From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from YQZPR01CU011.outbound.protection.outlook.com (mail-canadaeastazon11020132.outbound.protection.outlook.com [52.101.191.132]) (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 04BAB338F4E for ; Thu, 11 Sep 2025 17:15:12 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=fail smtp.client-ip=52.101.191.132 ARC-Seal:i=2; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1757610916; cv=fail; b=DUMVRj2TjLnMM1LAUr9/3Z5fXtZa0v+eoZUyzWfyiZ+2jBPU7/BkJbatSFKM06M+17oBKblB2HXd5T//IgjfmxIgkikbMoORryfHmDcGPoeo/F+A/DIaWw3QsCgiykqiIj2eW6mch3hN9FkN3qEfl4B+EhE2Lyir7WemMvD8+Nk= ARC-Message-Signature:i=2; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1757610916; c=relaxed/simple; bh=cU+WOg56dNDFqva4YwdqdJKLiP5jYXeIN67aOQMRB9w=; h=Message-ID:Date:Subject:To:Cc:References:From:In-Reply-To: Content-Type:MIME-Version; b=qoTl4TwgOUI500t0HDPE4owT31fqWTa8jtBQwI9tBE0zkZAVhZMM/KtJNQfAJjWegnohpYGPi1no8gyoCclEcsBeSn+85NzdIdz8i7fb5CRTeA45F/WGDCS4Zj+swsCYfsWYukQjhU+uFHIT8sJUTTYgzCw4yNlLlKnqoWyUdlk= ARC-Authentication-Results:i=2; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=efficios.com; spf=pass smtp.mailfrom=efficios.com; dkim=pass (2048-bit key) header.d=efficios.com header.i=@efficios.com header.b=V6zuetJw; arc=fail smtp.client-ip=52.101.191.132 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=efficios.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=efficios.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=efficios.com header.i=@efficios.com header.b="V6zuetJw" ARC-Seal: i=1; a=rsa-sha256; s=arcselector10001; d=microsoft.com; cv=none; b=oqP8O3CC4UYWRyzZTrhoOTfNhAdshIKpy8hxn5hZBEuuscXT+I5D5xBo4hAHSXzPP/d1itH46H0WhOIWI8AUfIUzavjezuhFQ8IAuCTBVnr+L7OSnBK5zH7csWGjW0e60hAlhkPWp4aM551qbJX8E1nIUPVC/fqLpzrtKJ7dRQZdt6IiJbHTOPvkL79A0hs43I0X+9LPfQT0ro+v/VbRvtlW39vOdxmuHCv5EGIHxOY8LLve9Bq7KlhYbFinqC+mD0Q2jdIjquk1iuN8sUQDx4T6MEn77h255SvQufdb1Hgl4J9yJ1MDrmbLwJbDc7aka9nyo3LP0I2b1/hi7NRPUA== 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=3jXGMfUQheXh+3zEWk5tBAfhEApQebXfjLjXfE+kp7w=; b=pX1fFPHWgXbWks+A6ocqJxES8wYmXcFRRPLP7Ms0+6lg6xBQyzpQYAhzhCj6suPs/4LSRbOTNMNHbGaikNAw3R4ovnIUC9tvSiAQI5xk7vUB3iVfOymgs/yLYcKG6QlGBQQz4lBviLGIkePTgU5Rto+IoWG9511gYOVYiAMjlomUot1S+Jg32SHQhZlfFDgHuVFRPI4Sm1DEdjDFeaNROtoBVPPqSPzs9qnRB5HnouUY1+itchCuL3gd9pXunoRYt1ygdCpOQm4Ypt5v44K+tyI77OIG+VDjIl2pQnATFV+hLAZY1bQqjwF8/4qrQHCKiCIY094/zmdut061Cc5lUg== ARC-Authentication-Results: i=1; mx.microsoft.com 1; spf=pass smtp.mailfrom=efficios.com; dmarc=pass action=none header.from=efficios.com; dkim=pass header.d=efficios.com; arc=none DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=efficios.com; s=selector1; h=From:Date:Subject:Message-ID:Content-Type:MIME-Version:X-MS-Exchange-SenderADCheck; bh=3jXGMfUQheXh+3zEWk5tBAfhEApQebXfjLjXfE+kp7w=; b=V6zuetJwE0RmjQkZGDDUf5xZ6UV5iWkAh9LtiGA2pto/7gpgTlU6FdTGAkWCWiSct9uR1PDXFjlucupAAjeVPxki0EQeKQUJT6jDSR+PfIVfEgVbgmwnnlxS1DrZbAfeBOWcC6FCJig5BXlqfheAvDBI2wLYZX8hHGyjcvX2XLE7zHAYnEsKQjkpTjleLVRVpuTf7y15z7r/leP5BQR7PlgS6NWcuZLPoSyrUp8qBNdn6+Q3P8Lhraq/nuB3Ex5jgyFpuqGSz7HoW/dMV5I5+kXXH0qIzxt7vSkUsKICETbELa6kmpBfVQ17mG2hxlHgLF0ikjxMu+qZaAf6FxjodQ== Authentication-Results: dkim=none (message not signed) header.d=none;dmarc=none action=none header.from=efficios.com; Received: from YT2PR01MB9175.CANPRD01.PROD.OUTLOOK.COM (2603:10b6:b01:be::5) by YQBPR0101MB5639.CANPRD01.PROD.OUTLOOK.COM (2603:10b6:c01:32::17) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.9094.22; Thu, 11 Sep 2025 17:15:10 +0000 Received: from YT2PR01MB9175.CANPRD01.PROD.OUTLOOK.COM ([fe80::50f1:2e3f:a5dd:5b4]) by YT2PR01MB9175.CANPRD01.PROD.OUTLOOK.COM ([fe80::50f1:2e3f:a5dd:5b4%3]) with mapi id 15.20.9094.021; Thu, 11 Sep 2025 17:15:10 +0000 Message-ID: <1cdacbf9-9564-4025-a5a8-777211a1fc2d@efficios.com> Date: Thu, 11 Sep 2025 13:15:09 -0400 User-Agent: Mozilla Thunderbird Subject: Re: [patch V4 26/36] rseq: Optimize event setting To: Thomas Gleixner , LKML Cc: Michael Jeanson , Jens Axboe , Peter Zijlstra , "Paul E. McKenney" , Boqun Feng , Paolo Bonzini , Sean Christopherson , Wei Liu , Dexuan Cui , x86@kernel.org, Arnd Bergmann , Heiko Carstens , Christian Borntraeger , Sven Schnelle , Huacai Chen , Paul Walmsley , Palmer Dabbelt References: <20250908212737.353775467@linutronix.de> <20250908212926.929787629@linutronix.de> <6e0bcd7c-f842-4db0-b30b-5f6857b45b66@efficios.com> <871podrn2i.ffs@tglx> From: Mathieu Desnoyers Content-Language: en-US In-Reply-To: <871podrn2i.ffs@tglx> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: YQZPR01CA0063.CANPRD01.PROD.OUTLOOK.COM (2603:10b6:c01:88::17) To YT2PR01MB9175.CANPRD01.PROD.OUTLOOK.COM (2603:10b6:b01:be::5) 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: YT2PR01MB9175:EE_|YQBPR0101MB5639:EE_ X-MS-Office365-Filtering-Correlation-Id: deddaf67-f876-400a-9af4-08ddf156c376 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|366016|376014|7416014; X-Microsoft-Antispam-Message-Info: =?utf-8?B?clhFVTVRcmZ4NmNFVmJpSUhkVi9kUDkxcEtTREU5NUhJQnJ0bHAveWNPdEFU?