Re: [PATCH v6 33/42] x86/resctrl: resctrl_exit() teardown resctrl but leave the mount point

[Reinette Chatre <reinette.chatre@intel.com>]

Hi James,

On 2/28/25 11:54 AM, James Morse wrote:
> Hi Reinette,
> 
> 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.

> 
> 
>> 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