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Fri, 22 Nov 2024 21:33:14 +0000 Message-ID: <053297de-b2bb-4077-8700-997564f93fbf@intel.com> Date: Fri, 22 Nov 2024 13:33:12 -0800 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v9 2/9] x86/resctrl: Prepare for per-ctrl_mon group mba_MBps control To: "Luck, Tony" CC: Fenghua Yu , Peter Newman , Jonathan Corbet , , James Morse , Jamie Iles , Babu Moger , Randy Dunlap , "Shaopeng Tan (Fujitsu)" , , , References: <20241114001712.80315-1-tony.luck@intel.com> <20241114001712.80315-3-tony.luck@intel.com> <515914f2-501d-4df3-894a-2d255d18be1c@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: MW4PR04CA0302.namprd04.prod.outlook.com (2603:10b6:303:82::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_|CY5PR11MB6462:EE_ X-MS-Office365-Filtering-Correlation-Id: 469854aa-1ff3-440c-245f-08dd0b3d4530 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|366016|376014|7416014|1800799024; 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This means that this >>> mount option cannot be used on systems that only support monitoring >>> of total bandwidth. >>> >>> Prepare to allow users to choose the input event independently for >>> each ctrl_mon group. >> > Hi Reinette, > >> The lack of detail on design and implementation leaves a lot for the >> reader to decipher. For example, >> * the change appears to create a contract that rdtgroup.mba_mbps_event >> is only valid if mba_sc is enabled, this is "documented" in the >> structure member comment but not connected to the rest of implementation, not >> here nor later in series. > > I'll add text documenting this to the commit comment here, and also a > comment in the code that defines mba_mbps_default_event. > >> * the patch uses *three* different checks to manage new variables: >> is_mbm_local_enabled(), is_mba_sc(), and supports_mba_mbps(). Reader is >> left to decipher that all checks are built on is_mbm_local_enabled() >> and thus it is ok to use these checks before using the value that is only >> assigned when is_mbm_local_enabled(). > > With some refactoring I've got that down to just one new additon of > "is_mba_sc()" (protecting the assignment of rdtgrp->mba_mbps_event > in rdtgroup_mkdir_ctrl_mon(). > Just to clarify, I am not stating that there should not be three different checks. Clearly if some initialization is done before resctrl mount then the mba_sc checks may not apply. My goal was to highlight the investigations a reader is forced to do without guidance from the changelog. ... >>> @@ -3611,6 +3613,8 @@ static int rdtgroup_mkdir_ctrl_mon(struct kernfs_node *parent_kn, >>> rdt_last_cmd_puts("kernfs subdir error\n"); >>> goto out_del_list; >>> } >>> + if (is_mba_sc(NULL)) >>> + rdtgrp->mba_mbps_event = mba_mbps_default_event; >>> } >>> >>> goto out_unlock; >>> @@ -3970,6 +3974,8 @@ static void __init rdtgroup_setup_default(void) >>> rdtgroup_default.closid = RESCTRL_RESERVED_CLOSID; >>> rdtgroup_default.mon.rmid = RESCTRL_RESERVED_RMID; >>> rdtgroup_default.type = RDTCTRL_GROUP; >>> + if (supports_mba_mbps()) >>> + rdtgroup_default.mba_mbps_event = mba_mbps_default_event; >>> INIT_LIST_HEAD(&rdtgroup_default.mon.crdtgrp_list); >>> >>> list_add(&rdtgroup_default.rdtgroup_list, &rdt_all_groups); >> >> I do not see the default resource group's mba_mbps_event ever being reset. This means >> that if the user mounts resctrl, changes mba_mbps_event, umount resctrl, remount >> resctrl, then the default resource group will not have the default mba_mbps_event >> but whatever was set on previous mount. Is this intended? No mention of this behavior in >> changelog. > > Good catch. You are correct that a changed event value in the default > CTRL_MON group is preserved across unmount/remount. This was not > intentional. Moving the initialization of rdtgroup_default.mba_mbps_event > into set_mba_sc() fixes this (with the added benefit of removing the > supports_mba_mbps() check). I am curious how this will turn out considering the fragmentation of the rdtgroup_default initialization. Reinette