= =?utf-8?B?RUVtSVVIQk8wQjJVOVJmSUVKcGVwV1ZCaUFPWTJ5WGxhQVUzKy9BamlXeUxm?= =?utf-8?B?U296R1J6K1VQcnhiZFFJU2ZMVDlnemZ0UTAzdzl3UHZscDZPV2FjMW05SUlG?= =?utf-8?B?RDZSRUIxMVVLVTZXa2JGQnpQWEJUc0RNckNubnB6MkQ0R3RiWjA0R2htTHY1?= =?utf-8?B?a3cwcEVna0FpZllBaXVZekpNaHlSU1I4RFI1OVpEbnNGZ1dDTTNTY0NadUVH?= =?utf-8?B?UGR2OENvWEx0OHBMalFBbWF1VThtUnJuamhlMUpROGorSG5PQkNZL0hSdlc2?= =?utf-8?B?Nk8wQS9VSU9LNFBRNFIrMDR3QmdpTkNWaXUyMVF2cmtGaE5LQVVhQnhZby91?= =?utf-8?B?OHN2TmJzc3VYRnNObDRZZUdIVVdka08xRUxCS3BUWHh6cFNyWlgxYkNxeTlD?= =?utf-8?B?cXYwdFlnMktBNEFha3YyaDBDL016Z0NPZmRLWUtQWStFemZTWW01TmsrNHdz?= =?utf-8?B?eWphNGIwV2QrMjNiSUw5ejQwMENHTUg5UUpUT2E2ZE1jb1dVaHYybzFxNEg3?= =?utf-8?B?U1lwajhEWEh2dEp1NExVZGliWEFHdlpISVpoYVI0aGtBTzlJMkp1QkcvaTBz?= =?utf-8?B?Q0RXOTZRU3FPN1lZQ0pnd3RIK3hqa09EeTk2R2lOSERlaGdiOEhNL3M4bjRk?= =?utf-8?B?c0tyUldCN2c4dkwvYjlkbkhKTmp4UGtsTDl3blEzR2pRMlppVGdTTG1kNm5T?= =?utf-8?B?QVR5VzJ5RzhwTjNLM092a0ZGcG9xSUpTaHNzRTYxd1poSEphd1ZZYnVpTjRm?= =?utf-8?B?S2V6VzVMdDJraktyQnZrdW0vOWk0RndsK3ZpRFVvUmhuTEtFMVBTeTZhLzdR?= =?utf-8?B?Ry9MRjlBM3Zjbndjb3oxeVZnSmJUOS9vcmpST21WbHBBRC9keTUydy9QeWFV?= =?utf-8?B?bEMzWGhkMERYcTBlRzk0Q1dDQ0taWjlmWUM2bjdORlBoWkJCcUdkYWJKMGFm?= =?utf-8?B?YzhHWHpHWHdtQW92eU1GMUkxektMSGJWcVRZNUl1bGprdVNmUEhDcmVpbHBR?= =?utf-8?B?WVNxNmwxZ1BXSDBKcXVXZVlkTTMyNllGaHRMelFVVytBTTJWNEwxMyt5cUM1?= =?utf-8?B?K1VReksrUFAyMmkrUDRMNjZmWWJYVkxod1JNSm8yckpWbzhkQ3lXK2w1UWxD?= =?utf-8?B?OTc0WnNlYmRPYW5BNDcyTUd3R2pQV3FBbnFpQ1pqcDhNSzZyWDJpa3ZYR0Nn?= =?utf-8?B?RVMwMnQzK1owR1Vha3MyMjluYm5hSnhUVzAzUVl5Z1ZMWFlLVWIwNVFFNGYr?= =?utf-8?B?SlBNbnQrVGxpTGJzR3ZNeFBwREhZcjhlL0tlRHhIdGJPTER4VmlHMzUwUnJE?= =?utf-8?B?dzRlR25hRGxMaUJZUnFWVy9JQ1c3NE1NMUZtTkdhZCtmK3hNeGROQUNWQXAy?= =?utf-8?B?RlRyU0lMNVVDbnJnSkc4d3lPc0xXTXdmVFROcXZsZGRON25UdVpjdDNiMmMr?= =?utf-8?B?RWtQL01mdkNyOGtJckYvZFBKRWlxM2hJNDVpOTlMMEF6QUE2UHRpNFM0YXIx?= =?utf-8?B?endXMFpqSFVBTEJmTzdtNk5XejV6NHJ5NGh0VW13dThCMnRuSGlNLzVpaUtn?= =?utf-8?B?bzBwQjRIOVdXTVRyWm5tYitFRXFocUV0UzRvN0VDSGR2bkNXMFFQcGZWTUVN?= =?utf-8?B?ekFwVzljQ212VXBGanJDa2NJT1RHaFR2TFlvc2dkWSt6ZWJnbWFQRGgvbnk1?