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Fri, 17 Jul 2026 22:56:41 +0000 Message-ID: Date: Fri, 17 Jul 2026 15:56:38 -0700 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/2] x86/resctrl, Documentation: Keep mbm_assign_mode "default" on boot To: Babu Moger , , CC: , , , , , , , , , , , , References: <8cb66e18e32e4087a9712c1e68ee6da614efe244.1784322818.git.babu.moger@amd.com> From: Reinette Chatre Content-Language: en-US In-Reply-To: <8cb66e18e32e4087a9712c1e68ee6da614efe244.1784322818.git.babu.moger@amd.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit X-ClientProxiedBy: MW4PR04CA0156.namprd04.prod.outlook.com (2603:10b6:303:85::11) To SJ2PR11MB8370.namprd11.prod.outlook.com (2603:10b6:a03:540::20) 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: SJ2PR11MB8370:EE_|SA2PR11MB5004:EE_ X-MS-Office365-Filtering-Correlation-Id: fea48001-521a-4dc4-b5b7-08dee456aa71 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|366016|7416014|376014|1800799024|23010399003|56012099006|11063799006|18002099003|22082099003|6133799003|10067099003; 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However, this can cause bandwidth monitoring > failures with existing userspace tools such as pqos. > > The pqos tool mounts the resctrl filesystem and creates 16 or more resctrl > groups by default. On systems with 32 or fewer ABMC counters, this default > configuration can consume all available counters, since each group requires > one counter for local MBM and another for total MBM. If additional > monitoring groups are created, counter resources are exhausted and pqos > tool reports memory bandwidth counters as zero for those groups. It is not obvious to me that this is a problem. If I understand correctly there are two scenarios possible with this pqos behavior: - ABMC is not in use ("mbm_assign_mode" is set to "default") - pqos can create 16 or more monitor groups - hardware still supports a limited number of counters with consequence that underlying counters reset at any time as the different monitoring groups need to be tracked. - pqos can read monitoring data of all 16 monitor groups, sometimes reading the events would return "Unavailable", sometimes reading the events return data. - *None* of the monitoring numbers returned are guaranteed to be accurate. - ABMC is in use ("mbm_assign_mode" is set to "mbm_event"): - pqos can create 16 or more monitor groups - only a subset of monitoring groups have counters assigned and these counters are guaranteed to only track the monitor groups/events they are assigned to - pqos can read monitoring data of all 16 monitor groups with two possibilities: - monitor group/event has counter assigned: monitoring numbers are guaranteed to be accurate - monitor group/event does not have counter assigned: monitoring numbers return 0 If my understanding is correct then the preference is to rather have wrong data than see 0? This does not sound right. What am I missing? The changelog starts with "this can cause bandwidth monitoring failures with existing userspace tools such as pqos". How could returning accurate memory bandwidth data be considered a failure? How does this issue manifest itself? > > Avoid this compatibility issue by leaving mbm_assign_mode in the "default" > mode during initialization. Users who want to use ABMC can continue to > enable it explicitly: > > echo mbm_event > /sys/fs/resctrl/info/L3_MON/mbm_assign_mode > > Update the resctrl documentation to reflect the new boot-time default and > adjust the mbm_assign_mode examples accordingly. > > Signed-off-by: Babu Moger > --- > There are plans to enable "mbm_event" by default once additional counters > are available. For now, keep the default mode to maintain compatibility > with existing tools. This is not ideal. resctrl should aim to provide a consistent user interface across kernel versions. Reinette