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Fri, 20 Feb 2026 17:26:00 +0000 Message-ID: <3d61f4fb-d190-41e5-bda7-18504f94e0e5@intel.com> Date: Sat, 21 Feb 2026 01:25:43 +0800 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 08/21] sched/cache: Calculate the percpu sd task LLC preference 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 , References: <41f8e91b70060e7697840163b80c3dc097aabb34.1770760558.git.tim.c.chen@linux.intel.com> <20260220110222.GP1395266@noisy.programming.kicks-ass.net> <20260220140241.GT1395416@noisy.programming.kicks-ass.net> Content-Language: en-US From: "Chen, Yu C" In-Reply-To: <20260220140241.GT1395416@noisy.programming.kicks-ass.net> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: KU0P306CA0078.MYSP306.PROD.OUTLOOK.COM (2603:1096:d10:2b::10) 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_|CH3PR11MB8137:EE_ X-MS-Office365-Filtering-Correlation-Id: 984f716a-2e72-4691-ed1a-08de70a51d9b 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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>>> int pref_llc; >>> >>> pref_llc = p->preferred_llc; >>> - if (pref_llc < 0) >>> + if (!valid_llc_id(pref_llc)) >>> return; >>> >>> rq->nr_llc_running++; >>> rq->nr_pref_llc_running += (pref_llc == task_llc(p)); >>> + >>> + scoped_guard (rcu) { >>> + sd = rcu_dereference(rq->sd); >>> + if (valid_llc_buf(sd, pref_llc)) >>> + sd->pf[pref_llc]++; >>> + } >>> } >>> >>> static void account_llc_dequeue(struct rq *rq, struct task_struct *p) >>> { >>> + struct sched_domain *sd; >>> int pref_llc; >>> >>> pref_llc = p->preferred_llc; >>> - if (pref_llc < 0) >>> + if (!valid_llc_id(pref_llc)) >>> return; >>> >>> rq->nr_llc_running--; >>> rq->nr_pref_llc_running -= (pref_llc == task_llc(p)); >>> + >>> + scoped_guard (rcu) { >>> + sd = rcu_dereference(rq->sd); >>> + if (valid_llc_buf(sd, pref_llc)) { >>> + /* >>> + * There is a race condition between dequeue >>> + * and CPU hotplug. After a task has been enqueued >>> + * on CPUx, a CPU hotplug event occurs, and all online >>> + * CPUs (including CPUx) rebuild their sched_domains >>> + * and reset statistics to zero (including sd->pf). >>> + * This can cause temporary undercount and we have to >>> + * check for such underflow in sd->pf. >>> + * >>> + * This undercount is temporary and accurate accounting >>> + * will resume once the rq has a chance to be idle. >>> + */ >>> + if (sd->pf[pref_llc]) >>> + sd->pf[pref_llc]--; >>> + } >>> + } >>> } >> >> FWIW, enqueue/dequeue must be with rq->lock held, and thus preemption >> disabled and IRQs off. That RCU section is completely pointless. > > That is, use rcu_dereference_all() and observe the warning go away. OK we will remove rcu_read_lock() and use rcu_dereference_all() directly. thanks, Chenyu