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Fri, 19 Dec 2025 23:05:36 +0000 Date: Fri, 19 Dec 2025 15:05:34 -0800 From: "Luck, Tony" To: Reinette Chatre CC: Aaron Tomlin , "Moger, Babu" , "Dave.Martin@arm.com" , "james.morse@arm.com" , "babu.moger@amd.com" , "tglx@linutronix.de" , "mingo@redhat.com" , "bp@alien8.de" , "dave.hansen@linux.intel.com" , "sean@ashe.io" , "linux-kernel@vger.kernel.org" Subject: Re: [PATCH v2 3/3] x86/resctrl: Add "*" shorthand to set minimum io_alloc CBM for all domains Message-ID: References: <20251215230257.1798865-1-atomlin@atomlin.com> <20251215230257.1798865-4-atomlin@atomlin.com> <95d219be-c89b-431a-974a-2e2b653f98e4@amd.com> <83cd3fe0-2dc6-4456-b845-4072b7ecb2a2@intel.com> <9967e08c-3aa4-47fc-9846-7f36a858df41@intel.com> Content-Type: text/plain; charset="us-ascii" Content-Disposition: inline In-Reply-To: <9967e08c-3aa4-47fc-9846-7f36a858df41@intel.com> X-ClientProxiedBy: SJ0PR03CA0289.namprd03.prod.outlook.com (2603:10b6:a03:39e::24) 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_|BL3PR11MB6410:EE_ X-MS-Office365-Filtering-Correlation-Id: f5f3ef19-445f-4248-4ece-08de3f531ed9 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|366016|376014|7416014; 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X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: =?us-ascii?Q?Gsrdf2RoNHu5Q78Qv1Wpk0uHabdAi1bGt/T0NZK2giQKDRgHZHG/cR/lbygY?= =?us-ascii?Q?FEhEOruvlPGp/OYJIbwe0wurlY6aScYWsFieO2n5BUsuBAn4796YJ9lz/yGJ?= =?us-ascii?Q?4GiYLDIO8tm3RbZQpM8QQXtYdv8IMorjh9zuONwthfnUEoVZbBMaxlrv1wEs?= =?us-ascii?Q?0YaWyohr1R2//2QEjSrd1VT3yDlO5CbG+Rcjberm1S84DtBZ3jwz/nxVYfCL?= =?us-ascii?Q?fu/pYYHS2bT6CYVR5Bkrvq6zcWCd0SUh7Tj7uS1jJmS7nCAqj9I3kDXspAWB?= =?us-ascii?Q?EeLcMoBVkd3DPJCDPxvOTs5dTEiuYRx4Dtosvp6zrxFQX/w4fKwMLWIuRY0p?= =?us-ascii?Q?arHHAbZb/NxqZ8s74zu4kLPpKe0gbVn6ePfOWYb97uYxCCZAfuXxdsrvAPQx?= =?us-ascii?Q?sag9lH4VuAj6pEFezh1iCungZ93byKEcPk6ml1pyb8gieXXAdF8yEsVl4uWJ?= =?us-ascii?Q?K0azicjRS/nHC83E/kKLWMhNmlw4uUEJEFfQIHwDkun6kFgxdaz5JEH42/JF?= =?us-ascii?Q?CIl6akFn8Emy0XvaZfdLXVRj9x4iRMBKrLYma4/YrD4uqFinqiyKnh45lhZA?= =?us-ascii?Q?TRmKu6wW/mxLzv/BEoX619HAKNgQPFCIYxw4ej7Hym0ax0TVFO+8HNYQD1ov?= =?us-ascii?Q?qNVZ7GbfmMALyU+kxzBFg+IpdC19b3ZXnpDeT0SI0y7E5shbgxKI/SIDYw9E?= =?us-ascii?Q?4L4xcPEM5sAfR3tvf67rtsQTGYBr4IX/1hS3Wd8kmIM942UtUuuqv3SNSsd6?= =?us-ascii?Q?U8UUA0kOEDKF9/J+5nlZZFfuYwEGwYt+VCv7D2EA61Vya9fddiCl9g86Scp1?= =?us-ascii?Q?Ne7SIXAnL0EkQ8KxC+X2QNvSLOFXIalLFWyUtzRqaugPBpNwg9ITU8yI0LHh?= =?us-ascii?Q?fwhTcXHNSOgfGrpF+cCy+YZhCMY96qbgA5znqoRCpSgq5b3kuDqXMKx+zSrL?= =?us-ascii?Q?Hvnwy+Wpk4QDQgjRupQ0QOutFATZbuVgsFLLHLA9//lsrm+BvFsfZJ+OcC/J?= =?us-ascii?Q?gqQHtHBwM/GY2af5W3O15bpsMQaJ6rSyk89ixndV9deLV3M5TrYJjd52Vtjd?= =?us-ascii?Q?/aOlsomxGit/CP7oV7q6oXMm14Anjhqs3TfY7vg30DMeU/hXhltP5/BmMGCo?= =?us-ascii?Q?mUGTHV1d+sFnJGkSKyEkG2V5Ja6t9kuDCORnIj+4/E0gJKFfHSKppStkMJL6?