From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from BL2PR02CU003.outbound.protection.outlook.com (mail-eastusazon11011058.outbound.protection.outlook.com [52.101.52.58]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 08288332EB0 for ; Tue, 10 Feb 2026 07:29:14 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=fail smtp.client-ip=52.101.52.58 ARC-Seal:i=2; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770708556; cv=fail; b=tE4C5p8KEz9s01OtaIbqlAcS969rkzvnu0kJs+qKA/+MkUV8TUztHWbq7u8wB7W4ZyNpj2/DTPY1+ujg6vMaUdVcg1niNMoS+7nMrLdaJidDAlvUO+V54yjBsfeDxXi6dQO2rPLEDLtt90cMkumPUFFnpgsHmkNUZ8KqTM5nMx8= ARC-Message-Signature:i=2; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770708556; c=relaxed/simple; bh=0rHYUHHcqQVJfZwQGNJx34EFum0SQ2gHqoQztS4UD1s=; h=Date:From:To:Cc:Subject:Message-ID:References:Content-Type: Content-Disposition:In-Reply-To:MIME-Version; b=cHucOb1wt3LA1Z5kP3dKl+LTBsrBvI6mZH6N22CtBItFHrC0xO2YzTzTAPZhbOKdWOOC+EW8hbp7/3B3Y+rOJUfh+Q1jgp5H8lNsILLeHKtA0qBE7oGM9XA5zI32o1DUkCZfxQqXAiY9b0S7H2sV1BZ9YO808FcgJy3xaFjeCHg= ARC-Authentication-Results:i=2; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=nvidia.com; spf=fail smtp.mailfrom=nvidia.com; dkim=pass (2048-bit key) header.d=Nvidia.com header.i=@Nvidia.com header.b=mg2FPfcz; arc=fail smtp.client-ip=52.101.52.58 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=nvidia.com Authentication-Results: smtp.subspace.kernel.org; spf=fail smtp.mailfrom=nvidia.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=Nvidia.com header.i=@Nvidia.com header.b="mg2FPfcz" ARC-Seal: i=1; a=rsa-sha256; s=arcselector10001; d=microsoft.com; cv=none; b=gYOi6L8BsSPqGjzWRTxcM2YEflkS6J67/UZPxebrgSjcXY3vabvVT+9Ta2pPDzGJCVW4KEqXQpw+O4cnqYriDOReixmmcGzQw8FIjJzo7u1zSS0YlSmu2c1fbVxD3GpItaq6zjpjjKRpwxyolmseuGJimJYPKmnugqf2W8C2YVYK2xYSG2kpG/4n+U8fVRokk7aC/VHBN8wzVxsbxKCSK2V9Hp4etbIlsYt5VVK5qxdG4pEV5XfkNF524e1kwE1AEt/r869HfQ6v/RnkkFopuse+Id6rqaOCANCR+IoBW0izILjMAh62usLCtlX5niFq/oIrUuQut3N1Xg4JRxd14w== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=microsoft.com; s=arcselector10001; h=From:Date:Subject:Message-ID:Content-Type:MIME-Version:X-MS-Exchange-AntiSpam-MessageData-ChunkCount:X-MS-Exchange-AntiSpam-MessageData-0:X-MS-Exchange-AntiSpam-MessageData-1; bh=uBoW5LeZNq3dVdvSTrTzHrWUvtr3v3JbtBQ3bao3IQw=; b=B0kX9aCzWII11g52R87LFLvj6dx8hboauii+xFHgJ3WQU0yy0X447nrgygdvUjMFNrvatqMonjc/0S7Jsk3+qbgpoib0ukJyMk0RSSFMlAJ0mb97imDCPfvCfx9YM52ma5lg/ZqN1dcks3JmTZJL9CIcJt1cFopNXHxCHB6vQl8G5beWW1Es6l0YnNII7zyngeoyvdHC4mcEnANdgLxKaHFzm1ygKr2fEObuNxPpJKeCzZMz7DijZVXBfC1iaN8nLiCJqiyarnRhEsuOYwBUWRfsF/Sa/nmAeItVEV57JBTrAIpJRxNC0HAVRpMvkHs29l0eK8nqmgweV7UMD41jmA== ARC-Authentication-Results: