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Mon, 18 Nov 2024 22:07:33 +0000 Message-ID: Date: Mon, 18 Nov 2024 14:07:29 -0800 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v9 08/26] x86/resctrl: Introduce the interface to display monitor mode To: , , , , , CC: , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , References: <121c79ea1abe6f21f726d086eba9fa61d31da3f1.1730244116.git.babu.moger@amd.com> <33cd0cc0-4f81-4a2d-a327-0c976219996a@amd.com> From: Reinette Chatre Content-Language: en-US In-Reply-To: <33cd0cc0-4f81-4a2d-a327-0c976219996a@amd.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit X-ClientProxiedBy: MW4PR03CA0165.namprd03.prod.outlook.com (2603:10b6:303:8d::20) 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_|SA3PR11MB7433:EE_ X-MS-Office365-Filtering-Correlation-Id: ae2336bb-2984-46a5-0e05-08dd081d66c9 X-LD-Processed: 46c98d88-e344-4ed4-8496-4ed7712e255d,ExtAddr 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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I wonder if we could perhaps always expose num_mbm_cntrs as a way for >> users to know if their platform may be impacted by this type of "unpredictability" (by comparing >> num_mbm_cntrs to num_rmids). > > There is some round about(or hacky) way to find that out number of RMIDs > that can be active. Does this give consistent and accurate data? Is this something that can be added to resctrl? (Reading your other message [1] it does not sound as though it can produce an accurate number on boot.) If not then it will be up to the documentation to be accurate. >>> + >>> + AMD Platforms with ABMC (Assignable Bandwidth Monitoring Counters) feature >>> + enable this mode by default so that counters remain assigned even when the >>> + corresponding RMID is not in use by any processor. >>> + >>> + "default": >>> + >>> + In default mode resctrl assumes there is a hardware counter for each >>> + event within every CTRL_MON and MON group. Reading mbm_total_bytes or >>> + mbm_local_bytes may report 'Unavailable' if there is no counter associated >>> + with that event. >> >> If I understand correctly, on AMD platforms without ABMC the events only report >> "Unavailable" if there is no counter assigned at the time of the query. If a counter >> is unassigned and then reassigned then the event count will reset and the user >> will get some data back but it may thus be unpredictable (to match earlier language). >> Is this correct? Any AMD platform in "default" mode may thus be vulnerable to >> "unpredictable" event counts (not just "Unavailable") ... this gets complicated > > Yes. All the AMD systems without ABMC are affected by this problem. > >> because users should be steered to avoid "default" mode if mbm_assign_mode is >> available, while not be made concerned to use "default" mode on Intel where >> mbm_assign_mode is not available. > > Can we add text to clarify this? Please do. Reinette [1] https://lore.kernel.org/all/35fc70fd-0281-4ac8-b32b-efa2f4516901@amd.com/