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Wed, 2 Sep 2026 05:39:41 +0000 Date: Wed, 2 Sep 2026 13:26:10 +0800 From: Chen Yu To: Peter Zijlstra CC: , , , , , , , , , , , , Subject: Re: [PATCH 4/4] sched/fair: Rework/fix task_h_load() Message-ID: References: <20260828074059.232353141@infradead.org> <20260828075558.660152190@infradead.org> Content-Type: text/plain; charset="us-ascii" Content-Disposition: inline In-Reply-To: <20260828075558.660152190@infradead.org> X-ClientProxiedBy: SI2P153CA0026.APCP153.PROD.OUTLOOK.COM (2603:1096:4:190::18) 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_|CHAPR11MB9632:EE_ X-MS-Office365-Filtering-Correlation-Id: 17de1995-353b-4f05-719c-08df08b495fa X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|7416014|376014|23010399003|366016|6133799003|10067099003|4143699003|56012099006|11063799006|22082099003|18002099003; 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> } > > + for_each_sched_entity_bl(se, cfs_rq) > + update_cfs_rq_h_load(group_cfs_rq(se), se, cfs_rq); > + It panics during bootup on a 192 Cores system, [ 9.445362][ T1687] Oops: general protection fault, kernel NULL pointer dereference 0x69: 0000 [#1] SMP NOPTI [ 18.987940][ T1687] CPU: 58 UID: 0 PID: 1687 Comm: systemd-udevd Not tainted 7.3.0-rc1-flat-hload+ #9 PREEMPTLAZY [ 19.022374][ T1687] RIP: 0010:pick_task_fair+0x43/0xd0 [ 19.175316][ T1687] [ 19.181239][ T1687] __pick_next_task+0x49/0x1b0 [ 19.189229][ T1687] __schedule+0x14b/0x6b0 [ 19.196711][ T1687] preempt_schedule+0x3a/0x60 [ 19.204543][ T1687] preempt_schedule_thunk+0x16/0x40 [ 19.212931][ T1687] _raw_spin_unlock_irqrestore+0x2b/0x30 [ 19.221833][ T1687] autogroup_move_group+0xc5/0x160 [ 19.230124][ T1687] sched_autogroup_create_attach+0xa7/0x180 [ 19.239280][ T1687] ksys_setsid+0x12c/0x170 [ 19.246753][ T1687] __do_sys_setsid+0xe/0x20 [ 19.254292][ T1687] do_syscall_64+0xbc/0x470 [ 19.307858][ T1687] entry_SYSCALL_64_after_hwframe+0x76/0x7e [ 19.316405][ T1687] RIP: 0033:0x7f347bfbcb9b It seems that the crash is a read of se->sched_delayed on a NULL se inside pick_next_entity(), so pick_eevdf() return NULL After the following top->down backlink traverse, for_each_sched_entity_bl(se, cfs_rq) update_cfs_rq_h_load(group_cfs_rq(se), se, cfs_rq); cfs_rq is not the root->cfs_rq anymore, but a middle cfs_rq(autogroup in above example). Meanwhile rq->cfs.curr remains NULL because the rq->cfs.curr has been dequeued if the task is runnable and queued: if (on_rq) __dequeue_entity(cfs_rq, se) se = &p->se; cfs_rq->curr = se; /*wrong cfs_rq*/ Then later _raw_spin_unlock_irqrestore triggers the scheduling it picks from rq->cfs_rq.curr with a NULL tree. Maybe we need to restore the rq->cfs_rq after the for_each_sched_entity_bl()? se = &p->se; cfs_rq = &rq->cfs; <-- cfs_rq->curr = se; thanks, Chenyu