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Thu, 23 Oct 2025 06:56:03 +0000 Message-ID: Date: Thu, 23 Oct 2025 14:55:51 +0800 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 17/19] sched/fair: Disable cache aware scheduling for processes with high thread counts To: Madadi Vineeth Reddy , Tim Chen CC: Peter Zijlstra , Ingo Molnar , "K Prateek Nayak" , "Gautham R . Shenoy" , Vincent Guittot , "Juri Lelli" , Dietmar Eggemann , Steven Rostedt , Ben Segall , "Mel Gorman" , Valentin Schneider , "Hillf Danton" , Shrikanth Hegde , "Jianyong Wu" , Yangyu Chen , Tingyin Duan , Vern Hao , Len Brown , Aubrey Li , Zhao Liu , Chen Yu , Adam Li , Tim Chen , , References: <1a63ee2a-9c1e-4aa3-adb0-012e0eae5dcf@linux.ibm.com> Content-Language: en-US From: "Chen, Yu C" In-Reply-To: <1a63ee2a-9c1e-4aa3-adb0-012e0eae5dcf@linux.ibm.com> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: PS2PR01CA0011.apcprd01.prod.exchangelabs.com (2603:1096:300:2d::23) To DM4PR11MB6020.namprd11.prod.outlook.com (2603:10b6:8:61::19) 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: DM4PR11MB6020:EE_|MN6PR11MB8146:EE_ X-MS-Office365-Filtering-Correlation-Id: cf474431-01df-4d4e-e038-08de12013b3e X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|376014|366016|7416014|1800799024; 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>> } >> >> +static bool exceed_llc_nr(struct mm_struct *mm, int cpu) >> +{ >> + int smt_nr = 1; >> + >> +#ifdef CONFIG_SCHED_SMT >> + if (sched_smt_active()) >> + smt_nr = cpumask_weight(cpu_smt_mask(cpu)); >> +#endif >> + >> + return ((mm->nr_running_avg * smt_nr) > per_cpu(sd_llc_size, cpu)); > > In Power10 and Power11 that has SMT8 and LLC size of 4, this would disable > cache aware scheduling even for one thread. > Using smt_nr was mainly due to concerns about introducing regressions on Power, as discussed in v3 https://lore.kernel.org/all/8f6c7c69-b6b3-4c82-8db3-96757f09245f@linux.ibm.com/ and https://lore.kernel.org/all/ddb9d558-d114-41db-9d4b-296fc2ecdbb4@linux.ibm.com/ It seems that aggregating tasks on an LLC with many SMT threads/smaller LLC size would pose a risk of cache contention. Additionally, with patch [19/19], users can tune /sys/kernel/debug/sched/llc_aggr_tolerance to adjust the threshold: return ((mm->nr_running_avg * smt_nr) > (scale * per_cpu(sd_llc_size, cpu))); > Also, llc_overload_pct already ensures the load on the preferred LLC doesn't > exceed certain capacity. Why is this exceed_llc_nr() check needed? Won't the > existing overload_pct naturally prevent excessive task aggregation by blocking > migrations when the destination LLC reaches ~50% utilization? > Using exceed_llc_nr() was because some short-duration tasks could generate low utilization but still cause cache contention (for some reason, the util_avg cannot track that properly), such as schbench. Therefore, we inhibit task aggregation for a large number of active threads. thanks, Chenyu