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Received: from BL1PR12MB5320.namprd12.prod.outlook.com (2603:10b6:208:314::17) by IA1PR12MB7758.namprd12.prod.outlook.com (2603:10b6:208:421::16) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.21.569.12; Mon, 2 Nov 2026 19:54:18 +0000 Received: from BL1PR12MB5320.namprd12.prod.outlook.com ([fe80::1876:4a6d:2cf5:b8d1]) by BL1PR12MB5320.namprd12.prod.outlook.com ([fe80::1876:4a6d:2cf5:b8d1%5]) with mapi id 15.21.0569.011; Mon, 2 Nov 2026 19:54:18 +0000 Message-ID: <4d7fed02-5ec9-49b9-bc52-83571fd368c7@amd.com> Date: Mon, 2 Nov 2026 13:54:14 -0600 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v6 2/2] x86,fs/resctrl: Keep mbm_assign_mode in default mode at boot To: Reinette Chatre , tony.luck@intel.com, bp@alien8.de Cc: x86@kernel.org, Dave.Martin@arm.com, james.morse@arm.com, corbet@lwn.net, skhan@linuxfoundation.org, rdunlap@infradead.org, tglx@kernel.org, mingo@redhat.com, dave.hansen@linux.intel.com, hpa@zytor.com, kas@kernel.org, rick.p.edgecombe@intel.com, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, linux-coco@lists.linux.dev, kvm@vger.kernel.org References: <3ebc7028-c559-4add-800a-614b29d66ba3@intel.com> Content-Language: en-US From: Babu Moger In-Reply-To: <3ebc7028-c559-4add-800a-614b29d66ba3@intel.com> Content-Type: text/plain; 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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: Looks like Boris is taking care of adding the below text. > > ... > 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 > Thank you. Babu