i=1; mx.microsoft.com 1; spf=pass smtp.mailfrom=nvidia.com; dmarc=pass action=none header.from=nvidia.com; dkim=pass header.d=nvidia.com; arc=none DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=Nvidia.com; s=selector2; h=From:Date:Subject:Message-ID:Content-Type:MIME-Version:X-MS-Exchange-SenderADCheck; bh=uBoW5LeZNq3dVdvSTrTzHrWUvtr3v3JbtBQ3bao3IQw=; b=mg2FPfczuZWZq1o0QwQYOfizFrMAlauclcyEQk/CR+4HfiBkfOYPfTTTWzIkCGtVg62aaQlpoaosa0SEtF4b1V7ivOhOZfGbR5jBwABN+G7OAxjWJjs32bvEzPxXw6UNUr4/XRcySDlR3AX54KcbDtA68Iwsfd+xNq0qFWGcsaLgXHQ6VHun5FdtFlRRIfLGbdUl8Cb7RTMNQQXJjRT6B9yRFx3SDrhQHBS1p6tzSe13wbjMbnQ14Eg/FfZ1lb6YD7PQn/NG+8fkfFkU4rszn4IgQBtQlSIj11QPnIC8mz4pFctHCRFS6GB6GCFePujt8SaYxR6WOT2SJPXBSTI5YQ== Authentication-Results: dkim=none (message not signed) header.d=none;dmarc=none action=none header.from=nvidia.com; Received: from LV8PR12MB9620.namprd12.prod.outlook.com (2603:10b6:408:2a1::19) by PH7PR12MB7186.namprd12.prod.outlook.com (2603:10b6:510:202::15) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.9587.19; Tue, 10 Feb 2026 07:29:10 +0000 Received: from LV8PR12MB9620.namprd12.prod.outlook.com ([fe80::299d:f5e0:3550:1528]) by LV8PR12MB9620.namprd12.prod.outlook.com ([fe80::299d:f5e0:3550:1528%5]) with mapi id 15.20.9587.017; Tue, 10 Feb 2026 07:29:10 +0000 Date: Tue, 10 Feb 2026 08:29:06 +0100 From: Andrea Righi To: Tejun Heo Cc: Emil Tsalapatis , David Vernet , Changwoo Min , Kuba Piecuch , Christian Loehle , Daniel Hodges , sched-ext@lists.linux.dev, linux-kernel@vger.kernel.org Subject: Re: [PATCH 2/2] selftests/sched_ext: Add test to validate ops.dequeue() semantics Message-ID: References: Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-ClientProxiedBy: MI2P293CA0001.ITAP293.PROD.OUTLOOK.COM (2603:10a6:290:45::14) To LV8PR12MB9620.namprd12.prod.outlook.com (2603:10b6:408:2a1::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: LV8PR12MB9620:EE_|PH7PR12MB7186:EE_ X-MS-Office365-Filtering-Correlation-Id: e9408844-7043-457c-85b9-08de687614df X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|376014|366016; X-Microsoft-Antispam-Message-Info: =?us-ascii?Q?yns06RtTWRn1IwofFc31JuOH6hPCuOt8sxXWI6dYX3+qMHYrG2rPH4398vSV?= =?us-ascii?Q?+hhPsna9OruPrE9k9FBIB3vKWBYYULEVSUP/nqvioEZu5HSU7avahdo+IRmR?