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Wed, 18 Feb 2026 15:34:55 +0000 Message-ID: Date: Wed, 18 Feb 2026 23:34:37 +0800 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 14/21] sched/cache: Respect LLC preference in task migration and detach 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: <82aeb78bbfb80cb6861b85e4db9d398f6c8e331b.1770760558.git.tim.c.chen@linux.intel.com> Content-Language: en-US From: "Chen, Yu C" In-Reply-To: Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 8bit X-ClientProxiedBy: KUZPR04CA0013.apcprd04.prod.outlook.com (2603:1096:d10:32::19) 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_|DS4PPF6915D992B:EE_ X-MS-Office365-Filtering-Correlation-Id: e2cbad3f-4b18-494e-f0f0-08de6f0343f8 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|7416014|366016|1800799024|376014; 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>> and only if nr_balanced_failed is high enough do we ignore that. >> (Peter Zijlstra) >> >> Let migrate_degrade_locality() take precedence over >> migrate_degrades_llc(), which aims to migrate towards the preferred >> NUMA node. (Peter Zijlstra) >> >> kernel/sched/fair.c | 64 +++++++++++++++++++++++++++++++++++++++++--- >> kernel/sched/sched.h | 13 +++++++++ >> 2 files changed, 73 insertions(+), 4 deletions(-) >> >> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c >> index 03959a701514..d1145997b88d 100644 >> --- a/kernel/sched/fair.c >> +++ b/kernel/sched/fair.c >> @@ -9973,8 +9973,8 @@ static enum llc_mig can_migrate_llc(int src_cpu, int dst_cpu, >> * Check if task p can migrate from source LLC to >> * destination LLC in terms of cache aware load balance. >> */ >> -static __maybe_unused enum llc_mig can_migrate_llc_task(int src_cpu, int dst_cpu, >> - struct task_struct *p) >> +static enum llc_mig can_migrate_llc_task(int src_cpu, int dst_cpu, >> + struct task_struct *p) >> { >> struct mm_struct *mm; >> bool to_pref; >> @@ -10041,6 +10041,47 @@ alb_break_llc(struct lb_env *env) >> >> return false; >> } >> + >> +/* >> + * Check if migrating task p from env->src_cpu to >> + * env->dst_cpu breaks LLC localiy. >> + */ >> +static bool migrate_degrades_llc(struct task_struct *p, struct lb_env *env) >> +{ >> + if (!sched_cache_enabled()) >> + return false; >> + >> + if (task_has_sched_core(p)) >> + return false; >> + /* >> + * Skip over tasks that would degrade LLC locality; >> + * only when nr_balanced_failed is sufficiently high do we >> + * ignore this constraint. >> + * >> + * Threshold of cache_nice_tries is set to 1 higher >> + * than nr_balance_failed to avoid excessive task >> + * migration at the same time. Refer to comments around >> + * llc_balance(). >> + */ >> + if (env->sd->nr_balance_failed >= env->sd->cache_nice_tries + 1) >> + return false; >> + >> + /* >> + * We know the env->src_cpu has some tasks prefer to >> + * run on env->dst_cpu, skip the tasks do not prefer >> + * env->dst_cpu, and find the one that prefers. >> + */ >> + if (env->migration_type == migrate_llc_task && >> + task_llc(p) != llc_id(env->dst_cpu)) >> + return true; > > `task_llc(p)` returns the LLC id of the CPU the task is currently running on, right? > Wouldn’t we need to check the task’s *preferred* LLC instead? > > Am I missing something? > No, you did not miss anything; this is indeed a bug. I realized this during our discussion yesterday: https://lore.kernel.org/all/22e975fd-4e31-498d-a016-2168721f532a@intel.com/ will fix it accordingly. Thanks! thanks, Chenyu