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Mon, 16 Feb 2026 06:58:27 +0000 Message-ID: <22e975fd-4e31-498d-a016-2168721f532a@intel.com> Date: Mon, 16 Feb 2026 14:58:10 +0800 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 05/21] sched/cache: Assign preferred LLC ID to processes To: Madadi Vineeth Reddy 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 , 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 References: <4a92b93edb669845e3bdca24c3ae3354b317c3eb.1770760558.git.tim.c.chen@linux.intel.com> <40508142-4a0c-4c00-839e-5807352071ee@linux.ibm.com> Content-Language: en-US From: "Chen, Yu C" In-Reply-To: <40508142-4a0c-4c00-839e-5807352071ee@linux.ibm.com> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: SI2PR02CA0032.apcprd02.prod.outlook.com (2603:1096:4:195::12) 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_|MW3PR11MB4666:EE_ X-MS-Office365-Filtering-Correlation-Id: 661c9992-baab-415d-590b-08de6d28c904 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|7416014|376014|366016; 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>> } >> + >> + mm_sched_llc = get_pref_llc(p, mm); >> + >> + if (p->preferred_llc != mm_sched_llc) >> + p->preferred_llc = mm_sched_llc; > > This writes to p->preferred_llc without using WRITE_ONCE(). If later patches read p->preferred_llc from > load balancing or migration paths on other CPUs, wouldn't this create a data race? > I suppose you are referring to data inconsistency between CPUs, as READ/WRITE_ONCE() make sure that the latest data is always read from/written to memory rather than from/to registers. > For example: > CPU 0: Task is running, account_mm_sched() writes p->preferred_llc > CPU 1: Load balancer reads p->preferred_llc to make migration decisions > > Should this use WRITE_ONCE() and READ_ONCE() at the read sites, unless all accesses are guaranteed to be > under rq->lock? > Actually, I found that p->preferred_llc is only read during task enqueue/dequeue in account_llc_enqueue() and account_llc_dequeue(), which are protected by the rq lock. However, after reviewing the code again, I noticed that in migrate_degrades_llc() (part of the load balance logic), we should check the task's preferred_llc instead of the task's current LLC obtained via task_llc(p). We will therefore switch to using READ/WRITE_ONCE for accesses to this variable in migrate_degrades_llc(). thanks, Chenyu