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Wed, 21 Oct 2026 16:39:48 +0000 Date: Thu, 22 Oct 2026 00:26:50 +0800 From: Chen Yu To: Luo Gengkun CC: , , , , , , , , , , , , Subject: Re: [PATCH v10 1/2] sched/cache: Reduce the overhead of task_cache_work by only scan the visisted cpus Message-ID: References: <20261009032943.95091-1-luogengkun@huaweicloud.com> <20261009032943.95091-2-luogengkun@huaweicloud.com> Content-Type: text/plain; charset="us-ascii" Content-Disposition: inline In-Reply-To: <20261009032943.95091-2-luogengkun@huaweicloud.com> X-ClientProxiedBy: SI2PR01CA0010.apcprd01.prod.exchangelabs.com (2603:1096:4:191::12) To DM4PR11MB6020.namprd11.prod.outlook.com (2603:10b6:8:61::19) X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DM4PR11MB6020:EE_|MN6PR11MB8101:EE_ X-MS-Office365-Filtering-Correlation-Id: eae01c31-3b1a-4e0f-ad4f-08df2f91eb8d X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|366016|376014|1800799024|7416014|23010399003|18002099003|22082099003|10067099003|4143699003|11063799006|261009223027099003|56012099006; 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However, most of these scans are meaningless, such as those > for CPUs that have never been visited or were accessed a long time ago. > > To address this problem, introduce visited_cpus to track the visited CPUs > and evict them once they have not been accessed for a duration exceeding > llc_epoch_affinity_timeout. > > Now that we know exactly which CPUs to scan from visited_cpus, we can remove > get_scan_cpumasks(). > > Signed-off-by: Luo Gengkun > Thanks Prateek for popping this patch up, and thanks Gengkun for the continuous optimization on this. I asked AI to launch a full hackbench test on a Xeon with 4 LLCs per node, especially to test the case where the LLC is saturated, under which scenario the contention for the visit_cpu mask is high. The test shows no regression. So from a code design point of view, this change is a good optimization to reduce the potential overhead, and also paves the way for the multiple preferred LLC/node work that Jianyong is doing IMO, Reviewed-by: Chen Yu Tested-by: Chen Yu thanks, Chenyu ========================================= Hackbench comparison BASE: cas_on_base TEST: cas_on_visit ========================================= MODE G FD | BASE(s) | TEST(s) | DIFF% | RESULT ------- -- ----+-----------------+-----------------+---------+---------- process 1 4 | 0.329/16.1% | 0.302/27.8% | 8.21% | IMPROVED threads 2 16 | 222.206/2.3% | 215.147/0.9% | 3.18% | IMPROVED threads 2 20 | 26.031/14.3% | 24.097/7.4% | 7.43% | IMPROVED threads 4 8 | 4.338/26.6% | 3.965/31.5% | 8.60% | IMPROVED