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Thu, 2 Oct 2025 12:51:49 +0000 Message-ID: <10d84ae7-4c33-4f58-80bb-612221d24ec2@intel.com> Date: Thu, 2 Oct 2025 20:51:33 +0800 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH v4 07/28] sched: Add helper function to decide whether to allow cache aware scheduling To: Peter Zijlstra CC: Ingo Molnar , K Prateek Nayak , "Gautham R . Shenoy" , Vincent Guittot , Juri Lelli , "Dietmar Eggemann" , Steven Rostedt , Ben Segall , Mel Gorman , "Valentin Schneider" , Libo Chen , "Madadi Vineeth Reddy" , Hillf Danton , Shrikanth Hegde , Jianyong Wu , Yangyu Chen , Tingyin Duan , Vern Hao , Len Brown , Tim Chen , Aubrey Li , Zhao Liu , Chen Yu , References: <701c7be7f0e69582d9ad0c25025ec2e133e73fbb.1754712565.git.tim.c.chen@linux.intel.com> <20251001131715.GO4067720@noisy.programming.kicks-ass.net> <89c777b7-33bd-400d-8b6f-4e6d697dc632@intel.com> <20251002115034.GS3419281@noisy.programming.kicks-ass.net> Content-Language: en-US From: "Chen, Yu C" In-Reply-To: <20251002115034.GS3419281@noisy.programming.kicks-ass.net> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 8bit X-ClientProxiedBy: TPYP295CA0058.TWNP295.PROD.OUTLOOK.COM (2603:1096:7d0:8::9) 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_|IA4PR11MB9347:EE_ X-MS-Office365-Filtering-Correlation-Id: 535ce9f0-63d7-409d-59ce-08de01b2737e X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|366016|7416014|376014; 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One side effect is that when the preferred LLC is saturated, >>>> more threads will continue to be stacked on it, degrading the workload's >>>> latency. A strategy is needed to prevent this aggregation from going too >>>> far such that the preferred LLC is too overloaded. >>> >>> So one of the ideas was to extend the preferred llc number to a mask. >>> Update the preferred mask with (nr_threads / llc_size) bits, indicating >>> the that many top llc as sorted by occupancy. >>> >>> >> >> Having more than one preferred LLC helps prevent aggregation from going >> too far on a single preferred LLC. >> >> One question would be: if one LLC cannot hold all the threads of a process, >> does a second preferred LLC help in this use case? Currently, this patch >> gives up task aggregation and falls back to legacy load balancing if the >> preferred LLC is overloaded. If we place threads across two preferred LLCs, >> these threads might encounter cross-LLC latency anyway - so we may as well >> let >> legacy load balancing spread them out IMO. > > Well, being stuck on 2 LLCs instead of being spread across 10 still > seems like a win, no? > > Remember, our friends at AMD have *MANY* LLCs. > I see, this makes sense. >> Another issue that Patch 7 tries to address is avoiding task >> bouncing between preferred LLCs and non-preferred LLCs. If we >> introduce a preferred LLC priority list, logic to prevent task >> bouncing between different preferred LLCs might be needed in >> load balancing, which could become complicated. > > It doesn't really become more difficult to tell preferred LLC from > non-preferred LLC with a asm. So why should things get more complicatd? > Besides distinguishing between preferred LLCs and non-preferred LLCs, we might also want to distinguish between the ith preferred LLC and the jth preferred LLC—this would help avoid task migration bouncing between them by using hysteresis. > > Anyway, it was just one of the 'random' ideas I had kicking about. > Reality always ruins things, *shrug* :-) Yes, multiple preferred LLCs is a promising direction if the data shows good results. We are planning to clean up the RFC patch and send a refreshed version to summarize the current status. Meanwhile, we will evaluate the multi-preferred LLC approach internally. Thanks for providing this idea. thanks, Chenyu