= =?utf-8?B?TC9qT2Y5RGNOWnM4a3VrdHVhbHN5b1V3UTBhV2xaOW5xSm9xeng3dnE4T1Ev?= =?utf-8?Q?CEQzE0atjAuJrnSkgTbRQ9iHFw1rYDdEZwCz0=3D?= X-Forefront-Antispam-Report: CIP:255.255.255.255;CTRY:;LANG:en;SCL:1;SRV:;IPV:NLI;SFV:NSPM;H:YT2PR01MB9175.CANPRD01.PROD.OUTLOOK.COM;PTR:;CAT:NONE;SFS:(13230040)(1800799024)(366016)(376014)(7416014);DIR:OUT;SFP:1102; X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: =?utf-8?B?dTNqMVBaYmxBWmtUZ21YRkwrMUlNTXJVZTBST2lseG1xblJZRk9jR25jQzNY?= =?utf-8?B?aWwxMjJBY2dzb3R6eXdNSnQ3V0ttOTlPSFlTaHpBbTVHMGd3aHdUaEZ3NEpR?= =?utf-8?B?RldXc2dSL0ozeXNCdFRzUy9nd0hyYzBaSWlxaEtraG45NDVSVHBXTVg3NW5v?= =?utf-8?B?VnBYTkdnY0xVL04vbmdkNHAzMi9TL3lxelpERTVlOURCTFpzZ25MWkxuWG5Y?= =?utf-8?B?VWJBUjFIM2txc3hpbS9aSElFaGkvM0tRU0dLWWRpU3Z0RXFnMjNWZHBvQzBD?= =?utf-8?B?OG0xcjRtT2FWbXRtN1FiQXhLbW9QaVpPMTdVZkx6ek9UczZWOHh0aE5jWXFa?= =?utf-8?B?c0ZpZ05kRm1hK3VnU2phTkVUQTlTcWdyL0lYTk1neGpqZ3hBVXZkaGFHeXcx?= =?utf-8?B?TnpZR0VVKzJyMGhiSVBEbEQxQ1AvSWxwZzVsTlowT0xzNmJQeUE4OStta25m?= =?utf-8?B?UjhDTXdEWFgzT2dUQ2pwT0lBRldPL3ZsTUNpZWtubTdMazR5MWxDUkRnOUpG?= =?utf-8?B?c2piME51d0o1T1BwYVFOUlRqaE9hWnBqYURuaFRyZmMxMWxFaWtMeXgzZDdV?= =?utf-8?B?K1BTWmhtdzloa0Z4UzAyK2FNbUtTSVhkdmJvcG4rdXhMeFBLT2F1L1oxeFFG?= =?utf-8?B?cHRac0RjUk9pVHZ3cG9mNktLSUR0cWN2OXhRaWYwNG14a0NhL3hLVUxwRUFu?= =?utf-8?B?WWZFeDRzM1BMcFl3NjkwNFljbEplRmdHODB5N3FwSFVoRVA0SUczSmhpbW4z?= =?utf-8?B?NkxMbjliNGdEVkhHVXlNL1oyczNEZWxjR0FaRVVvaUUzQTAwOEtnSHFxVVU2?= =?utf-8?B?dTZ5OUx2QTQyYlFucnkrZWx6eVl4alNtRjJHTFJxdVdkR1UwcVlPS24vNWFh?= =?utf-8?B?VkRzbHA3cDQwMkpaMU5KTXhYeXR4VUlkSkxzbFlMaEFweW1PcWI2ZE5sZWpw?= =?utf-8?B?SFpxOEJscFVJTjFJOUR5VmQydkJhbHhxS2JST2NPV1VLM1RwTXZhSDlFTGJr?= =?utf-8?B?OVk0U0V6NVRNWHlrQzM2QVBGMDJlZTkwekdMQnFYQXBjSHphcjB1ZWVDWHNJ?= =?utf-8?B?RkY2dDFWMmRGVEhZakJqZjVEU2Q5OFRFZnphQ2NISXFtTEhzMHNOSzVKWkpx?= =?utf-8?B?MVpoeXhybmEzMHpLdVBwZmVYek5BZnc1a3ZnNDBycUhKNGFKQ0V6VitZOXp6?= =?utf-8?B?dm43dk9JZDh1ZWxVU1gvQW83a0xQRm5DWWtwWGZ5TlZsZU1EZDhKTWU0VUtZ?= =?utf-8?B?Q3FUeHZkUnVEb1lsalNPcWp6dlc5cmRsMlZUT3pQdlJSSm4rVUFNNlVSNkYx?= =?utf-8?B?Mm52TnQ4UWhxVWJZZEs1d3U3dTM0cGVSbjdqd291MWpPdXNPeDNBTVdRYm5p?= =?utf-8?B?WTZQdDZLQTNJaGpJRC94TzVKN1FGbkhUN3NhbmZKYkE4TTR5aWNqNDhKWUhX?= =?utf-8?B?c2V4UEw2UlhHdDVnUFFVY3QyekZBM2p1Uk9oWHhmeG9uemJCK0RpTytvbjRi?= =?utf-8?B?SGFLa3RKbnB0MVNhakFFeWJwYkNtWlRxcUQ4TUw0L3d5RVNZZUwrOUN3Z2RL?