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Mon, 29 Sep 2025 15:38:16 +0000 Message-ID: Date: Mon, 29 Sep 2025 08:38:13 -0700 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] fs/resctrl,x86/resctrl: Factor mba rounding to be per-arch To: Dave Martin CC: , Tony Luck , "James Morse" , Thomas Gleixner , "Ingo Molnar" , Borislav Petkov , Dave Hansen , "H. Peter Anvin" , "Jonathan Corbet" , , References: <20250902162507.18520-1-Dave.Martin@arm.com> <5be54a14-a7ba-49ba-8ddc-db532f2cf318@intel.com> <9dba03c5-cf45-4510-ab6c-2a945e73fd1c@intel.com> Content-Language: en-US From: Reinette Chatre In-Reply-To: Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit X-ClientProxiedBy: MW4P223CA0021.NAMP223.PROD.OUTLOOK.COM (2603:10b6:303:80::26) To SJ2PR11MB7573.namprd11.prod.outlook.com (2603:10b6:a03:4d2::10) 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: SJ2PR11MB7573:EE_|DM4PR11MB6502:EE_ X-MS-Office365-Filtering-Correlation-Id: d8dbfbd3-391a-4a10-0070-08ddff6e3503 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|7416014|376014|1800799024|366016; X-Microsoft-Antispam-Message-Info: =?utf-8?B?UXJuaElyd2NBRmRUTlVJQjZrUkM0UUZJR04yOTNmNzJKUjVmV2FhTFZ6ZW03?= =?utf-8?B?VnAwY1dXb1lUTmorTWZyYTlqZjZrM0hBaXdoV1VqanVGSEFXSVVZdERscTJq?= =?utf-8?B?TTRvRS9RekE5bFVLVCt4cXRiTnJtcVJldjJybHFSNkZyU2paeFdnRWtURUxB?= =?utf-8?B?enVlT2VXZTlLeDQwM25nTGc5NFRPZUcrYldtVTkrVzVIRFZhQWlqOGxXOHRa?= =?utf-8?B?L1FqTTVyNE5ORXQ2TFh5WmlZc0JaK0JGbkVGS1dTek9QT0RZTWIrMWVGUTBv?= =?utf-8?B?R3hqdWZhcW9EaDJHb1Q3ZEMxbkN0UkszMmVMMkk5R3ZtVit6WjVpZXI1Nk1G?= =?utf-8?B?NU5lcVloWjdIelc0aldrS1g0QXBjaWlyNWRpeFM0TkMvYVBNTk96M0JGQnNV?= =?utf-8?B?Q2pub2M5ZTg0V3VkZHA2YmVnMlBlaVdqQTA2RTViMlIzYWhXbHNvNlZhVDVK?= =?utf-8?B?dHN4dUVDZ08zSUN1ekIxckJUQUZuRW93aFRaMmNWczhOY3NMQUFjeEp2b2Zq?= =?utf-8?B?NkU1QzhFY1VnZnZMVDZhL1FqOEpQMFdKdGZRUWFheCtMUk56cTROOWNIVzFD?= =?utf-8?B?RDA2TklPNHIzNDdWSW9NR3N4ajV2bStvRVlkKzVaTFFydUR1dTFKV0RrMHRu?= =?utf-8?B?U2tPYVlYUEZ6M09YcWUwRnZHQmtKdWdWdXo2Z05BUnBOdzZHYnRuOEhkUm1P?= =?utf-8?B?QWhvci94QVN3QW9id3NEYktwUmFtMjZiWU1aTVEwamN2dHF0YnpBWTRqd3o0?= =?utf-8?B?OU9FY0lNcnh3Wk56b21XamRxWmJaVVVTUTkxRlpGK3VHcWdxMWdLc2w2cGVr?= =?utf-8?B?dFdFakVsRmtSaUdWanBqTGpFeHg3TCtVRXJkakJkbWpkZEpJMm11M0YwWnk2?= =?utf-8?B?c285a2JqUnNDeDA1UFFPak1pRTk4Wjl0L1c5YWFmNXRCeTVQcGx5YWYwYnA2?= =?utf-8?B?RDFNQmVIb2paWlVGdnI1YjNlR000VjY2QWU1VmdEc3RsV04rbGxBeFpKcm1X?= =?utf-8?B?bEdlc1IxakVnU0U1eFJlaVdaTW9nKzFnYWpVdkxQVGpjem81RGVTcHRzeVdz?= =?utf-8?B?MENGRks3ZUQ4R1BUQW5Iblh6aGtWa0F6ZGpWUW8xMW00TG1VNW10RzVkdnZY?= =?utf-8?B?dS95cnRCSXg2SFFwUnpQcThRL280a1REUXJYVDN6aTlFK05USWxvZmkyWEpu?= =?utf-8?B?UXQwaEFXSElpMUdMemg4Vk1yNHVmVUZhRmsvTFlvai9wSFRoQkR5aDRGc2dm?= =?utf-8?B?REpXd1VFRjBmMmo0UERnMlJoTTk0bXRnYXFSWC9oVEJOLzRpeFAzdUZ6R3JW?= =?utf-8?B?TVdIdllsczdZTXprMG03dWdZK3htMHFwNGdnQ3NaLy9uUld3TGtKR2VvTDN1?= =?utf-8?B?ZFo4SWFrd0dGVnM4cnZydHMzdEpXdkJWZlJ4VHZVeDJxQ3VNZTAySkl1OEFl?= =?utf-8?B?RDNmM0NTankrUzdQUFNobVNvOWdCRmVOZklCTHlXTUxIelhnRUJtNDNWSHYr?= =?utf-8?B?QnFWY1NvTXRhVTV3cGFObzVhenloOVhoZWhJWWJWakJmVC8ycGNZb2czQ045?= =?utf-8?B?WXBFQzNJZ2Y3c2pYbi84S0QzUmtPU0pkZDZSOStsTFdmMmwwdEh2Y0NpSmw0?= =?utf-8?B?Qk1JRStLaFFxTFdWT0RYdWxtLy9Fclc0RHNaZFd5Z29pTGdTd1BSZ3grZEkw?= =?utf-8?B?VzNqdHk5b2hmV0w0MU5na0tQdzllRTFmOWV0V1dXU1A5NldpVWtwRmZVelVz?= =?utf-8?B?dlFCaE1NSzFzOVIyWVRaUFZpWWwyT2IyNjcvM2Fuamo3aXJuMVZRWkRKNDN1?= =?utf-8?B?dW4wbjR4VG5Fd1E4dzJodlZjdWxDOFIvVXFlUGd5MXN2MkZoLy9jeW0vN05w?= =?utf-8?B?YXZ3K3BRUmVBQWVpaVJVSUZhRk8zbWM5bHYweFhyVXRkK2c9PQ==?= X-Forefront-Antispam-Report: CIP:255.255.255.255;CTRY:;LANG:en;SCL:1;SRV:;IPV:NLI;SFV:NSPM;H:SJ2PR11MB7573.namprd11.prod.outlook.com;PTR:;CAT:NONE;SFS:(13230040)(7416014)(376014)(1800799024)(366016);DIR:OUT;SFP:1101; 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Knowing this will help >>>> me to keep things consistent when the two worlds meet. >>> >>> I think this has just evolved over time. On the x86 side, MBA is a >>> specific architectural feature, but on the MPAM side the architecture >>> doesn't really have a name for the same thing. Memory bandwidth is a >>> concept, but a few different types of control are defined for