growth between two samples fits in unsigned long (< 2^32 on 32-bit, <
2^64 on 64-bit); the 32-bit bound is the practically-reachable one that
motivates this helper.
The variants are only safe for monotonically-incremented counters.
Non-monotonic page-count readers must keep using the existing clamped
helpers so transient negative reads still present as zero.
This is a prerequisite for a later patch which replaces the producer-side
anon_cost/file_cost accumulators with a read-side accumulator in
prepare_scan_control() that samples monotonic per-LRU vmstat counters
(PGROTATE_*, NR_VMSCAN_WRITE, WORKINGSET_RESTORE_*) via
lruvec_page_state_monotonic() and folds their unsigned modular deltas into
lruvec->cost[].count.
Link: https://lore.kernel.org/20260727162550.2032-1-usama.arif@linux.dev
Link: https://lore.kernel.org/20260727162550.2032-2-usama.arif@linux.dev
Link: https://gist.github.com/uarif1/a4eb33a86c5b2d7bbc55b42f0956e884 [1]
Signed-off-by: Usama Arif <usama.arif@linux.dev>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
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>
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