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Howlett" , "Mike Rapoport" , , To: "Johannes Weiner" , "Andrew Morton" , "Vlastimil Babka" From: "Zi Yan" Subject: Re: [PATCH 4/4] mm: page_alloc: fix non-movable reclaim storm in defrag_mode X-Mailer: aerc 0.21.0 References: <20260626182215.1107966-1-hannes@cmpxchg.org> <20260626182215.1107966-5-hannes@cmpxchg.org> In-Reply-To: <20260626182215.1107966-5-hannes@cmpxchg.org> X-ClientProxiedBy: YQBPR0101CA0132.CANPRD01.PROD.OUTLOOK.COM (2603:10b6:c01:5::35) To IA0PR12MB8374.namprd12.prod.outlook.com (2603:10b6:208:40e::7) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: IA0PR12MB8374:EE_|PH8PR12MB7373:EE_ X-MS-Office365-Filtering-Correlation-Id: 34e5a745-9e2f-4739-2ae2-08ded3b0d9de X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|7416014|376014|23010399003|1800799024|366016|18002099003|22082099003|11063799006|4143699003|56012099006|6133799003; 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Tracing confirmed > that this is unmovable/reclaimable requests spinning in the allocator > and direct reclaim, causing excessive amounts of swap. > > The initial plan for defrag_mode was to rely on kswapd/kcompactd to > produce blocks, and if those are overwhelmed under high pressure, let > the allocator fall back (__rmqueue_steal()) after its retry loops. > However, that retrying results in more reclaim on some of these > workloads than we'd hoped, sometimes excessively so, spurred on by the > !costly order conditions in should_reclaim_retry(). > > The storms are dependent on the request type. Reclaim will inevitably > make room in existing movable blocks, since that's where the LRU pages > live. So if movable requests retry on reclaim, they make progress. > > When non-movable requests spin in reclaim that isn't productive. They > cannot use the individually freed pages, and the process is unlikely > to accidentally free whole blocks to meet the ALLOC_NOFRAGMENT bar. > They spin and overreclaim excessively, which tanks performance and > triggers userspace guards like swap exhaustion or pressure based OOM. > > To fix this, send non-movable requests, regardless of order, into > pageblock reclaim/compaction. This way, they help move things along to > meet the ALLOC_NOFRAGMENT bar. After this patch, the reclaim storms > and excess OOM rates are no longer observed in production. > > The longer-term plan is still to have all requests, including the > movable ones, help make blocks to spread the cost of defragmenting > more evenly and fairly; combined with proper watermarking to reduce > allocation latencies in the common case. However, doing this naively > unearths scaling and concurrency limitations in compaction that need > to be addressed first. Promoting just non-movables for now is the > minimally viable bug fix for the above issue. > > Fixes: f38356df6474 ("mm: page_alloc: introduce defrag_mode") Should be Fixes: e3aa7df331bc ("mm: page_alloc: defrag_mode"). Since I cannot find f38356df6474 in the tree. > Signed-off-by: Johannes Weiner --=20 Best Regards, Yan, Zi