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emacs 28.2 From: Alistair Popple To: "Huang, Ying" Cc: Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-cxl@vger.kernel.org, nvdimm@lists.linux.dev, linux-acpi@vger.kernel.org, "Aneesh Kumar K . V" , Wei Xu , Dan Williams , Dave Hansen , Davidlohr Bueso , Johannes Weiner , Jonathan Cameron , Michal Hocko , Yang Shi , Rafael J Wysocki , Dave Jiang Subject: Re: [PATCH RESEND 1/4] memory tiering: add abstract distance calculation algorithms management Date: Thu, 27 Jul 2023 13:42:15 +1000 In-reply-to: <878rb3xh2x.fsf@yhuang6-desk2.ccr.corp.intel.com> Message-ID: <87351axbk6.fsf@nvdebian.thelocal> Content-Type: text/plain X-ClientProxiedBy: SY2PR01CA0046.ausprd01.prod.outlook.com (2603:10c6:1:15::34) To BYAPR12MB3176.namprd12.prod.outlook.com (2603:10b6:a03:134::26) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: BYAPR12MB3176:EE_|PH0PR12MB7837:EE_ X-MS-Office365-Filtering-Correlation-Id: f807bd33-a63c-4c4a-6527-08db8e53dc44 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; X-Microsoft-Antispam-Message-Info: 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 X-Forefront-Antispam-Report: CIP:255.255.255.255;CTRY:;LANG:en;SCL:1;SRV:;IPV:NLI;SFV:NSPM;H:BYAPR12MB3176.namprd12.prod.outlook.com;PTR:;CAT:NONE;SFS:(13230028)(4636009)(346002)(396003)(39860400002)(136003)(376002)(366004)(451199021)(5660300002)(38100700002)(66556008)(83380400001)(8936002)(9686003)(7416002)(8676002)(6916009)(6486002)(316002)(478600001)(66946007)(41300700001)(54906003)(6666004)(6506007)(4326008)(26005)(186003)(2906002)(66476007)(6512007)(86362001);DIR:OUT;SFP:1101; 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I >>> don't think that it's good to make dax/kem to know every possible >>> types of memory devices. >> >> Do we expect there to be lots of different types of memory devices >> sharing a common dax/kmem driver though? Must admit I'm coming from a >> GPU background where we'd expect each type of device to have it's own >> driver anyway so wasn't expecting different types of memory devices to >> be handled by the same driver. > > Now, dax/kmem.c is used for > > - PMEM (Optane DCPMM, or AEP) > - CXL.mem > - HBM (attached to CPU) Thanks a lot for the background! I will admit to having a faily narrow focus here. >>> And, I don't think that we are forced to use the general notifier >>> chain interface in all memory device drivers. If the memory device >>> driver has better understanding of the memory device, it can use other >>> way to determine abstract distance. For example, a CXL memory device >>> driver can identify abstract distance by itself. While other memory >>> device drivers can use the general notifier chain interface at the >>> same time. >> >> Whilst I think personally I would find that flexibility useful I am >> concerned it means every driver will just end up divining it's own >> distance rather than ensuring data in HMAT/CDAT/etc. is correct. That >> would kind of defeat the purpose of it all then. > > But we have no way to enforce that too. Enforce that HMAT/CDAT/etc. is correct? Agree we can't enforce it, but we can influence it. If drivers can easily ignore the notifier chain and do their own thing that's what will happen. >>> While other memory device drivers can use the general notifier chain >>> interface at the same time. How would that work in practice though? The abstract distance as far as I can tell doesn't have any meaning other than establishing preferences for memory demotion order. Therefore all calculations are relative to the rest of the calculations on the system. So if a driver does it's own thing how does it choose a sensible distance? IHMO the value here is in coordinating all that through a standard interface, whether that is HMAT or something else. - Alistair