= =?us-ascii?Q?Dd7QRFxMQpx+LiGspWGjvZA1Nrg3wC4vzB+G9MKnAIDjR6g9SOy1xKlhHeJ9?= =?us-ascii?Q?GQeFo8xNxiwPXtnIha6tJz3t+iOXyQvTSnzB7/ny6oE5JyYfDDZKTdtXNVK3?= =?us-ascii?Q?Uh6u8l97r9EnlgsFdTwW8mzDI45LwC5x/pQ7i0BjtUedaQDnfrsEEaOLOSd1?= =?us-ascii?Q?URX5cvwnLrK17llB7i4Kzj/7d+aXo5+Zu/5lmHt5Qw7zq05dzXSYb0IRSJk/?= =?us-ascii?Q?RJaZsSC0XmGkrtcBtziKi6dusnRbR6+skyQaHkhNwJLjLe0L6zacUxe9yWKn?= =?us-ascii?Q?NVplQM/+cVAPEuG59RrvHF8yWdrObyB4EGJ9PF95tvJ71PA88SIs+yX/Gb7E?= =?us-ascii?Q?+ycgaU2nG62p4ZxbtjA678c4w8NOyG93QDOlxaGcPJqIii/BSa1eiONEKPM1?= =?us-ascii?Q?29ZS9zfdkoY0bPtZsKMEqTvr16b79S0IA+5uxeCtBpVXwLuq1+znfB/Roow0?= =?us-ascii?Q?X+rm9b9cky/ZmXATb97bFtUbyXUZVa24ebK9Up0/Ax6YHs1oZ1Gkvp9LkKr+?= =?us-ascii?Q?aZvZEkysBNUBSteRINvUwWqylA68BIwbRYT4i6JfAOii397ggohCsX5XSPZL?= =?us-ascii?Q?PMgf+RZBlxdTYA/fp8fkoOxOV5OmCgCKIXr+mELToQ/UkcMKzfloy5VtVyNM?= =?us-ascii?Q?8LjR+8IVuyq+MKMnfY7/sKrLaJHWNbxt8hyv9k4DI2JRm80MQv0DHSY27exe?= =?us-ascii?Q?0+SKn+p1c5pGlAK1PXHF3BuCyrbh1TPiXt0nnFTud8OaU6ubhY9TbGyMSGkD?= =?us-ascii?Q?3eiriHGqqlQfy6rd6K42CaZmcYTMRtTsSX5g2GsQX+ozJmbylU/g7p4dXRCF?= =?us-ascii?Q?uKqUSi1L9shbXMEcdhEJHSzhjSM3eEV1vmkxfBDwEQktCWheJ+n6IB9fOlVW?= =?us-ascii?Q?uVLN5CDGTbS99v9XP5GYnPaSRk9hsp4H8m5dtofFIPIyJtvQB6cmhmsCkWFn?= =?us-ascii?Q?WiEpsLQteLPLI399foYqxlNvhCZ6CLq3ue0dfwQazf897ZCgGoTK764r/azJ?= =?us-ascii?Q?dXstDTCtCbudXwWbD3AeNoFnc4WFvI6hbwyArAI7jrWN4GH53xYIrwIpGGtK?= =?us-ascii?Q?1ak7JkN/jtnPBovO+G+GilzDyhRC+BJ6qwGdA+lq3tuR6lO8f7/l/GZvhxjG?= =?us-ascii?Q?WlycwqQjTwRgMu7AFGH94CBbF/mvQeog0T/mIimr1+ZqQ/egEkBGhySpWTX2?= =?us-ascii?Q?pR3INVE43gJv3hb4NLdi8NRGphth4pUx2c50vanJy6kOLv2JuoIFP1igJ1r2?= =?us-ascii?Q?5KEvcL3MOeUO4FzV8SAvAfSsfaO+GxlPMPLQLdWrqguV0dUJKTF4zL0pjewT?= =?us-ascii?Q?PZTAogF/5/j62W7afBZC7AlTnYFDSMNq9zUnx+LZ2ogJeZ5/Q1HFFEKQVMb6?= =?us-ascii?Q?2e2Dp8Pbnif/llZFcIIp+xt3KyMYSdTf/wuypQefiqJzQyZEvl4JI7GmjDkT?= =?us-ascii?Q?Rh1sA8EWjvW/T5VeEbI=3D?= X-Forefront-Antispam-Report: CIP:255.255.255.255;CTRY:;LANG:en;SCL:1;SRV:;IPV:NLI;SFV:NSPM;H:LV8PR12MB9620.namprd12.prod.outlook.com;PTR:;CAT:NONE;SFS:(13230040)(1800799024)(376014)(366016);DIR:OUT;SFP:1101; X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: =?us-ascii?Q?V9WHFEuZEbvMk/NiZjgzyr3d7BSaH82IY00JVnZEP+/s45ELq88ZzCAdWi/t?= =?us-ascii?Q?CY2ZanQFabQs0kYIFAoC0WyOlczRyJet+TRWhs+mV+QCjIuEM7uR/d1k1Hh6?= =?us-ascii?Q?EjcI3hk4ulbcfYCAnHnKmMPCMGzWHqEPklG76GNl33zHtFcMVI5XgdhGWt+5?= =?us-ascii?Q?TRD8NZTVoB2t6rlE3O5lLwKjbSDNZ/CKKZLwmFnIbS72d4yDwtlGmPhQrA6F?= =?us-ascii?Q?yhY5Kecd+iByXwE7y9tRrg5kNYT/vUduaBs2Mmb32FtJx4COCKuJ0kDLmwce?