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Mon, 19 Aug 2024 14:52:54 +0000 Message-ID: <96880c73-7f0b-4a62-8f9f-11042dec92c7@intel.com> Date: Mon, 19 Aug 2024 07:52:50 -0700 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v6 19/22] x86/resctrl: Introduce the interface to switch between monitor modes From: Reinette Chatre 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> <56aae68f-b590-42ec-a6b5-de3af97a8091@intel.com> Content-Language: en-US In-Reply-To: <56aae68f-b590-42ec-a6b5-de3af97a8091@intel.com> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 8bit X-ClientProxiedBy: MW4PR03CA0063.namprd03.prod.outlook.com (2603:10b6:303:b6::8) 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_|DM4PR11MB6261:EE_ X-MS-Office365-Filtering-Correlation-Id: 9eb9d29a-2cb0-4cc5-bd74-08dcc05e9b54 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|1800799024|376014|7416014; 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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? To expand on this we may have to rework the interface if the counters can be assigned to events other than MBM. James: could you please elaborate how you plan to use this feature and if this interface works for the planned usage? Peter: considering the previous example [1] where soft-ABMC was using the "mbm_control" interface I do not think it is ideal to only use the "t" and "l" flags while llc_occupancy is also enabled/disabled via this interface. We should consider (a) renaming the control file to indicate larger scope than MBM, (b) add flags for llc_occupancy. What do you think? I believe this is in line with stated goal from [1]: "I believe mbm_control should always accurately reflect which events are being counted." Reinette [1] https://lore.kernel.org/lkml/CALPaoCi1CwLy_HbFNOxPfdReEJstd3c+DvOMJHb5P9jBP+iatw@mail.gmail.com/