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Fri, 30 Oct 2026 22:08:12 +0000 Message-ID: <3ebc7028-c559-4add-800a-614b29d66ba3@intel.com> Date: Fri, 30 Oct 2026 15:08:09 -0700 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v6 2/2] x86,fs/resctrl: Keep mbm_assign_mode in default mode at boot To: Babu Moger , , CC: , , , , , , , , , , , , , , , References: Content-Language: en-US From: Reinette Chatre In-Reply-To: Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit X-ClientProxiedBy: MW4P222CA0017.NAMP222.PROD.OUTLOOK.COM (2603:10b6:303:114::22) To SJ2PR11MB8370.namprd11.prod.outlook.com (2603:10b6:a03:540::20) X-Mailing-List: linux-coco@lists.linux.dev List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: SJ2PR11MB8370:EE_|PH5PR11MB880693:EE_ X-MS-Office365-Filtering-Correlation-Id: dccf91c8-b0e8-42f2-255b-08df36d24a17 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|23010399003|7416014|376014|366016|1800799024|11063799006|4143699003|56012099006|261009223027099003|261026121527099003|261028144727099003|261021121427099003|261022075727099003|261022113327099003|10067099003|261026140227099003|261022105027099003|18002099003|22082099003|6133799003; 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Hardware counters are scarce on these platforms, so > mbm_event mode suits workflows that monitor a subset of groups at a time > and rotate assignments as needed. > > x86 resctrl enables the mbm_event mode by default on hardware that supports > it. This breaks the pqos tool [1], which assumes the historical default > mode. It creates 16 or more monitoring groups and uses two counters per > group. On a platform with 32 counters per domain, that exhausts the > mbm_event counters pool, causing additional groups to read "Unassigned". > pqos interprets a non-numeric event read as zero bandwidth and reports 0 > MB/s for those groups: > > $ sudo pqos -m all:0-4 > CORE IPC MISSES LLC[KB] MBL[MB/s] MBR[MB/s] > 0 0.69 16k 192.0 0.0 0.0 > 1 0.40 3k 0.0 0.0 0.0 > 2 0.44 1k 64.0 0.0 0.0 > 3 0.39 1k 64.0 0.0 0.0 > 4 0.43 1k 0.0 0.0 0.0 > > Leave mbm_assign_mode in default mode during initialization. Default mode > uses one counter per monitoring group. On existing AMD platforms, the > default active counters pool is 64 and may be larger on newer hardware. > The active counters pool is the number of counters that the hardware can > actively track in default mode. > > Deployments within the active counters pool receive accurate bandwidth > measurements in the default mode. Users can create more monitoring groups > (for example, 4096 on current hardware) than there are active counters in > the pool. Beyond that pool, readings may be misleading or report as > "Unavailable", with no user-visible indication. > > Default mode is still limited once the number of monitoring groups exceeds > that active counters pool. After hardware re-allocates a counter, a read > may return "Unavailable". pqos still treats "Unavailable" as zero. > Successive reads can be a count, "Unavailable", then another count. pqos > treats those jumps as wraparound and can report an inconsistent rate: > > $ sudo pqos -m all:0-4 > CORE IPC MISSES LLC[KB] MBL[MB/s] MBR[MB/s] > 0 0.76 60k 32.0 0.0 0.0 > 1 0.45 1k 32.0 0.0 0.0 > 2 1.57 107k 64.0 0.0 17592186044184.9 > 3 1.62 276k 4928.0 3.2 0.5 > > Users that need stable measurements beyond the active counters pool should > use mbm_event mode and rotate assignments as needed. Enable it with: > > # echo mbm_event > /sys/fs/resctrl/info/L3_MON/mbm_assign_mode > > Fixes: 0f1576e43adc ("x86/resctrl: Configure mbm_event mode if supported") > Closes: https://github.com/intel/intel-cmt-cat/issues/311 > Signed-off-by: Babu Moger > Cc: stable@vger.kernel.org > Link: https://github.com/intel/intel-cmt-cat # [1] > --- To help future readers understand how we landed on this I think it may be useful to add a link to the decision that resulted in this fix. For example: ... Leave mbm_assign_mode in default mode during initialization as decided in [2]. ... Link: https://lore.kernel.org/20260804191608.GDanI6eK2Q07w6pNcb@fat_crate.local # [2] | Reviewed-by: Reinette Chatre Reinette