= =?us-ascii?Q?DKcFYDPzSvRZRNFyGZYln3bO/OD0885FstuH+Qw/ksf5YSfnwDRpA820PpGG?= =?us-ascii?Q?3zyA6YRpHOVJ5Jcxc5DHG39c7ZoClEA2ipv1xBKy79gYoQhDxxtlDkULtZcX?= =?us-ascii?Q?H5uA/cE0oMUwT98KpJFc4Igwjf+5YxTvfGCo1xyqpvcBEdeD+U9KLKx9rw+G?= =?us-ascii?Q?skK0QOKhmxInwVd5K2WWicM3jkW7a1lT1RnB5sZzeoqNIiIC5YWjmQ1D91Ip?= =?us-ascii?Q?IukExCT7Xt6tjkd73Yp5EKlnODz1Cx6rKYTO3B/mG1ZpKkCQaSHx8rcJ/Iwa?= =?us-ascii?Q?s+oGf9TLn4Ou0mkB/PkrHVcwojRLWIrZsmYihyRYUZFT5JGJvwz8OKrlBjyf?= =?us-ascii?Q?KXOcf4h96kN2VWMwScr6VE9c0XCbr+fmyoqyr6Mk3BhsMfBXd8b/9bDN/E0F?= =?us-ascii?Q?+kYkUw7diBnUtrnwrsbAtm/LfmtlSUuHR/IYWroKNx3xvYwcwqaddq3/WTDi?= =?us-ascii?Q?QZbbWbgrxgLnwlM5zT6O07tkyl1MpKJ2JmO1didWK5sVSPTINn+BWP429xSF?= =?us-ascii?Q?Qgbxs+KsAqix9vxmxmFDZde1Xmbjw5/15r4p/sbHLPJxdEZasU5vGeia3jSQ?= =?us-ascii?Q?rSyCXrrOMIkIG/HMQun6ehCQ9VANX+3/JQxXVx6PlGTGi/iRVRnVk7fBG/sB?= =?us-ascii?Q?wqvaEBEVVsTDkcffJTN2cDMMwmdg8w/50AlYti7XSOeX6O/HqHtcjkX9PxWD?= =?us-ascii?Q?qtU15zgvnpT3wrvVvkNQZQPjCRkO/XOyROJVfFCW0w821NqnPOUSbA8cMzx4?= =?us-ascii?Q?lPfoPxxD8Xry88+HjXVSUpMpdFeyEGhLIb+egDYb/fLe5cvz6+pOJoIT4PAm?= =?us-ascii?Q?Gp+0uGtxtvIi4OE0NCPtqT6bMgCOfT96BKnw7buHRRumpJwCwH0WLViTY8Ug?= =?us-ascii?Q?h0FD3bTmMGXhlWjNeho6DAKByPoJi2FP2YcWdSNrBBTu3paPEYz27+ggFIWj?= =?us-ascii?Q?rSl2Y011BUS0ojselyW08/A8bwZzOweHZhRKZjcqGr17C4tBUuGja4occtZ6?= =?us-ascii?Q?dScGSP61vyGpDJlPl7QKrKfpsWIgDr0yYHYVVcPqiBp1btbQ8cyvNxqKXgKq?= =?us-ascii?Q?OSQdzlGfhRXopZ5+qSdD9q4bUkRKEa1S4SiW8OigvydR1crDtnp69QF4LPA5?= =?us-ascii?Q?Kq66OlSenRqeb+9q0eg03sU2c9mC0wQJom5FxymqmttBFvLXgtnM1reH3APl?= =?us-ascii?Q?mAxiTbBi1SB3HIKHRl9wTWbHmYgl0SvGpTJg/7yGafHilTMO8lgOTC5dMexP?= =?us-ascii?Q?aGgUe+O3io+Che8cjqVrv+iDVYs3d6udTcXbN9cPvPGFkrCUSevBER6k8L5d?= =?us-ascii?Q?M8lH7x/PId/YQ6+FpzrzblX/SYZeam7CQX7R5SSCx4wrzhztglXjW3CXuliF?= =?us-ascii?Q?hZaELf6dB5NbM6AU2r6UhTwJRr+ZHdfQUhxiu0wkE8wRbt7L?= X-OriginatorOrg: Nvidia.com X-MS-Exchange-CrossTenant-Network-Message-Id: e9408844-7043-457c-85b9-08de687614df X-MS-Exchange-CrossTenant-AuthSource: LV8PR12MB9620.namprd12.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 10 Feb 2026 07:29:10.2347 (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: E8V12mj0VLSkIRMSyZbQKjC5TLlaknbontX8XJMVWa56Wcy6mQP5ImG+vxAgTgpXWknJbXIccmCkdVa1zu7fcQ== X-MS-Exchange-Transport-CrossTenantHeadersStamped: PH7PR12MB7186 On Mon, Feb 09, 2026 at 02:42:04PM -1000, Tejun Heo wrote: > Hello, Andrea. > > On Mon, Feb 09, 2026 at 11:22:19PM +0100, Andrea Righi wrote: > ... > > Ok, what