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X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: =?us-ascii?Q?+sOvuIoUvgqasy+3oUtc5kc9SDo5bUpsAlBzENgIi388M7OBtIb3jzWBJCUi?= =?us-ascii?Q?KrRsy0EWd/weEmsksluK7y2QO1hP9DNG491UZxaBaQyk+Cf3MBIo5EhtGldR?= =?us-ascii?Q?FIo5dpciT7hk26Tf0s75jFHKPw59K+ilSnJYRMSrpnvicXG5z1P4RgZO4FAC?= =?us-ascii?Q?XqSDlgSYxjF9pLKpWDPH/i4nxX2O1S5Mi6sZLorMJaNXg0FR03viAcxODQ7e?= =?us-ascii?Q?sfxiiKhks2nfbiDRP7GJgxb80iN6iv3QRN9q8jdcrbOSNkOciQKLRIZ1C7jL?= =?us-ascii?Q?xxbO3XlTiA925lg6pvGt8JfZKP9FpDqYHIY70RlRxWN8RsoDd+tvaooR4lM5?= =?us-ascii?Q?wOx0wZ8c9Kme589Dsu+23Urr65plqyXH0qvU+CaySeIiZSKvUGpBgLbhs8cB?= =?us-ascii?Q?Lt7HQtDVGd/PWY+OGA+ACpVsiUZ80f3jSTiscraxabaXYlgWpUxDQnGzDGia?= =?us-ascii?Q?0ItTblwjLWOpwhpcsCl5fIWarhgNAdZu0ApRpo2wo2KM4g+/AyG921LtuSgX?= =?us-ascii?Q?LArI0AQ/nTjYezLJJG7FGp2ABfyGM3WJzV5kIi+6dtRsSfcex9F9i4nFEfel?= =?us-ascii?Q?17TCzd76TnR6KtXdoQOXyB/M10MijKrKsXqaUYfogx8DBD77rtcbk+r1xFYO?= =?us-ascii?Q?q0zEzLJ4EI9qJNUREXYB3QRaNM6ySBcm1Has2m0LzJmzefysXo+ZapU8aNY1?= =?us-ascii?Q?97RqPYuro37fCchcjTPu32H8bNix8g7jqSGgJxCQDNU7ShrvDHr1YQJtI8d9?= =?us-ascii?Q?QC7Gl/eQnv+HIlIK33YsqPPduWrgjX8ge2qmfR4LsS/7QwXcTXyuvoBEMtob?= =?us-ascii?Q?1zojcF6vAJyr1SDv334mUxtP9OXK023xD9DSOsnEFUOmMG+GVdf2ZGCO4762?= =?us-ascii?Q?DUXY7/8hMLuV+QVgePwXzwdvntxTCaRKGZu/XHDMtb5cOCqOsVcYkVxgDJ3s?= =?us-ascii?Q?l15l/0qMBTXff4uESRiqlxoPRX5cKg0GhNiNhpQgUmnTONX3ZpXKZqyqoX+t?= =?us-ascii?Q?pvU10m9vOtOeKQcAgBj2r5hIZUFkviv/7Bon90EW9hhKsC1/yFKSQVKcrRCO?= =?us-ascii?Q?lgdiGf5Ie815oDEIfj0eKtZr1MCza2U1yzq9qOnIGroNplKGw9K8k+bRxZDj?= =?us-ascii?Q?vMkTG/mRVepHhzsvfFBTm6HGc2sVMpeKmiIrldSN+fCLMu3IMAgmBWVcywtM?= =?us-ascii?Q?WDGL41Lt9fDRKp9XGE93knHw/3OS+kXYaiGT2GTTj2V78lS1jAg87NCfMMcv?= =?us-ascii?Q?SGW9pWEmncH53sVQwRBVsuexnoI2cQkXTvb0e7XrzDFPGzo52T/02szcDlOd?= =?us-ascii?Q?Cb3xiFOmJ4jPftj0X2uT39PvmA/hXUDr3183oEHc/AMozOxcPLQo2ejaN/AN?= =?us-ascii?Q?mbYoETytsdmTTrRX4X9R90wFWt8A6L0cdKBujqWtHAH1YcUKP18wGI7u9Oe9?= =?us-ascii?Q?ukzCOF/k7fYC2WJ9xsHn5QAErsaxBpm9Q3KtXSo2n76jS6rM4+lBGOkyovzg?= =?us-ascii?Q?nupRrhD4NwiPNsXqJgGPL8BZUDJKaEsk3dMZueTGxYVoBEaIg6iq/XPU0hUU?= =?us-ascii?Q?X8Kg2m9nTebj2drUUwis0FT5NrHUHKMGcDHyGBt41FH+7hr8EHEVfxPedqu7?= =?us-ascii?Q?Hx7V6O7SkVIjtORA6IKiR0DAzW4rwSUjqGPu958qZsU6lazDZNjZ7+p0LkN0?= =?us-ascii?Q?VCXNIDcoMwcLfoOmUUTDiUWeya5gxO3j7eJ4Jgjq/w1WfeVZ?= X-OriginatorOrg: Nvidia.com X-MS-Exchange-CrossTenant-Network-Message-Id: 362d0896-8e1f-470f-c1ed-08de6bffc923 X-MS-Exchange-CrossTenant-AuthSource: LV8PR12MB9620.namprd12.