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Fri, 10 Apr 2026 08:59:41 +0000 Message-ID: Date: Fri, 10 Apr 2026 16:59:19 +0800 User-Agent: Mozilla Thunderbird Subject: Re: [Patch v4 17/22] sched/cache: Avoid cache-aware scheduling for memory-heavy processes To: Peter Zijlstra , Tim Chen CC: Ingo Molnar , K Prateek Nayak , "Gautham R . Shenoy" , Vincent Guittot , Juri Lelli , "Dietmar Eggemann" , Steven Rostedt , Ben Segall , Mel Gorman , "Valentin Schneider" , Madadi Vineeth Reddy , Hillf Danton , Shrikanth Hegde , Jianyong Wu , Yangyu Chen , Tingyin Duan , Vern Hao , Vern Hao , Len Brown , Aubrey Li , Zhao Liu , Chen Yu , Adam Li , Aaron Lu , Tim Chen , Josh Don , Gavin Guo , Qais Yousef , Libo Chen , , "Luck, Tony" , Reinette Chatre References: <339bb2636c7306e17540268a9295a8e673b92804.1775065312.git.tim.c.chen@linux.intel.com> <20260409124642.GC3126523@noisy.programming.kicks-ass.net> Content-Language: en-US From: "Chen, Yu C" In-Reply-To: <20260409124642.GC3126523@noisy.programming.kicks-ass.net> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: KU2P306CA0023.MYSP306.PROD.OUTLOOK.COM (2603:1096:d10:3b::15) 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_|CH3PR11MB7841:EE_ X-MS-Office365-Filtering-Correlation-Id: 006bc559-723f-4b32-8d88-08de96df8051 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|7416014|376014|366016|1800799024|56012099003|18002099003|22082099003; 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If RSS >> exceeds the LLC size, skip cache-aware scheduling. >> >> Note that RSS is only an approximation of the memory footprint. >> By default, the comparison is strict, but a later patch will allow >> users to provide a hint to adjust this threshold. >> >> According to the test from Adam, some systems do not have shared L3 >> but with shared L2 as clusters. In this case, the L2 becomes the LLC[1]. > > This is pretty terrible. If you want LLC size, add it to the topology > information (and ideally integrate with RDT) and make proportional to > cpumask size, such that if someone cuts the domain in pieces, they get > proportional size etc. > If I understand correctly, do you mean the following: 1.Introduce a generic arch_get_llc_size() as a wrapper around the existing get_cpu_cacheinfo_level(), which returns the llc_size. Both the scheduler and RDT can use arch_get_llc_size(). 2. The sched domain stores llc_size in sd->res_size = llc_size * sd_span / arch_llc_span, and the cache_aware_scheduler uses sd->res_size for the comparison. We will adjust the code accordingly. > Also, if we have NUMA_BALANCING on, that can provide a much better > estimate for the actual size. > > Just using RSS seems like a very bad metric here. > Got it. Currently we lack accurate memory footprint metrics in the kernel. If we support user-provided hints in the future, we can leverage RDT llc_occupancy metrics(Is it legal to use RDT's metrics directly in the kernel? It would switch from MSR-read to MMIO read thus less overhead). For now, let me have a try how to leverage NUMA fault-in stats. If NUMA balancing is off, I need to think more on how to avoid over-aggregation for memory-intensive workloads. thanks, Chenyu