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Thu, 19 Feb 2026 15:07:53 +0000 Message-ID: <66a23acb-973e-4987-a006-18c50e18d3aa@intel.com> Date: Thu, 19 Feb 2026 23:07:36 +0800 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 00/21] Cache Aware Scheduling To: Peter Zijlstra , Qais Yousef CC: Tim Chen , 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 , Libo Chen , References: <20260219140828.a7pyzupun7lsdw34@airbuntu> <20260219144108.GI1282955@noisy.programming.kicks-ass.net> Content-Language: en-US From: "Chen, Yu C" In-Reply-To: <20260219144108.GI1282955@noisy.programming.kicks-ass.net> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: SGBP274CA0020.SGPP274.PROD.OUTLOOK.COM (2603:1096:4:b0::32) 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_|PH8PR11MB6999:EE_ X-MS-Office365-Filtering-Correlation-Id: e6a6f2c8-a7d4-43c3-04a6-08de6fc8a799 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|376014|7416014|366016; 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But I must ask, why are you deferring >> to load balance and not looking at wake up path? LB should be for corrections. >> When wake up path is doing wrong decision all the time, LB (which is super slow >> to react) is too late to start grouping tasks? What am I missing? > > There used to be wakeup steering, but I'm not sure that still exists in > this version (still need to read beyond the first few patches). It isn't > hard to add. > Please let me explain a little more about why we did this in the load balance path. Yes, the original version implemented cache-aware scheduling only in the wakeup path. According to our testing, this appeared to cause some task bouncing issues across LLCs. This was due to conflicts with the legacy load balancer, which tries to spread tasks to different LLCs. So as Peter said, the load balancer should be taken care of anyway. Later, we kept only the cache aware logic in the load balancer, and the test results became much more stable, so we kept it as is. The wakeup path more or less aggregates the wakees(threads within the same process) within the LLC in the wakeup fast path, so we have not changed it for now. Let me copy the changelog from the previous patch version: " In previous versions, aggregation of tasks were done in the wake up path, without making load balancing paths aware of LLC (Last-Level-Cache) preference. This led to the following problems: 1) Aggregation of tasks during wake up led to load imbalance between LLCs 2) Load balancing tried to even out the load between LLCs 3) Wake up tasks aggregation happened at a faster rate and load balancing moved tasks in opposite directions, leading to continuous and excessive task migrations and regressions in benchmarks like schbench. In this version, load balancing is made cache-aware. The main idea of cache-aware load balancing consists of two parts: 1) Identify tasks that prefer to run on their hottest LLC and move them there. 2) Prevent generic load balancing from moving a task out of its hottest LLC. " thanks, Chenyu