= =?us-ascii?Q?ITp8A2mR99UvoLaGLOwEP75ETS9vTvbPlP+3ljASb+8iwObNuzGL2FsgEKxi?= =?us-ascii?Q?kO/WNFNrWEaRs4ge8Hexd9YZYC7w2yqUlDYGQKwhlmuGNw8rbWMHbFQVvGuL?= =?us-ascii?Q?SU3j9KIN14jzZoueFwhSCm4ogPvlziKKR7FyJ8uaCucIhw9S6pUi/6a4gb6o?= =?us-ascii?Q?SpRu8TYmowCgjSYRbnLYNWs6+bnRWrkO6TtQmRBAP2H2FWQ3uMiLqtpggURV?= =?us-ascii?Q?Hx059G28nu/O1JdkyOyNI3Nl5Dn28CA50XPUsmj0FqNAQIJVwJO/XmdQ06vX?= =?us-ascii?Q?LloQOUcEIjwrGBP4Vbu4s2FqTBjLESmb+Ia7XW7NbuLU3ffvttjcaPXbGR7s?= =?us-ascii?Q?WX1Y42e9h0ncscIAFt29cNuq6T0mmtCdfSLyzxU0qzvdxA4d5ueCaNmLxBin?= =?us-ascii?Q?jlmAF4wqoNRtn5iCjC06hAb8CW5Qpu6jPz3rXawYv40oFLiozqAGWB8CsySb?= =?us-ascii?Q?fwP5BY/bBCOIpJJMFz7pcq0icZFs5DHHC/wItN7rMRea0MWNk7kYE2Gp7NYb?= =?us-ascii?Q?fynmY6Mopg=3D=3D?= X-MS-Exchange-CrossTenant-Network-Message-Id: f5f3ef19-445f-4248-4ece-08de3f531ed9 X-MS-Exchange-CrossTenant-AuthSource: SJ1PR11MB6083.namprd11.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 19 Dec 2025 23:05:36.6879 (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: P+G6CnmjZ82evX6m66LA40SzyidAYLvKSGYrFF/taCgwlxPssr7UbKoK5Xy93VsopIFn61pthCHu/gM3HOs1pA== X-MS-Exchange-Transport-CrossTenantHeadersStamped: BL3PR11MB6410 X-OriginatorOrg: intel.com On Fri, Dec 19, 2025 at 02:00:32PM -0800, Reinette Chatre wrote: > Hi Tony, > > On 12/19/25 12:42 PM, Luck, Tony wrote: > > On Fri, Dec 19, 2025 at 10:28:32AM -0800, Reinette Chatre wrote: > >> Hi Aaron and Tony, > >> > >> On 12/18/25 4:44 PM, Aaron Tomlin wrote: > >>> On Thu, Dec 18, 2025 at 02:59:59PM -0800, Reinette Chatre wrote: > >>>> If I remember correctly the idea was to limit this feature to io_alloc to > >>>> avoid needing to deal with L2 asymmetric domains [1]. > >>> > >>> Hi Reinette, > >>> > >>> You are quite right; I am in complete agreement with your assessment. The > >>> primary intention behind limiting the scope to io_alloc was indeed to avoid > >>> the complexities associated with L2 asymmetric domains. > >>> > >>> Are we all in alignment to focus this feature entirely on io_alloc for the > >>> time being? If so, I will be pleased to prepare a follow-up series that > >> > >> From my side I am ok to limit this to io_alloc. Of course, this does not prevent > >> cache allocation from supporting this syntax in the future. > >> > >> Tony: did you perhaps imply with examples in [2] that '*' only be supported by > >> L3 and MB, not L2? Can it be guaranteed that L3 will never be asymmetric? Not that > >> it is a blocker though, as discussed earlier there are ways [3] to support > > > > I'd forgotten the L2 asymmetry issue. If we wanted to enable "*" more > > generally, then resctrl would have to limit it to symmetric resources > > or to allow setting values that work for all domains in an asymmetric > > resource). But that seems more complexity in the kernel for something > > than can easily be handled in user space. E.g. to reset L3 to ffff > > > > # sed -n -e '/L3:/s/\(=[0-9a-f][0-9a-f]*\)/=ffff/gp' schemata > schemata > > ack. > > Can I interpret this as you being ok to limit support for '*' syntax to io_alloc (for now?)