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Wed, 10 Jun 2026 07:09:32 +0000 Message-ID: <8bd94a5f-1460-4bc3-a2b2-68a298200ad8@intel.com> Date: Wed, 10 Jun 2026 15:09:22 +0800 User-Agent: Mozilla Thunderbird Subject: Re: [RFC] mpam,x86,fs/resctrl: Generic schema description Proof of Concept To: Reinette Chatre CC: Borislav Petkov , Thomas Gleixner , "Dave Hansen" , Peter Newman , "x86@kernel.org" , "linux-kernel@vger.kernel.org" , Ben Horgan , Tony Luck , Dave Martin , James Morse , Drew Fustini , Babu Moger , Fenghua Yu , Chen Yu References: <29c95b69-e1a4-46b1-ab8b-45c09308b924@arm.com> <7f2d0cc3-266c-4fa3-a9ba-7352325c76d1@intel.com> <3ef279c9-c4b3-48cb-8235-da8a871b54f9@arm.com> <5cffdb88-924e-43e7-84f2-ea19d82910fb@intel.com> <0c957e0a-0188-40c0-96e6-4e5d787a8ae6@arm.com> Content-Language: en-US From: "Chen, Yu C" In-Reply-To: Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 8bit X-ClientProxiedBy: TPYP295CA0023.TWNP295.PROD.OUTLOOK.COM (2603:1096:7d0:a::7) To DM4PR11MB6020.namprd11.prod.outlook.com (2603:10b6:8:61::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: DM4PR11MB6020:EE_|SABPR11MB9939:EE_ X-MS-Office365-Filtering-Correlation-Id: 80946457-03e8-4c9f-7972-08dec6bf3880 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|23010399003|376014|366016|1800799024|7416014|3023799007|18002099003|22082099003|6133799003|56012099006|11063799006|4143699003; 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Just going by >>>> the MPAM specification, a system keeping MIN at 0 and just setting MAX from MB, >>>> (MIN=0, MAX=MB) should behave the same as one always setting both, (MIN=MB, >>>> MAX=MB). In the MIN=0 case there is never any high preference traffic and in the >>>> MIN=MAX_MB case there is never any medium preference traffic. It seemed best to >>>> not rely on any platform specific heuristics to try and guess what's better and >>>> just wait til the time we could support MB_MIN in resctrl (and leave the >>>> decision up to the user). My expectation was that this would be the simplest >>>> course of action. >>> >>> This sounds fair. Two observations: >>> - The hierarchy exposed by resctrl may be different on systems that have the "same" >>> controls. >>> For example, on an MPAM system (if I understand correctly) the user may see: >>> info/ >>> └── MB/ >>> └── resource_schemata/ >>> ├── MB/ >>> │   └── MB_MAX/ >>> └── MB_MIN/ >> >> Yes, this matches my understanding. >> >>> >>> Compared with a possible implementation on Intel that looks like: >>> info/ >>> └── MB/ >>> └── resource_schemata/ >>> ├── MB/ >>> │   └── MB_OPT/ >>> ├── MB_MAX/ >>> └── MB_MIN/ >> >> Not sure if my understanding is correct here... >> In the kernel today is it rdt max that backs MB? (Ignoring the sw controller) > > resctrl does not have support for the RDT "MAX" controller yet. Since resctrl was > created as part of enabling RDT the resctrl MB control maps exactly to RDT's > original percentage based memory delay value that is an approximate. Newer hardware > support three controls: optimal, minimum, and maximum. These controls have finer > granularity than what the default percentage based control supports so emulation > is needed. > So far I assumed that on these systems the default MB control would be emulated > by the new "optimal" control but after these exchanges I can see there being an > argument for it to be emulated by the new "maximum" control also. Apart from it > implying a cap there is also the idea that the "maximum" control is more likely to > be available on all platforms. > Regarding the region-aware RDT case, I wonder if we actually need to emulate the legacy MB control using MB_MAX. First, when we refer to the "legacy" for region-aware RDT, I suppose it corresponds to "MSR access" plus "percentage-based control". case 1: If the platform does not support region-aware RDT (no ERDT table is detected), the MB is naturally the "legacy" MB, and the info directory would look like: info └── MB └── resource_schemata └── MB case 2:If the platform supports region-aware RDT (i.e., ERDT parsing succeeds), then the structure looks like below: info └── MB └── resource_schema └── MB <=== legacy └── MB_REGION0_OPT └── MB_REGION1_OPT └── MB_REGION0_MIN └── MB_REGION1_MIX └── MB_REGION0_MAX └── MB_REGION1_MAX The OS supports runtime switching between the legacy MB and region-aware MB, via a MB scope control like: /sys/fs/resctrl/info/MB/resource_schemata/mode [legacy] native If the user wants to use the legacy interface, the system will fall back to the default [MSR-based, percentage]; if the user wants to use the new interface, the region-aware interfaces will be used. Since the two modes are mutually exclusive, there seems to be no need to use MB_REGION0_MAX to emulate legacy MB. thanks, Chenyu