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Sat, 1 Mar 2025 02:35:05 +0000 Message-ID: Date: Fri, 28 Feb 2025 18:35:03 -0800 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v6 33/42] x86/resctrl: resctrl_exit() teardown resctrl but leave the mount point To: James Morse , , CC: Thomas Gleixner , Ingo Molnar , Borislav Petkov , H Peter Anvin , Babu Moger , , "D Scott Phillips OS" , , , , , , Jamie Iles , Xin Hao , , , , David Hildenbrand , Rex Nie , "Dave Martin" , Koba Ko , Shanker Donthineni , Shaopeng Tan , "Tony Luck" References: <20250207181823.6378-1-james.morse@arm.com> <20250207181823.6378-34-james.morse@arm.com> <91026839-2f2e-4562-aa77-6901148c89ad@intel.com> Content-Language: en-US From: Reinette Chatre In-Reply-To: Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit X-ClientProxiedBy: MW3PR06CA0011.namprd06.prod.outlook.com (2603:10b6:303:2a::16) To SJ2PR11MB7573.namprd11.prod.outlook.com (2603:10b6:a03:4d2::10) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: SJ2PR11MB7573:EE_|SJ0PR11MB4847:EE_ X-MS-Office365-Filtering-Correlation-Id: c4b93536-1a77-4431-ab3c-08dd5869ad11 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|366016|7416014|376014; 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MPAM has an error interrupt which indicates >>> the MPAM driver has gone haywire. Should this occur tasks could run with >>> the wrong control values, leading to bad performance for important tasks. >>> The MPAM driver needs a way to tell resctrl that no further configuration >>> should be attempted. >>> >>> Using resctrl_exit() for this leaves the system in a funny state as >>> resctrl is still mounted, but cannot be un-mounted because the sysfs >>> directory that is typically used has been removed. Dave Martin suggests >>> this may cause systemd trouble in the future as not all filesystems >>> can be unmounted. >>> >>> Add calls to remove all the files and directories in resctrl, and >>> remove the sysfs_remove_mount_point() call that leaves the system >>> in a funny state. When triggered, this causes all the resctrl files >>> to disappear. resctrl can be unmounted, but not mounted again. > >>> diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c >>> index 6e30283358d4..424622d2f959 100644 >>> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c >>> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c >>> @@ -4371,9 +4375,12 @@ int __init resctrl_init(void) >>> >> >> Could you please add the kerneldoc you proposed in >> https://lore.kernel.org/lkml/f2ecb501-bc65-49a9-903d-80ba1737845f@arm.com/ ? > > Huh. The way that is indented means I copied it out the file - I'm not sure went wrong > there. Thanks for fishing out the link! > > >>> void __exit resctrl_exit(void) >>> { >>> + mutex_lock(&rdtgroup_mutex); >>> + rdtgroup_destroy_root(); >>> + mutex_unlock(&rdtgroup_mutex); >>> + >>> debugfs_remove_recursive(debugfs_resctrl); >>> unregister_filesystem(&rdt_fs_type); >>> - sysfs_remove_mount_point(fs_kobj, "resctrl"); >>> >>> resctrl_mon_resource_exit(); >>> } >> >> It is difficult for me to follow the kernfs reference counting required >> to make this work. Specifically, the root kn is "destroyed" here but it >> is required to stick around until unmount when the rest of the files >> are removed. > > This drops resctrl's reference to all of the files, which would make the files disappear. > unmount is what calls kernfs_kill_sb(), which gets rid of the root of the filesystem. My concern is mostly with the kernfs_remove() calls in the rdt_kill_sb()->rmdir_all_sub() flow. For example: kernfs_remove(kn_info); kernfs_remove(kn_mongrp); kernfs_remove(kn_mondata); As I understand the above require the destroyed root to still be around. > > >> Have you been able to test this flow? I think you mentioned >> something like this before but I cannot find the details now. > > Yes: > https://web.git.kernel.org/pub/scm/linux/kernel/git/morse/linux.git/commit/?h=mpam/snapshot%2bextras/v6.14-rc1&id=8c96f858b25aa42694c5db56a2afe255ed8262dd > > This is a debugfs file that schedules the threaded bit of the MPAM error interrupt > handler. I figure its MPAM specific because there is no way into this code on x86. > (the aim is to get the CI to tickle this) Thank you. Reinette