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b=GjFNhMyEpcfDf5d+h4efUchZW5E07M1C3iJM3cQ86zGjIG5u5fx91VzpUZPViFtv65+Sqj1grS/NvycAAf8tWIh2HqJdVWhhSrqKyG5rIjNwcITQdcbPcjHdGkZ53ko66WCevxAK8VWJmvYC2oW0h08/Vsbjuoj4zhmxF4spB2w= Authentication-Results: dkim=none (message not signed) header.d=none;dmarc=none action=none header.from=amd.com; Received: from BL1PR12MB5320.namprd12.prod.outlook.com (2603:10b6:208:314::17) by DS2PR12MB9688.namprd12.prod.outlook.com (2603:10b6:8:27d::6) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.21.245.13; Tue, 28 Jul 2026 19:46:38 +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 15.21.0270.009; Tue, 28 Jul 2026 19:46:37 +0000 Message-ID: <2c098892-5d4b-402b-bdfb-59e3bf164c8d@amd.com> Date: Tue, 28 Jul 2026 14:46:34 -0500 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/2] x86/resctrl, Documentation: Keep mbm_assign_mode "default" on boot To: Borislav Petkov Cc: tony.luck@intel.com, reinette.chatre@intel.com, x86@kernel.org, Dave.Martin@arm.com, james.morse@arm.com, corbet@lwn.net, skhan@linuxfoundation.org, tglx@kernel.org, mingo@redhat.com, dave.hansen@linux.intel.com, hpa@zytor.com, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, eranian@google.com, peternewman@google.com References: <8cb66e18e32e4087a9712c1e68ee6da614efe244.1784322818.git.babu.moger@amd.com> <20260728191818.GBamkAeh7BD1d6Gd8X@fat_crate.local> Content-Language: en-US From: Babu Moger In-Reply-To: <20260728191818.GBamkAeh7BD1d6Gd8X@fat_crate.local> Content-Type: text/plain; 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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. >> >> Avoid this compatibility issue by leaving mbm_assign_mode in the "default" >> mode during initialization. > > Question: why doesn't the default mode suffer from this counter exhaustion? Or > if it does, why is the whole issue better in the default mode? > ABMC mode (also referred to as mbm_event mode) relies on ABMC counters, and this system provides only 32 such counters. By comparison, the default mode uses RMIDs, with up to 4096 RMIDs available. However, even though the RMID space is much larger, the hardware can track only 64 RMIDs concurrently. As a result, if pqos attempts to monitor more than 64 groups, the reported bandwidth measurements may become inaccurate. That said, I have not been able to reproduce any bandwidth inaccuracies on the systems I tested. My point is that this may not be a significant issue for the vast majority of users. The possibility of reduced accuracy when monitoring a large number of groups is the primary concern that Reinette has raised. Thanks Babu