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

[James Morse]

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@xxxxxxx/ ?

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.


> 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