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Wed, 7 Jan 2026 22:27:57 +0000 Date: Wed, 7 Jan 2026 14:27:56 -0800 From: "Luck, Tony" To: Reinette Chatre CC: Borislav Petkov , Fenghua Yu , "Wieczor-Retman, Maciej" , Peter Newman , James Morse , Babu Moger , Drew Fustini , Dave Martin , "Chen, Yu C" , "x86@kernel.org" , "linux-kernel@vger.kernel.org" , "patches@lists.linux.dev" Subject: Re: [PATCH v17 13/32] x86,fs/resctrl: Add an architectural hook called for each mount Message-ID: References: <20251217172121.12030-14-tony.luck@intel.com> <20260105191711.GBaVwON5nZn-uO6Sqg@fat_crate.local> <20260105200435.GCaVwZU2gFV3LhJnMR@fat_crate.local> <4525e857-c52a-4e5d-bd74-120f66a707e3@intel.com> Content-Type: text/plain; charset="us-ascii" Content-Disposition: inline In-Reply-To: X-ClientProxiedBy: BYAPR21CA0019.namprd21.prod.outlook.com (2603:10b6:a03:114::29) To SJ1PR11MB6083.namprd11.prod.outlook.com (2603:10b6:a03:48a::9) 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: SJ1PR11MB6083:EE_|LV2PR11MB5975:EE_ X-MS-Office365-Filtering-Correlation-Id: cd225abf-0117-41db-f28f-08de4e3c023f X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|366016|7416014|376014; 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X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: =?us-ascii?Q?jd6Vrm8clJL94dCOr9K6ofD8404qUwIfTxhR0oPbWwh1kgD1+DO8j3qEPvKk?= =?us-ascii?Q?LZT20IJKbkQRxEGD6HdPorgveMO3lLBAwakoVV9VylORwdCtXjf3jDyFaa8O?= =?us-ascii?Q?0awxq4QZY6bwTeHckUuTHP0PMCk9l5hkCR/937vnw3cKJVEo/caOIEuguJTr?= =?us-ascii?Q?onqtmqQ0K3vXg19E/yKhH3No9RsYK8dxHxnRBZu0q0o64VvrJmeF1WBUvgUO?= =?us-ascii?Q?A0zhIOxqjUpyBz0zLvN+3uAqieaZmrASNPLCKihMaPJaZ3VM56e6xUOid7vT?= =?us-ascii?Q?qeWnIs3xkIs4JcqbAaqBtq27QDGy94yVaYPXtaEjASsaQuEhUDt2RRBUPUDL?= =?us-ascii?Q?0U4wMRH7prd7QV4QXKXtRyvYE395nE+Q39R6V7Iyt4xpuO4qLkZ35ddt4n39?= =?us-ascii?Q?sJ/l0/Y3TOkZYdmG1fUq6qHZnrVw9KJYaXN0OBAF2V9y6zmFVMU/ZRGZv7Yj?= =?us-ascii?Q?I9yGpfrJQD5KUXwRBDWeH9wxmmn2jBH317WFcrcILrIpLsgwqAsWNYmPukZe?= =?us-ascii?Q?OCaBGz4Bq40BYkL1+X9kjuZZG18o5sNhWa1ZdB5CgAvjr+ESjfby1s647lVB?= =?us-ascii?Q?rdiYBt6ONx57xg0d5FaRtkBbSx3iH/P/RUUmlkAk2YTZOgXr/E1J5TJKtPAm?= =?us-ascii?Q?p8LxKH9pr3wxb4+Re4YfKeEAmyaN+PEVP9BcjkvvtcZANU/6RAMJk+KjauxR?= =?us-ascii?Q?CicJwfYsLFtoppaYZHGCmi8mEZZLpIc8N7y/cFEeIX8Ml0Ku+4Xod3IGK9IM?= =?us-ascii?Q?7LXY3L/xQ0x7WrIRTRS5PE6QIpZc7Jl9CptYqH0ztN/ag9MzFEdk3HA2k6cj?= =?us-ascii?Q?XXc03p5Uo+/g/+99VYFm5BWmMoxiAVYSK4jrc1L9c8gVUzZnYDU0eVob7qgj?= =?us-ascii?Q?yN6z6DSeCFoQNT0aKYcAW56YuJ64u3hsK5vU0JmRi0fEAMsrXs5uulqEch06?= =?us-ascii?Q?uEXv8Z3WuXhtgMYXkc7SvoW+m2XWLBHtUfz1OmkpQ+j/8PS8kit+M3ln4hnp?= =?us-ascii?Q?ZZRf5iE4fF5xl21dpxiGf69WgdcGKCkJ9rS7yFHuwqrgCTisfIkECgWTP8jE?= =?us-ascii?Q?ETORUkCPRXHJAQ6AlHPSkZeB7Dj+JIGilCfmKAvoGBLSefT+G3QR/AWSqfSS?= =?us-ascii?Q?Qlgrdt6a+kyFXf08t18ORMVLLdhByuoCRKPux634a2QtUDITrttAq9lpCuNL?