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From: Shakeel Butt To: Usama Arif Cc: 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, 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 Subject: Re: [PATCH v5 3/3] mm/vmscan: reduce lru_lock contention via vmstat-derived scan-balance cost Message-ID: References: <20260727162550.2032-1-usama.arif@linux.dev> <20260727162550.2032-4-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=us-ascii Content-Disposition: inline In-Reply-To: <20260727162550.2032-4-usama.arif@linux.dev> X-Migadu-Flow: FLOW_OUT On Mon, Jul 27, 2026 at 09:23:25AM -0700, 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 I see you already added my Ack here. Sounds good.