= =?utf-8?B?anQvWW5zNGpJR0I3UStueWllV1JTSVQ0Ulh1WHVRNDhXcnk0VFNxR2hTaHF5?= =?utf-8?B?dFh4a0xyaU9LQTlHQ3NUOVBKaEZOV3c2SXZWR09pd0tyWVNKYW1FWXBPT3hk?= =?utf-8?B?eUNPblh1KzNtTWZJNXNuLzE2bjBYRVV6cW9ISDNsOWNyOXBUMEdsdlFJak1j?= =?utf-8?B?dEhCcmFSNDlqN3pzVlJoUGtzaFlSSjJyR0F3NDFhZFVLTXMrNC9kcGRiVTRz?= =?utf-8?B?YnNlamg2eWVYZUM0OHBUY3BVOWhmZFlSYWRDamFHOXlYUk1idE4xRVorYWM0?= =?utf-8?B?ZHc4bjhiWmhqdEs1V3BhWE15ZE9QeDBPQ3R5RXFKNXNCWTdNaEhaZGZoMzNJ?= =?utf-8?B?ZkZxdUVZRE5IZ3pQd1JLbHFubmg0aEhscVRoVEkvRHFhejgxZWZ3ZllHTUlv?= =?utf-8?B?Rnl3WFp1K3Zna0lZUkwvb1piNy9yekpkWktFZCtsdFVCaERpa2E1cUttZ1Jh?= =?utf-8?B?TVVYT1E2SDJvTXlLYjFpTHNrZHpMZ295YWhYaGFXM2I2WUxnSEdNUlZScHdj?= =?utf-8?B?OEJvTnV0cGJHdGVGNEZET3FQcFRyM3pML3dHZCtiQ1B1Y2NtcDZiZGFVTzR0?= =?utf-8?Q?MpsNIWvKJ+DOv9GuJAFl4qc=3D?= X-OriginatorOrg: efficios.com X-MS-Exchange-CrossTenant-Network-Message-Id: deddaf67-f876-400a-9af4-08ddf156c376 X-MS-Exchange-CrossTenant-AuthSource: YT2PR01MB9175.CANPRD01.PROD.OUTLOOK.COM X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 11 Sep 2025 17:15:10.6992 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: 4f278736-4ab6-415c-957e-1f55336bd31e X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: 4jeePBuwmp6QUuycgBNVuc+k1DMMU4C++zA3VZi+g7yeio2P09cQGTR9gM7k90RQIRyAGXBpH2umJ99oUYaWHBwI+K8gbpOr5/0pW4h4pP0= X-MS-Exchange-Transport-CrossTenantHeadersStamped: YQBPR0101MB5639 On 2025-09-11 12:06, Thomas Gleixner wrote: > On Thu, Sep 11 2025 at 10:03, Mathieu Desnoyers wrote: >> On 2025-09-08 17:32, Thomas Gleixner wrote: >>> static inline void rseq_handle_notify_resume(struct pt_regs *regs) >>> { >>> - if (current->rseq.event.has_rseq) >>> + /* '&' is intentional to spare one conditional branch */ >>> + if (current->rseq.event.sched_switch & current->rseq.event.has_rseq) >> >> I wonder.. except for the corner case of rseq unregistration, >> when can we have sched_switch set but not has_rseq ? >> >> We could remove a load from the fast path and the AND if we >> clear the sched_switch flag on rseq unregistration. > > We probably could. Though I doubt it matters much and I opted for > correctness instead of premature optimization. Note that this concerns code that documents an explicit " & " as intentional to spare a branch, which led me to assume that optimizing it was important. Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. https://www.efficios.com