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Fri, 16 Aug 2024 18:09:16 +0000 Message-ID: <56aae68f-b590-42ec-a6b5-de3af97a8091@intel.com> Date: Fri, 16 Aug 2024 11:09:12 -0700 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v6 19/22] x86/resctrl: Introduce the interface to switch between monitor modes To: Peter Newman CC: James Morse , Babu Moger , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , References: <784eaa900b9e0778ddc534c04c7ded9466bfd19b.1722981659.git.babu.moger@amd.com> <983fded5-48f8-439d-8afe-45b60841985a@arm.com> <8c93c091-39b3-4219-b892-9df2747eb91a@intel.com> From: Reinette Chatre Content-Language: en-US In-Reply-To: Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 8bit X-ClientProxiedBy: MW4PR04CA0357.namprd04.prod.outlook.com (2603:10b6:303:8a::32) To SJ2PR11MB7573.namprd11.prod.outlook.com (2603:10b6:a03:4d2::10) Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: SJ2PR11MB7573:EE_|SJ0PR11MB5151:EE_ X-MS-Office365-Filtering-Correlation-Id: 1b812bae-1b92-4491-184a-08dcbe1e8a58 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|7416014|376014|366016; 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"soft-ABMC"?). >> >> This series adds support for ABMC on recent AMD hardware to address the issue described >> in cover letter. This issue also exists on earlier AMD hardware that does not have the ABMC >> feature and Peter is working on a software solution to address the issue on non-ABMC hardware. >> This software solution is expected to have the same interface as the hardware solution but >> earlier discussions revealed that it may introduce extra latency that users may only want to >> accept during periods of active monitoring. Thus the option to disable the counter assignment >> mode. > > Sorry again for the soft-RMID/soft-ABMC confusion[1], it was soft-RMID > that impacted context switch latency. Soft-ABMC does not require any > additional work at context switch. No problem. I did read [1] but I do not think I've seen soft-ABMC yet so my understanding of what it does is vague. > The only disadvantage to soft-ABMC I can think of is that it also > limits reading llc_occupancy event counts to "assigned" groups, > whereas without it, llc_occupancy works reliably on all RMIDs on AMD > hardware. hmmm ... keeping original llc_occupancy behavior does seem useful enough as motivation to keep the "legacy"/"default" mbm_assign_mode? It does sound to me as though soft-ABMC may not be as accurate when it comes to llc_occupancy. As I understand the hardware may tag entries in cache with RMID and that has a longer lifetime than the tasks that allocated that data into the cache. If soft-ABMC permanently associates an RMID with a local and total counter pair but that RMID is dynamically assigned to resctrl groups then a group may not always get the same RMID ... and thus its llc_occupancy data would be a combination of its cache allocations and all the cache allocations of resource groups that had that RMID before it. This may need significantly enhanced "limbo" handling? Reinette