= =?us-ascii?Q?JPoWTk0kDhB76WuXfgmcvmjmRk5cOoLmLk4GzM09Yc4GVZ60YdKJCqKkzngV?= =?us-ascii?Q?1Ho/dElCxaL5U0fK0iflLwMcGV1CdWjhnWSq6klQbqScUI3Ib7Zxq6zxyYTf?= =?us-ascii?Q?QiIgTui8K4ev+Yd9zX3Dkjqh8uMgRUSioeXMqfoDgqhVf4jkfrfKonwfTCav?= =?us-ascii?Q?1pdGfBHrgk6dR7AvIo4pDgjysodhwLcgxWi3AWaDrKrhpAMGr5vmtk+1R+y/?= =?us-ascii?Q?C9d8Kv0+Rddt456MytUcjKQIEGdGdDBb4+Sxa659lyuxoWUhiyNcVFjMUfEz?= =?us-ascii?Q?VEp99b02eA1216xBZl70oZ1ZGpkC+C9vkHk5bvH/og6h+FmtJgkgIx3iMjv2?= =?us-ascii?Q?HAM7+BM+Ic1abHi3lFIomfWgz8wGfCAvvdDNkaxAKlBqnbatJxqha2t6C/0X?= =?us-ascii?Q?3Ysxr0MX9Z3jPQvEj7FAQ1XRbP779+noN/60iv2mqQBzVey4Rwb13lHVSDBu?= =?us-ascii?Q?78fYOVumnAjnfYb+0roYqMG/fBdRVFt450MKAyLuIys8yFHLofHTdH4mQGyN?= =?us-ascii?Q?dJOcJjW5BQ=3D=3D?= X-MS-Exchange-CrossTenant-Network-Message-Id: cd225abf-0117-41db-f28f-08de4e3c023f X-MS-Exchange-CrossTenant-AuthSource: SJ1PR11MB6083.namprd11.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 07 Jan 2026 22:27:57.7642 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: 46c98d88-e344-4ed4-8496-4ed7712e255d X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: dxtYrs2nUzXDbYa+Zwqgnd7LHzYIECM7DcT6rYYaEuFQljRYpb7vlu0AvfMa0UyzTjw4CKbsmuPTu3V1BXqIbA== X-MS-Exchange-Transport-CrossTenantHeadersStamped: LV2PR11MB5975 X-OriginatorOrg: intel.com On Wed, Jan 07, 2026 at 02:09:35PM -0800, Reinette Chatre wrote: > Hi Tony, > > If these DO_ONCE macros are ever used heavily in run-time code, it might > > be better for once_lock and once_mutex to be statically defined in each > > invocation of the DO_ONCE() and DO_ONCE_SLEEPABLE() macros. But the fact > > that the static key protects the spinlock/mutex from being called may > > mean that it is practically hard to hit problems. > > Which problems do you have in mind? One problem I see is that since these "once" > functions are globally forced to be serialized this may cause unnecessary delays, > for example during initialization. I do not think this impacts the resctrl intended > usage since resctrl_arch_pre_mount() is not called during initialization and is > already ok with delays (it is on a "slow" path). Reinette Yes. Unnecessary delays due to serialization. But that only happens if the first call to a DO_ONCE*() instance overlaps with another first call. It might be quite hard to hit that during boot unless there are many uses of DO_ONCE*() Looking at this some more, DO_ONCE() is overkill for mounting resctrl. The static key part is there so that DO_ONCE*() can be safely used in some hot code path without adding overhead of checking some "bool done" type variable and branching around it. I don't see anyone except validation executing resctrl mounts at multiple times per second. But it does make the code easier to read with a single line with obvious meaning instead of multiple lines with declarations, initializations, and if () conditions. -Tony