From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id D037D26FA7D for ; Thu, 6 Mar 2025 19:29:00 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1741289343; cv=none; b=XQrE22X1ZS7ZiVVMwZFAGzmULtKwE+BCLizA4el1JM9TkM6xkux1oftSlKKOJx0K4qQRPLjK8xIDktudNWq2vs5Kr7HndFOWYVLVmjq74p23YUp4ibhAAN95m1E7GjGWKtqVfU/gH8b7GsqPaQW3FE/uaINBs1Bs0PV3dyA44r8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1741289343; c=relaxed/simple; bh=a9kHyK8nwvFJXwd1zDqQ2DnutTrv0U8n+tTp0ncr3NM=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=b1Q6XPuCEf64GCo45MYkfvneMn+hm2WQNKK44PaHb/sVuaYOU7Y5PTRfBO5irFthPciQcjk6aARwyg4Oebz1Fh6YloruOLK+XUdJpOVjMfWPJEWilFfdpBvno3Aj3CYkayz8jGgWoJ0sb2FQouKUoOEzTh4TvPGXAFhKMQ1gzxw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id A2BE9169E; Thu, 6 Mar 2025 11:29:12 -0800 (PST) Received: from [10.1.197.49] (eglon.cambridge.arm.com [10.1.197.49]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id C2BED3F5A1; Thu, 6 Mar 2025 11:28:51 -0800 (PST) Message-ID: <0b2c17fe-3998-4def-81eb-a910d79fbc0f@arm.com> Date: Thu, 6 Mar 2025 19:28:49 +0000 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v6 33/42] x86/resctrl: resctrl_exit() teardown resctrl but leave the mount point To: Reinette Chatre , x86@kernel.org, linux-kernel@vger.kernel.org Cc: Thomas Gleixner , Ingo Molnar , Borislav Petkov , H Peter Anvin , Babu Moger , shameerali.kolothum.thodi@huawei.com, D Scott Phillips OS , carl@os.amperecomputing.com, lcherian@marvell.com, bobo.shaobowang@huawei.com, tan.shaopeng@fujitsu.com, baolin.wang@linux.alibaba.com, Jamie Iles , Xin Hao , peternewman@google.com, dfustini@baylibre.com, amitsinght@marvell.com, 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-GB From: James Morse In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Hi Reinette, On 01/03/2025 02:35, Reinette Chatre wrote: > On 2/28/25 11:54 AM, James Morse wrote: >> On 20/02/2025 04:42, Reinette Chatre wrote: >>> On 2/7/25 10:18 AM, James Morse wrote: >>>> resctrl_exit() was intended for use when the 'resctrl' module was unloaded. >>>> resctrl can't be built as a module, and the kernfs helpers are not exported >>>> so this is unlikely to change. 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. Right - because rdt_get_tree() has these global pointers into the hierarchy, but doesn't take a reference. rmdir_all_sub() relies on always being called before rdtgroup_destroy_root(). The point hack would be for rdtgroup_destroy_root() to NULL out those global pointers, (I note they are left dangling) - that would make a subsequent call to rmdir_all_sub() harmless. A better fix would be to pull out all the filesystem relevant parts from rdt_kill_sb(), make that safe for multiple calls and get resctrl_exit() to call that. A call to rdt_kill_sb() after resctrl_exit() would just cleanup the super-block. This will leave things in a more predictable state. >>> 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) Thanks, James