From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 7520F481241; Tue, 4 Aug 2026 17:17:14 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785863845; cv=none; b=EC15MW1hOKN+hXWpSeL5mduavQJbNQO3pMniwqvdp02mmq1qApW2Pu4B7UZRQjFMDfjqqtcGeKGLEOymFIVwAeyi60z2668XDN7M0OB1ZE1zhPTpCnujn3xTK7k8qABMhDo1WAWr5vlIF9Hf9i3LET0e4i+6pi+QDysNiR9a5nI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785863845; c=relaxed/simple; bh=c2Jx0jK9GHxlZxEEFjwuvHIuqw/Osa5R7BEFv8oO/78=; h=Message-ID:Date:MIME-Version:Subject:To:References:From: In-Reply-To:Content-Type; b=L81ZoEk8UI5R9zW8MZtSd4VAI71tuSSWRzduKg6W7/QjQ76kjv8K/6HOvma5gllaw/RfovExcjAc3B0HKZJiHrfak7yttrRpvO+DYQ9H+3nFSA7ARcreBeS/aHZu8S4mxfDur9x91xcn7gDwYCVnd9rG11N8lcx89McJCVrI6CE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=AwBfps2x; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="AwBfps2x" Received: by smtp.kernel.org (Postfix) with ESMTPSA id B0D601F000E9; Tue, 4 Aug 2026 17:17:06 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1785863831; bh=J19P9P6+ypat/OvYRKtICP+Obahh1CzQhSviP26PwVM=; h=Date:Subject:To:References:From:In-Reply-To; b=AwBfps2xwIykpE1Z2NV6S4OZDKZOxIC4vaRgBzCHqVlRGKyU2UJHFFDDTNZ7o2sza frhAZK+MTE0MsyGoY1llOM3iJWO527/XtMoo0HE0HkjGpAiBtpNaex5sz8AAg0kkeT SbepXc+0fw4p/vKTGYKwhHKayur0/FR+SO5skpa9HMNomgQUakdNWDpId3HaZyRc0e ACHKKHrxYd5+IXQRT8DYBLlgIUgydvZ4byJHJEkTPlTCXtjqCpl3tRXxeHyja3PpQY dXqCxEFlHQvdPQPRRw66sj6V6Tqw+za0zoN1znkD+myqYRSnKpSs/JsHPSYzTO4vHG OR5N0J8xIV2ow== Message-ID: Date: Tue, 4 Aug 2026 19:17:04 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v5 3/3] mm/vmscan: reduce lru_lock contention via vmstat-derived scan-balance cost Content-Language: en-US To: Usama Arif , Andrew Morton , david@kernel.org, ljs@kernel.org, liam@infradead.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 References: <20260727162550.2032-1-usama.arif@linux.dev> <20260727162550.2032-4-usama.arif@linux.dev> From: "Vlastimil Babka (SUSE)" Autocrypt: addr=vbabka@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 7/27/26 18:23, Usama Arif wrote: > 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. > > Acked-by: Shakeel Butt > Acked-by: Johannes Weiner > Signed-off-by: Usama Arif Acked-by: Vlastimil Babka (SUSE)