From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-172.mta1.migadu.com (out-172.mta1.migadu.com [95.215.58.172]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 8AC0D31A572 for ; Mon, 13 Jul 2026 16:34:51 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783960493; cv=none; b=VVX6v/I5tbdAtY3RsRATFBtH9zdE/RnMW5bJAT6v3Thg/B6vsLHHWLL2QPY/lsvpL0eFiYI4e8+LVdiS9NerbKy48Mqre+aVoNUNIN92pnfZ1AW8tF3BAzCQ1P4gnzl3D0NZm4kbBz4iletZ+wy8vi2Bp+6v+21IcIwA9sXL8fM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783960493; c=relaxed/simple; bh=PxmHAwDQbClWeCLoWqc9/9f6I+Cuqfvjaw6ziNUyfjo=; h=From:To:Cc:Subject:Date:Message-ID:MIME-Version:Content-Type; b=OYki27eQa+Ch3zehHNaxtkuxnU/QfrS3iPnTBI0DmOTpmiqg2d+6BDbeyzE50hAFeaVd4Ooqhz7QAhgD7esAsq6MJIfdygrU8Wr5aw0r0VRcWatbIA4oQedaAvGDHUbe1jPdK6bjGpG26KMy8Er6SA67DcS4CjuGc6Kc1RBXBo8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=haNkL20k; arc=none smtp.client-ip=95.215.58.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="haNkL20k" X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1783960489; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding; bh=XGVzZk+EZbjQt/HAvFA/+T9A3SExi5ynlWA6au9+tbw=; b=haNkL20kKANlYAa3K530QRSumtDpsJOOmJ7WWp1z+ekDmIZu6uxpyI/FPsopzKnfJLJbnU uvnI8Ebv+PdEiIt6zc6p7Y1ikMfAnYwqzIho94yJcOo9oq1EVfWSlVAgo9caBqRn7nOUID 2AJXdXvpYLB07RdsFgwYRn+oZoBXD84= From: Usama Arif To: Andrew Morton , david@kernel.org, ljs@kernel.org, liam@infradead.org, vbabka@kernel.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com, kasong@tencent.com, qi.zheng@linux.dev, shakeel.butt@linux.dev, axelrasmussen@google.com, yuanchu@google.com, weixugc@google.com, chrisl@kernel.org, nphamcs@gmail.com, baoquan.he@linux.dev, youngjun.park@lge.com, hannes@cmpxchg.org, roman.gushchin@linux.dev, muchun.song@linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, rientjes@google.com, kernel-team@meta.com Cc: Usama Arif Subject: [PATCH v2 0/2] mm/vmscan: reduce lru_lock contention via vmstat-derived scan-balance cost Date: Mon, 13 Jul 2026 09:34:15 -0700 Message-ID: <20260713163443.3562378-1-usama.arif@linux.dev> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT 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 producer-local inputs (PGROTATE_*, PGRECLAIM_PAGEOUT_*) become per-LRU vmstat counters; WORKINGSET_RESTORE_* already captures the refault input. prepare_scan_control() reads the raw cost signal lock-free from those vmstats and folds the delta 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 2 unsigned longs + a spinlock, which is a small cost. == 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]. == Series structure == 1. mm/vmstat, mm/memcontrol: add _monotonic vmstat readers Non-clamping siblings of lruvec_page_state() / node_page_state() / global_node_page_state() for callers that snapshot a monotonically-incremented counter and subtract two samples. Unsigned modular subtraction on the raw wrapping value stays correct across a signed-long overflow that the existing clamp would otherwise turn into a huge spurious delta (a real hazard on 32-bit). 2. mm/vmscan: reduce lru_lock contention via vmstat-derived scan-balance cost The optimization described above; samples PGROTATE_*, PGRECLAIM_PAGEOUT_* and WORKINGSET_RESTORE_* via lruvec_page_state_monotonic() from patch 1. == Alternatives considered == 1. cost_lock for both producer and consumer (no vmstat indirection): Keep the producer loop, just swap lru_lock for a new per-lruvec cost_lock. Decouples cost from LRU manipulation, but producers still synchronously contend on cost_lock, the parent_lruvec() walk is still required (O(memcg-depth) acquisitions per recording, now on cost_lock), and lru_reparent_memcg() still needs explicit cost-splice. We can do much better and this series removes the producer lock entirely and gets hierarchy propagation for "free" via rstat. 2. Attempt to switch to using MGLRU's scan model: MGLRU has no anon_cost/file_cost at all. It replaces the cost heuristic with generation-based aging: per-LRU sequence numbers (min_seq/max_seq) age folios into generations, and the older-generation type is the one to scan. So lru_note_cost_unlock_irq() / lru_note_cost_refault() are simply not called when lru_gen_enabled() — by design it sidesteps every concern this patch addresses. But MGLRU is not a substitute for fixing classic LRU: - It relies on a lot of things including per-lruvec generation lists, bloom filters, mm_struct walk infrastructure, working-set protection tiers and a whole sysfs interface. Replacing classic LRU's cost recording with the MGLRU model would mean dragging in all of that. - It changes scan-balance semantics, not just the locking, so it's a heuristic change we would need to evaluate separately. There are known regressions (database/anon-heavy workloads sensitive to swappiness, or file-cache-dominated workloads where MGLRU's bloom-filter protection differs from classic refault tracking). This series keeps classic LRU's scan-balance model and fixes where its cost history is sampled and aged. 3. Atomic cost counter: lrusize/4 halving has no clean atomic form, and the parent walk still has to run explicitly. Reusing vmstats gives per-CPU aggregation AND rstat hierarchy propagation for free. 4. Drop cost_lock from the existing patch and reuse lru_lock in the consumer (prepare_scan_control()): Saves 1 lock space per lruvec but re-couples the cost path to LRU manipulation, though just from the consumer side this time. prepare_scan_control() runs at the start of every shrink_lruvec() cycle, so under sustained memory pressure it would take lru_lock on the hot path and block isolate_lru_folios() / move_folios_to_lru() / folio_add_lru() i.e. when reclaim is in flight. A dedicated cost_lock is never taken by anyone except the consumer cost calculation. [1] https://gist.github.com/uarif1/a4eb33a86c5b2d7bbc55b42f0956e884 v1 -> v2: https://lore.kernel.org/all/20260706122954.3552990-1-usama.arif@linux.dev/ - Sample via the newly introduced lruvec_page_state_monotonic() to fix a 32-bit delta underflow when the underlying signed long wraps past LONG_MAX (Johannes and sashiko) RFC -> v1: https://lore.kernel.org/all/20260626122009.75334-1-usama.arif@linux.dev/ - Document in coverletter and commit message how the read-side vmstat accumulator improves cost-model aging across reclaim gaps (Johannes) - Fully decay the cost_accum below lrusize / 4 using a while loop (sashiko) Usama Arif (2): mm/vmstat, mm/memcontrol: add _monotonic vmstat readers mm/vmscan: reduce lru_lock contention via vmstat-derived scan-balance cost include/linux/memcontrol.h | 8 +++++ include/linux/mmzone.h | 11 ++++-- include/linux/swap.h | 3 -- include/linux/vmstat.h | 16 +++++++++ mm/memcontrol-v1.c | 4 +-- mm/memcontrol.c | 40 +++++++++++++++++++++ mm/mmzone.c | 1 + mm/swap.c | 69 ------------------------------------ mm/vmscan.c | 72 ++++++++++++++++++++++++++++++++------ mm/vmstat.c | 15 ++++++++ mm/workingset.c | 5 --- 11 files changed, 153 insertions(+), 91 deletions(-) -- 2.53.0-Meta