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Mon, 3 Aug 2026 04:20:13 +0000 Message-ID: Date: Mon, 3 Aug 2026 12:20:03 +0800 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] sched/cache: honor migrate_llc_task semantics in active load balance To: Lu Wang CC: , , , , , , , , , , , , "chen.yu@linux.dev" References: <20260801122252.2476258-1-wanglu.priv@gmail.com> Content-Language: en-US From: "Chen, Yu C" In-Reply-To: <20260801122252.2476258-1-wanglu.priv@gmail.com> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: SI2PR04CA0013.apcprd04.prod.outlook.com (2603:1096:4:197::6) 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_|DS4PR11MB084795:EE_ X-MS-Office365-Filtering-Correlation-Id: 7df1f8cd-510d-42b1-8e55-08def1168351 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|376014|7416014|23010399003|366016|1800799024|56012099006|6133799003|10067099003|11063799006|22082099003|18002099003; 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The CPU stopper > callback constructs a fresh lb_env, so preserve the migration type on > the runqueue across the asynchronous boundary. > > For CAS-directed active balance, reject a candidate whose preferred LLC > does not match the destination LLC. This keeps the fallback from moving > a task away from its preferred LLC. > [ ... ] > static inline bool > migrate_degrades_llc(struct task_struct *p, struct lb_env *env) > { > @@ -10796,7 +10813,7 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env) > * 4) too many balance attempts have failed. > */ > if (env->flags & LBF_ACTIVE_LB) > - return 1; > + return !migrate_llc_task_wrong_dst(p, env); > When checking whether active load balance is needed in need_active_balance(), alb_break_llc() is called to determine if the balance would break LLC locality (i.e., prevent a task from being migrated away from its preferred LLC). Later in need_active_balance(), if LLC locality is not broken and the task is of migrate_llc_task type, we force an active load balance. This means that once active load balance reaches can_migrate_task(), we want it to always return true - this helps the case where the task is the only running one on the src_cpu. Given this logic, I do not see the need for the duplicated LLC-locality check in can_migrate_task() for active load balance. Am I missing anything here? thanks, Chenyu