it, with >>> different names. >>> >>> So, for the MPAM driver "mba" is more of a software concept than >>> something in a published spec: it's the glue that attaches to "MB" >>> resource as seen through resctrl. >>> >>> (This isn't official though; it's just the mental model that I have >>> formed.) >> >> I see. Thank you for the details. My mental model is simpler: write acronyms >> in upper case. > > Generally, I agree, although I'm not sure whether that acronym belongs > in the MPAM-specific code. > > For this patch, though, that's irrelevant. I've changed it to "MBA" > as requested. > Thank you. ... >>>> Considering the two statements: >>>> - "The available steps are no larger than this value." >>>> - "this value ... is not smaller than the apparent size of any individual rounding step" >>>> >>>> The "not larger" and "not smaller" sounds like all these words just end up saying that >>>> this is the step size? >>> >>> They are intended to be the same statement: A <= B versus >>> B >= A respectively. >> >> This is what I understood from the words ... and that made me think that it >> can be simplified to A = B ... but no need to digress ... onto the alternatives below ... > > Right... > > [...] > >>> Instead, maybe we can just say something like: >>> >>> | The available steps are spaced at roughly equal intervals between the >>> | value reported by info/MB/min_bandwidth and 100%, inclusive. Reading >>> | info/MB/bandwidth_gran gives the worst-case precision of these >>> | interval steps, in per cent. >>> >>> What do you think? >> >> I find "worst-case precision" a bit confusing, consider for example, what >> would "best-case precision" be? What do you think of "info/MB/bandwidth_gran gives >> the upper limit of these interval steps"? I believe this matches what you >> mentioned a couple of messages ago: "The available steps are no larger than this >> value." > > Yes, that works. "Worst case" implies a value judgement that smaller > steps are "better" then large steps, since the goal is control. > > But your wording, to the effect that this is the largest (apparent) > step size, conveys all the needed information. Thank you for considering it. My preference is for stating things succinctly and not leave too much for interpretation. > >> (and "per cent" -> "percent") > > ( Note: https://en.wiktionary.org/wiki/per_cent ) Yes, in particular I note the "chiefly Commonwealth". I respect the differences in the English language and was easily convinced in earlier MPAM work to accept different spelling. I now regret doing so because after merge we now have a supposedly coherent resctrl codebase with inconsistent spelling that is unpleasant to encounter when reading the code and also complicates new work. > (Though either is acceptable, the fused word has a more informal feel > to it for me. Happy to change it -- though your rewording below gets > rid of it anyway. (This word doesn't appear in resctrl.rst -- > evertying is "percentage" etc.) > >> >>> If that's adequate, then the wording under the definition of >>> "bandwidth_gran" could be aligned with this. >> >> I think putting together a couple of your proposals and statements while making the >> text more accurate may work: >> >> "bandwidth_gran": >> The approximate granularity in which the memory bandwidth >> percentage is allocated. The allocated bandwidth percentage >> is rounded up to the next control step available on the >> hardware. The available hardware steps are no larger than >> this value. > > That's better, thanks. I'm happy to pick this up and reword the text > in both places along these lines. Thank you very much. Please do feel free to rework. > >> I assume "available" is needed because, even though the steps are not larger >> than "bandwidth_gran", the steps may not be consistent across the "min_bandwidth" >> to 100% range? > > Yes -- or, rather, the steps _look_ inconsistent because they are > rounded to exact percentages by the interface. > > I don't think we expect the actual steps in the hardware to be > irregular. > Thank you for clarifying. Reinette