From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-177.mta1.migadu.com (out-177.mta1.migadu.com [95.215.58.177]) (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 D151F2459E5 for ; Fri, 26 Jun 2026 12:20:27 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.177 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782476429; cv=none; b=h3u1nb1Mcp1EwVOjy5DMFBHnfrBO6tDiyByIie1Qkb2V0M7MSk6fGB1hykoqYLszXTo0XjJZ/zTGa9O5XTSgGzjzZK9D7HarIUtZQTgWuCW+P9Ey9wBLu4zEWvs7GxirGhuhu2zT9AQDJl9RgW23YC2p5iZlIdc507VRDtPWwvg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782476429; c=relaxed/simple; bh=liHUnt6/fGFEFl8xrFW6e+gj0j+4SEcKN+8BgIc25Iw=; h=From:To:Cc:Subject:Date:Message-ID:MIME-Version:Content-Type; b=EfTzjJ+yEsdGB/0533QacVXBsJ8dxqoNhJSPK0Bfdz3QtEJWjnxUW1tsAXGtscddZEYdMBPimezAjZcefxCgParWTUIjkBDDSJXp+jY0n/1ooLNQDR92um3NySW8PhUKRcqxHanUGBJIRiPae7ed+dPIVV1eHVnmLl008YNrc9A= 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=JtuLUD3h; arc=none smtp.client-ip=95.215.58.177 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="JtuLUD3h" 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=1782476415; 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=nKnqyFpkGqJ5qqN1vo0VqOJxDpoK/HcDMxENgN5Bb08=; b=JtuLUD3hLJaijfheOV7umvsIyWwvW98MolEhw1ZGvfSBbwIKF+bG8XdOtypvHIOlRlDnFZ tBUtI2/7O5QuO+wOKwCoIn5Oqb3EQZQuwc/Fm5GMVOlRfwR/QWCZmtpId2qbQ4VTCoa9ef d2o63X8gbL+y6nj8lkOPnl2DiGRD/mU= 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: [RFC 0/1] mm/vmscan: reduce lru_lock contention via vmstat-derived scan-balance cost Date: Fri, 26 Jun 2026 05:19:46 -0700 Message-ID: <20260626122009.75334-1-usama.arif@linux.dev> Precedence: bulk X-Mailing-List: cgroups@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 patch 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. Trade-offs: - Signal freshness is slightly worse: cost reads see rstat- aggregated values that can lag until periodic / reader-triggered flushing. Decay timing is also coarser since multiple producer events may be batched into one read-side halving check. The cost signal is a heuristic feeding the anon-vs-file split, it's not a precise control loop — it's deliberately smoothed by the lrusize/4 halving. Producing/consuming it with a tiny lag should not be perceptible. - Per-lruvec footprint grows by 2 unsigned longs + a spinlock, its 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]. == 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 patch preserves classic-LRU semantics. 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 calucation. [1] https://gist.github.com/uarif1/a4eb33a86c5b2d7bbc55b42f0956e884 Usama Arif (1): mm/vmscan: reduce lru_lock contention via vmstat-derived scan-balance cost include/linux/mmzone.h | 11 +++++-- include/linux/swap.h | 3 -- mm/memcontrol-v1.c | 4 +-- mm/memcontrol.c | 4 +++ mm/mmzone.c | 1 + mm/swap.c | 69 ------------------------------------------ mm/vmscan.c | 64 +++++++++++++++++++++++++++++++++------ mm/vmstat.c | 4 +++ mm/workingset.c | 5 --- 9 files changed, 74 insertions(+), 91 deletions(-) -- 2.53.0-Meta