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 14 Feb 2026 19:32:27.3929 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: 43083d15-7273-40c1-b7db-39efd9ccc17a X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: qdHmO/+G5TWbayeXuYOKe9cwtWuU3GelJv4sJ3tkIUEs+7rZDMy3/zwR7xBJ+BTok8RNjV5VAP1hQTzHb3S3Yg== X-MS-Exchange-Transport-CrossTenantHeadersStamped: CH3PR12MB7666 On Sat, Feb 14, 2026 at 07:56:12AM -1000, Tejun Heo wrote: > Hello, Andrea. > > On Sat, Feb 14, 2026 at 11:16:34AM +0100, Andrea Righi wrote: > > I ran more tests and I don't think we can simply rely on p->scx.sticky_cpu. > > > > In particular, I don't see how to handle this scenario using only > > p->scx.sticky_cpu: a task starts an internal migration, a sched_change > > occurs, and ops.dequeue() gets skipped because p->scx.sticky_cpu >= 0. > > Oh, that shouldn't happen, so move_remote_task_to_local_dsq() does the > following: > > deactivate_task(src_rq, p, 0); > set_task_cpu(p, cpu_of(dst_rq)); > p->scx.sticky_cpu = cpu_of(dst_rq); > > raw_spin_rq_unlock(src_rq); > raw_spin_rq_lock(dst_rq); > ... > activate_task(dst_rq, p, 0); > > It *looks* like something get can get while the locks are switched; however, > the above deactivate_task() does WRITE_ONCE(p->on_rq, TASK_ON_RQ_MIGRATING) > and task_rq_lock() does the following: > > for (;;) { > raw_spin_lock_irqsave(&p->pi_lock, rf->flags); > rq = task_rq(p); > raw_spin_rq_lock(rq); > /* > * move_queued_task() task_rq_lock() > * > * ACQUIRE (rq->lock) > * [S] ->on_rq = MIGRATING [L] rq = task_rq() > * WMB (__set_task_cpu()) ACQUIRE (rq->lock); > * [S] ->cpu = new_cpu [L] task_rq() > * [L] ->on_rq > * RELEASE (rq->lock) > * > * If we observe the old CPU in task_rq_lock(), the acquire of > * the old rq->lock will fully serialize against the stores. > * > * If we observe the new CPU in task_rq_lock(), the address > * dependency headed by '[L] rq = task_rq()' and the acquire > * will pair with the WMB to ensure we then also see migrating. > */ > if (likely(rq == task_rq(p) && !task_on_rq_migrating(p))) { > rq_pin_lock(rq, rf); > return rq; > } > raw_spin_rq_unlock(rq); > raw_spin_unlock_irqrestore(&p->pi_lock, rf->flags); > > while (unlikely(task_on_rq_migrating(p))) > cpu_relax(); > } > > ie. TASK_ON_RQ_MIGRATING works like a separate lock that protects the task > while it's switching the RQs, so any operations that use task_rq_lock() > which includes any property changes can't get inbetween. Yeah, that makes sense, so the scenario I was thinking it was happening can't happen. I guess I'm missing some ops.dequeue() events then or there's a race somewhere, because I can see tasks being enqueued without a corresponding ops.dequeue(). I'll add some debugging and keep investigating. Thanks! -Andrea