From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from CH4PR04CU002.outbound.protection.outlook.com (mail-northcentralusazon11013027.outbound.protection.outlook.com [40.107.201.27]) (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 DD5CE2C234A for ; Sun, 8 Mar 2026 08:54:33 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=fail smtp.client-ip=40.107.201.27 ARC-Seal:i=2; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1772960075; cv=fail; b=r1941oKZgMKLxV+ssKbNR3QHrtA2ih30bzUjUKMk/pX46d1JVZROk43qNHjMW7hqpAjrjYxJSwmOGpY6icvlCIou/s7zzml4d4vF2RQLu6tg7Nm0wq5WJ4zlTv1VlC8MXuxUyhHpSwXC7qqivbC2BQoKw3Kdahyue8GwfGk49pY= ARC-Message-Signature:i=2; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1772960075; c=relaxed/simple; bh=2LM95hIEJnxyWFVDqhhPc4NgvmucrYwXHlLcsvMI27Y=; h=Date:From:To:Cc:Subject:Message-ID:References:Content-Type: Content-Disposition:In-Reply-To:MIME-Version; b=nbtOVAwqieaNnr9zJ2yrMVSInbfhAL8vw5sT5ca9JqwyHffM+G5UfdGZ0aHDh2E5zaxGbst7MDu6hYLx9H+jh2vDO95PtRS/sYo4uCR6dpTwmHUvuRpRS7QVEQTzVSfU1H8RdDGjnd66LtNtLqwxwTyvc6daPJWTeYMpN6IFNy4= 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=W0N4YaVL; arc=fail smtp.client-ip=40.107.201.27 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="W0N4YaVL" ARC-Seal: i=1; a=rsa-sha256; s=arcselector10001; d=microsoft.com; cv=none; b=aOwFgfEestWa084z6rRzjESvn5cVjmiAIUthdRxg8uWyfUsO1pF7TxDfPDhu1Lu++hjuLimyux+WZMYuvQme9RXDrDKzxmbg4lgDlR3nRcAXKpmzwnJM9DjvLvTKoWn8DjpCQcB+kqLYaA/FYnqxB8IAN6hHG+qHMyY5RzoWn4F7H/RcjlUL4TaYxn5+k3g5/uVZ9tcjuPTRpfzT1QRymjVT/pcEP98l6gB5vD1OzsE0tH6TvWGPUEm8au0Zh9jSG34s5PSqQYB9uQgxvFRiLt2SwQLm/gv49I53SocbRKRHT6lCM1X0IDGTwOWNHn8deWwfYuEfYJWjVuQasCRZrg== 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=YFMOOsxZn56KMVj5lxxyxd9gMGvy+5fHR23eVEfj0M0=; b=umzcHK2bSD2N7GYgTeCDh4zCIHCBHp4qrA/txVIoGybn9QJf/4u2lGAsgA4OkwBHs32qQVukCshdS1pHfy1eFww0M2eDYuMU0/Ve6APPlbj0WqlHR4YI+xYTtCfzFnG+r0/RpYYhzpmVx9CGhOWhT0Xfz9qmthjXPbPm4spWPBX1WbDqyUleDj0VNzRJ8Fkc+MQTOk/dlQ6+wjxOojTd1mUGnkXf/pYisPBg8btIR7AxT2hLAcgReJnDW2LQacECNU0B+M5AGVUrisl8mlnq+PNGZGvRXl0dzblSDTl758dcdoPjYSeJEG9MrlL8aoOTeDt7uBtDsDkZKcM4d8WclQ== 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=YFMOOsxZn56KMVj5lxxyxd9gMGvy+5fHR23eVEfj0M0=; b=W0N4YaVLKUpmuRsoDYdP3sbUkPusOy05AaZ0YOXwo1vaWDQYNiMXYEgoPT8cHUsuPAAHeOm97TRjnnwTswauPISon2wNuFbN6qHKdbOzPsFtJ1C9MEA8kK5ULgfn6gfVJyis8lQGR7AfQdIDCjDeMBmXzL1Y8PbpB45z/ondlQNoZSlEJFiUK4PaKd9Y7K2j3X6qn+njh+gSLs0rG/b6hagdf3G+3GWbn2uar3gkFwbJNFeqqHn3Qv+D0lKSj9z+BxKXPCpkBHeyn9e99iuFf44dTi9LYdYGfwSF+l/Y7vdJfS7Ne9Bk2SstT7CfpHRVfMdDIaLb5BWwkvPJ8QvJTw== 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 LV0PR12MB999091.namprd12.prod.outlook.com (2603:10b6:408:32c::16) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.9700.7; Sun, 8 Mar 2026 08:54:31 +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 15.20.9654.022; Sun, 8 Mar 2026 08:54:31 +0000 Date: Sun, 8 Mar 2026 09:54:23 +0100 From: Andrea Righi To: Tejun Heo Cc: David Vernet , Changwoo Min , sched-ext@lists.linux.dev, Emil Tsalapatis , linux-kernel@vger.kernel.org Subject: Re: [PATCHSET sched_ext/for-7.1] sched_ext: Implement SCX_ENQ_IMMED Message-ID: References: <20260307002817.1298341-1-tj@kernel.org> Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-ClientProxiedBy: MI1P293CA0027.ITAP293.PROD.OUTLOOK.COM (2603:10a6:290:3::7) 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_|LV0PR12MB999091:EE_ X-MS-Office365-Filtering-Correlation-Id: e4b6fc64-82e4-4c58-e1a5-08de7cf0500f X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|376014|1800799024|366016; X-Microsoft-Antispam-Message-Info: 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 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)(376014)(1800799024)(366016);DIR:OUT;SFP:1101; X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: =?us-ascii?Q?5UVKji/+n+52tOug0VmLOjF5vja724Q7JURswHUwAELdlvADy5xpQKrowh9r?= =?us-ascii?Q?dR8ResvTVyQ9kNuWhDZmSVL4ZOVgXcCJZzSAEJIJEWyZAzHKFPB9D4PLb3g6?= =?us-ascii?Q?YbCVptYKUiVs8b0/cjcG9l3WD4B2pjgDO3Oo2WPMfwF9YIw/NYhIkKCK7mf5?= =?us-ascii?Q?Qf7YgaLAYL46eGiYJ30cjzgQrZhZsMdBI8HhO6rzb8wxOltxEXoVSsppV703?= =?us-ascii?Q?+puBgqKY16MctqKkSwGL0TkeC7WTJJkaVAJMIJ+1MBDQBg89GZS8ir93DX/8?= =?us-ascii?Q?PjaGOYjA57Zp0AVv7czujHrdE4TeasnKxibuIJXCovigzWm+gJGXJoEUS2kd?= =?us-ascii?Q?CTUgyzjviXozUsi/g9RrYuafHdHY5Uyo1x5lXaown3Cs1GvjpLa7ALJIxdq0?= =?us-ascii?Q?DLcAJ6IMCfAP2mZAkEFzYxOiWlYUGMHt+HFdIiwjfF3dIIERqYuMw1YjSExr?= =?us-ascii?Q?oLQJK6dhSofaIQlCYnLfkISV55fowhSAc7psP3i6PbN33wje+yYcYLY1eqIn?= =?us-ascii?Q?NfGvIUCkGOpG7Smkh2j9AG1F4P1rj8+gan8nWmeRm2Fwiq7iEBRvaMuFI2bT?= =?us-ascii?Q?GpVpznUTnkOVPq4o0l2XpRmz6ZbAhDNhpra98/MfX6yF6kDuWjItg/NQvSc1?= =?us-ascii?Q?qv6WV7x64MsrTJOXR4CC/Fan4ToOlbMhWEHojpIPnsOJIqsg9a4rrXsJsdeR?= =?us-ascii?Q?gYLsYIadRaxWjfUVLi4/E9I3hYBcic+hbmAsbE6qvVaRueLkRmUy5kAnMAuC?= =?us-ascii?Q?X+bSAsbVCPI8L8ZGpH4pIDzZhkQb4AGj29EpQStlzV8QbQC/m1Sx2HElIAzX?= =?us-ascii?Q?pHCQqdg88Rk56GcNsN38qnP4jQhVZssL6+vfWWRJAGzRpUvAlyVBrkFT/8Ty?= =?us-ascii?Q?BHvT60bp0fedZHgncvU+xi1Gz0p06q/LNUyJ5nx68kANgD1db4cNqZzRMSj+?= =?us-ascii?Q?LuQw0snXg82sghMx8cckOO3l/0BUVx0fS21Sf+0kGmammOwKlZ0hYSaYAj9u?= =?us-ascii?Q?TYE8dTT1LUmQVeQxBntDGKr+qNzdu+H+ushnBEvkPgIMlKtma5Z2rEepV2yZ?= =?us-ascii?Q?pn4QPu3EtjAuO84n/grgwCCg1fSxKJykvh/1dQxk20wo6VRrIxImoiq0GOeM?= =?us-ascii?Q?enNK+OtUZVWjKReqlRHdiB9fsXZdWh5omSIHow5pr5U7kd9cQscaM8gLWbpU?