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Tue, 11 Aug 2026 10:23:14 +0000 Message-ID: <14f72439-d8f4-47aa-b3ce-6b8485ae46ea@intel.com> Date: Tue, 11 Aug 2026 18:23:05 +0800 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] sched/cache: honor migrate_llc_task semantics in active load balance To: Lu Wang CC: , , , , , , , , , , , , References: <20260801122252.2476258-1-wanglu.priv@gmail.com> <20260809105343.1189051-1-wanglu.priv@gmail.com> Content-Language: en-US From: "Chen, Yu C" In-Reply-To: <20260809105343.1189051-1-wanglu.priv@gmail.com> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: TP0P295CA0058.TWNP295.PROD.OUTLOOK.COM (2603:1096:910:3::17) 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_|SA0PR11MB4544:EE_ X-MS-Office365-Filtering-Correlation-Id: 12a8fc39-5c9c-41f7-c59f-08def7928d96 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|376014|7416014|366016|23010399003|1800799024|6133799003|11063799006|4143699003|56012099006|10067099003|18002099003|22082099003; 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The CPU stopper > callback constructs a fresh lb_env, so select the stopper callback when > queueing active balance to preserve the migration semantics across the > asynchronous boundary. > Worthy adding a [Problem Statement] section to describe what issue this patch tries to fix (with polish): src_rq has two runnable tasks, p1 and p2. p1 prefers dst_rq (dst_llc), while p2 prefers src_rq (src_llc). In this case, migrate_llc_task is set because src_rq has at least one task, p1, that wants to migrate to dst_rq. In ALB, can_migrate_task() finds p2 and returns true for it, thus moving p2 out of its preferred LLC. > 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. > > Fixes: e4c9a4cb244a (\"sched/cache: Add migrate_llc_task migration type for cache-aware balancing\") > Suggested-by: \"Chen, Yu C\" > Signed-off-by: Lu Wang > --- > Changes in v2: > - Select the stopper callback at kick time to preserve > migrate_llc_task semantics in active load balance without passing > migration_type across the stopper, which affects delayed-dequeue tasks. Maybe move the above reasoning about why the new stopper function is needed into the commit log for future reference. > @@ -10645,6 +10646,20 @@ alb_break_llc(struct lb_env *env) > return false; > } > > +/* > + * Returns true if p's preferred LLC does not match the destination CPU > + * under migrate_llc_task semantics. Passive LB passes migrate_llc_task > + * in migration_type, while active LB carries it in LBF_ACTIVE_LB_LLC. "while active LB carries LBF_ACTIVE_LB_LLC in env->flags to avoid overwriting env->migrate_type." Per my understanding, Reviewed-by: Chen Yu thanks, Chenyu