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Fri, 20 Feb 2026 15:22:43 +0000 Message-ID: Date: Fri, 20 Feb 2026 23:22:29 +0800 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 04/21] sched/cache: Make LLC id continuous To: Peter Zijlstra CC: 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 , , Tim Chen , "Gautham R . Shenoy" , Vincent Guittot , Ingo Molnar , K Prateek Nayak References: <60a05a3f50d14a7bf3b968f62cca87893c5c552c.1770760558.git.tim.c.chen@linux.intel.com> <79755f7d-cc68-4189-b6d8-850378e54017@amd.com> <95923e36-4117-4209-97aa-a92b60f2bd49@intel.com> <6a73ac0d-1fc9-49a7-87a2-65951ab2c844@amd.com> <20260219154829.GJ1395266@noisy.programming.kicks-ass.net> Content-Language: en-US From: "Chen, Yu C" In-Reply-To: <20260219154829.GJ1395266@noisy.programming.kicks-ass.net> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: TP0P295CA0045.TWNP295.PROD.OUTLOOK.COM (2603:1096:910:4::16) 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_|CY5PR11MB6092:EE_ X-MS-Office365-Filtering-Correlation-Id: 27b69e6b-d67c-489f-55ae-08de7093e4b0 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|7416014|376014|366016|1800799024; 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[ SMT: 14,15 ] >> >> and the following series of events: >> >> o All CPUs in LLC are offline to begin with (maxcpus = 1 like scenario). >> >> o CPUs 10-15 are onlined first. >> >> o CPU8 is put in a separate root partition and brought online. >> (XXX: I'm not 100% sure if this is possible in this order) >> >> o build_sched_domains() will bail out at SMT domain since the cpumap >> is covered by tl->mask() and tl_llc = tl_smt. >> >> o llc_id calculation uses the tl_smt->mask() which will not contain >> CPUs 10-15 and CPU8 will get a unique LLC id even though there are >> other online CPUs in the LLC with a different llc_id (!!!) > > Yeah, so partitions (including isol_cpus) could wreck things here, since > this is purely about the sched_domains. > > You can create N single CPU partitions (isol_cpus does this) and end up > with the same 'problem' that online one at a time loop did. Except this > time it would not be 'wrong'. Since they are single CPU domains, you > also don't get load-balancing, so who cares I suppose. But it will > inflate max_lid. > > But suppose you create N/2 partitions (where N is the number of CPUs in > the physical LLC), then you get many individual 'LLC's and > load-balancing inside them. I suppose this is correct, although it does > inflate max_lid somewhat beyond what you would normally expect. > > However, most of that space would be wasted, since you're not actually > allowed to migrate to them. > Besides wasting space, after removing CPUs from all N/2 partitions and merging them into the root partition, each CPU would still have a distinct llc_id from the other CPUs in the same LLC domain, because we do not reassign llc_id values to CPUs in current version. This issue should be resolved by switching to the new dynamic llc_id allocation and release method. thanks, Chenyu