= =?us-ascii?Q?AqdpyfyeQTYZJnJgXVY3VioC8/0WcDaPrpBfcljhy0yEh2Sv8gScKm6SV2Yl?= =?us-ascii?Q?H0X8kGMsQdSwGMR5HEo2MYqYPLSyaCTZDXsuLCKuOXsnBxI81MtjLOyMZ97n?= =?us-ascii?Q?KHj3sm7McIvuE59WgZ/+SR3zU4J7PyxF6Ljq+/pZjADc252lQkC+x/gWYilS?= =?us-ascii?Q?UC1VvYozRy9IkQfmmBokoCYtTgGvthSc0FKjFald2CXOVVVQebxyHZrb6+Ac?= =?us-ascii?Q?NHiBrsXhBTNFoKayT6MyzY/2CY6Tc9wTBrD3sW3AyShuPfmKxrg9ZKf54b+K?= =?us-ascii?Q?sx7nPD0YPjjbpeCkrpmIsqOaMQjQtBgkrl5nN9KA7A0gDZ0i+gOJU/oNCSCx?= =?us-ascii?Q?wDk2zupxd3zkbNw9l/gqJPx61GpCDydRyawji51+RM15b7qTH7NTiVOjbmVO?= =?us-ascii?Q?0SeepePlQvFHVDBfQ4FQ3za4G69Yf44Z4iziyTrQlBTB6frwKvc8bNVtXqkC?= =?us-ascii?Q?BwnHYkD/GeGvL246YYEw8l9GkXPTqhUR/HxQg0gH+PMNH2+wdJvAQ+uOTjRF?= =?us-ascii?Q?FTA5Nt3PUg=3D=3D?= X-OriginatorOrg: Nvidia.com X-MS-Exchange-CrossTenant-Network-Message-Id: e4b6fc64-82e4-4c58-e1a5-08de7cf0500f X-MS-Exchange-CrossTenant-AuthSource: LV8PR12MB9620.namprd12.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 08 Mar 2026 08:54:31.3522 (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: BD74ut83gDDeWdsxPtWyASWliAPtBMF+y+FTVohhSPVe6Orm6UMAeGc95BIUqcIXtL1rpIrWWqWpQ4aQwFkEJw== X-MS-Exchange-Transport-CrossTenantHeadersStamped: LV0PR12MB999091 On Sat, Mar 07, 2026 at 02:19:46PM -1000, Tejun Heo wrote: > Hello, > > On Sat, Mar 07, 2026 at 11:36:46PM +0100, Andrea Righi wrote: > > This looks interesting, but I'm trying to understand the typical use case > > of this feature. > > > > I agree that we need some kernel support to "atomically" determine when a > > CPU is available (it can't be done fully in BPF). Initially I thought the > > main target for ENQ_IMMED was to improve latency-sensitive workloads, but > > this actually hurts latency, due to the additional re-enqueue cost and in > > this case it might be better to be "less perfect" and not use ENQ_IMMED. > > I don't see how it'd worsen latency. You atomically get the CPU or not. If > you don't, the only thing you can do is reenqueueing to find an alternate > cpu if available. If you don't do that, the task would end up waiting for > the CPU which is now busy doing something else to open up in the local DSQ. Yeah, I need to do more tests with this. I did a quick test with scx_cosmos enabling SCX_OPS_ALWAYS_ENQ_IMMED and in ops.enqueue() taking the migration attempt path (find another idle CPU) when SCX_ENQ_REENQ is set, and I'm noticing 5-10% regression in avg fps / tail latency. Maybe that's a too simplistic solution. I think with the global SCX_OPS_ALWAYS_ENQ_IMMED I may end up skipping some direct dispatches from ops.select_cpu(), that is probably what is hurting performance in my case, but it's just a guess, I'll investigate more. Thanks, -Andrea