you're saying is that a direct dispatch from ops.select_cpu() is > > just a shortcut for work that would otherwise happen at the head of > > ops.enqueue(). > > > > So, while ops.select_cpu() itself is not "being in scheduler custody", the > > semantic operation of dispatching a task is still the scheduler taking > > control of the task. As a result, a dispatch to a user DSQ from > > ops.select_cpu() should be treated the same as a dispatch to a user DSQ > > from ops.enqueue() for the purpose of triggering ops.dequeue(). The fact > > that this happens in ops.select_cpu() rather than ops.enqueue() is an > > implementation detail, not a semantic boundary. > > Yes. > > > Under this interpretation, storing a task in BPF internal data structures > > from ops.select_cpu() should not trigger ops.dequeue(), since the task has > > not been put under scheduler control yet. However, dispatching a task to a > > Also, ops.select_cpu() putting the task in a BPF struct doesn't affect > what's happening in the enqueue path. ops.enqueue() will still be invoked > and the task will be transferred to BPF side iff ops.enqueue() does not > perform a direct dispatch. Imagine the following (unlikely but possible) > scenario: > > CPU A CPU B > > ops.select_cpu() puts task in a BPF > data structure > ops.dispatch() sees the task, dequeues it and > dispatches it to CPU B's local DSQ. > > finish_dispatch() runs but the task is still > SCX_OPSS_NONE and dispatch attempt is ignored. > > ops.enqueue() runs and returns without > doing anything. Task transitions to > SCX_OPSS_QUEUED. > > Afterwards, the kernel considers the task to be owned by BPF but the BPF > side thinks the task has already been dispatched. It just doesn't make much > sense to do BPF enqueue operation from ops.select_cpu(). The only reason it > works for direct dispatch is because the kernel defers the operation to the > enqueue time behind the scene. Makes sense. Storing a task in a BPF internal data structure from ops.select_cpu() doesn't prevent ops.enqueue() from being called and it can introduce racy behavior. So, putting a task in a BPF queue at that point is just extra overhead and provides no real benefit. In other words, the only thing that makes sense for ops.select_cpu() is direct dispatch, attempting any other form of enqueue from there is pointless because the ops.enqueue() path will be invoked anyway. We should probably document this behavior to make it explicit. Thanks, -Andrea