? > As I understand the intended implementation discussed here will indeed just allow setting values > that will work for all domains ... all or nothing. So, if L3 does become asymmetric along the > way then the implementation does seem to remain reasonable. I still don't see a good reason for the kernel to handle any of this. Having some resctrl control files accept wildcards while others don't seems like a confusing interface. Is there something I'm missing that makes io_alloc harder for users to handle than L3 or MB in schemata? Babu's simpler implementation for io_alloc makes me more comfortable with this. But I'm still unconvinced that wildcards must be handled in kernel code. > > >> '*' when domains may be asymmetric. That proposal is only reasonable if considering the > >> feature as "let user set same CBM on all domains" that just happens to support the "reset > >> to min" use case for L3 io_alloc. I assume even on asymmetric domains the min would be the > >> same? If there is indeed a requirement to support "reset to defaults" for general cache > >> allocation then this feature would not work for asymmetric domains as you highlighted in [4]. > > > >> Although, a "reset to defaults" for cache allocation use case may be better handled by > >> removing and recreating the resource group since the defaults will take into account > >> any exclusive allocations? > > > > Removing the directory would free the RMID and allocate a new one when you > > recreated it. Losing cache occupancy information completely, and disturbing > > memory bandwidth monitoring. Also leaving the user to hunt down tasks > > that were reassigned to the default CLOS and reassign them. So too many > > side effects. > > I agree. Even so, without knowing more details about use case it is difficult to reason about > user space expectations here. This is regarding a "reset to defaults" use case that was raised in > [4]. Did you raise that concern based on insights into requests from users to support this? > I think we would need to create new syntax if resctrl needs to support such use case. I don't have any requests from users. > > Reinette > > > >> > >> Reinette > >> > >> > >>> reflects this consensus once our wider discussion has concluded. > >>> > >>>> > >>>> [1] https://lore.kernel.org/lkml/SJ1PR11MB60833A27A1B8057CDDFB1B2BFCCFA@SJ1PR11MB6083.namprd11.prod.outlook.com/ > >> [2] https://lore.kernel.org/all/SJ1PR11MB6083CCA2A7904E459B1AA35DFCA8A@SJ1PR11MB6083.namprd11.prod.outlook.com/ > >> [3] https://lore.kernel.org/lkml/f4a043d2-9cb0-41c9-a45d-31f96fd007d5@amd.com/ > >> [4] https://lore.kernel.org/lkml/SJ1PR11MB60836AB4270419338FBB4D1EFCCEA@SJ1PR11MB6083.namprd11.prod.outlook.com/ > > > > -Tony > -Tony