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Fri, 25 Oct 2024 22:00:02 +0000 Message-ID: Date: Fri, 25 Oct 2024 15:00:00 -0700 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v7 1/4] x86/resctrl: Make input event for MBA Software Controller configurable To: "Luck, Tony" , "Yu, Fenghua" , Peter Newman , Jonathan Corbet , Shuah Khan , "x86@kernel.org" CC: Shaopeng Tan , James Morse , Jamie Iles , Babu Moger , Randy Dunlap , "linux-kernel@vger.kernel.org" , "linux-doc@vger.kernel.org" , "patches@lists.linux.dev" References: <20241003191228.67541-1-tony.luck@intel.com> <20241003191228.67541-2-tony.luck@intel.com> From: Reinette Chatre Content-Language: en-US In-Reply-To: Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit X-ClientProxiedBy: MW4PR04CA0242.namprd04.prod.outlook.com (2603:10b6:303:88::7) 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_|PH0PR11MB5079:EE_ X-MS-Office365-Filtering-Correlation-Id: e4be587a-92b7-4824-809e-08dcf540603c X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|376014|1800799024|366016|7416014; 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But that got complicated because of the amount of state >> that needs to be updated when switching events. >> >> Could you please clarify which state you referred to? I wonder if it may be the >> struct mbm_state state maintained by mbm_bw_count()? mbm_bw_count() is lightweight >> and I see no problem with it being called for all supported MBM events when >> the software controller is enabled. With state for all supported events always available >> it seems simpler to runtime switch between which events guide the software controller? >> >> Thinking about it more, it seems possible for the user to use different >> MBM events to guide the software controller for different resource groups. >> >> If it is possible to do runtime switching in this way I do think it will simplify this >> implementation while not requiring the user to remount resctrl to make changes. You >> mentioned [3] that "a separate patch series" may be coming to address this but doing this >> now seems simpler while avoiding any future work as well as confusing duplicate ABI >> ... unless you were referring to other issues that needs to be addressed separately? > > Yes, the state maintained by mbm_bw_count() was the piece that worried me. After > a user switch to a different event there would be no bandwidth data until two updates > passed by (one to get a baseline, second to compute bandwidth). So update_mba_bw() > would need to be aware of this liminal period to avoid making updates with no data to > back them up. > > Your solution is elegant. The cost to maintain bandwidth data for each event is indeed > very low. So there are no weird transition cases. update_mba_bw() can immediately > compare bandwidth for the new event against the target bandwidth and make appropriate > adjustments. Thank you for considering it. > > This requires a new file in each CTRL_MON directory when mba_sc is enabled so > the user can make their selection. > > Note that technically it would be possible to make a different selection for each domain. > But that seems like an option without an obvious use case and would just complicate > the syntax of the new file. I did not consider this possibility. I agree with your assessment. > > Maybe name this new file "mba_sc_event"[1] with contents that match the names of > the mbm_monitor events as listed in /sys/fs/resctrl/info/L3_MON/mon_features? I do like that the content is connected to existing user interface by using events from mon_features. What do you think of connecting the filename to existing user interface (the mount option) also by, for example, being named "mba_MBps_event"? > > So default state when resctrl is mounted with the software controller enabled would > have: > > $ cat /sys/fs/resctrl/mba_sc_event > mbm_local_bytes > > User could switch to total with > > # echo mbm_total_bytes > /sys/fs/resctrl/mba_sc_event > > On systems where mbm_local_bytes is not supported default would be mbm_total_bytes. > > New CTRL_MON directories would also default to mbm_local_bytes if it is supported. This sounds good to me. Thank you very much for considering the change. Reinette