Re: [PATCH v8 04/21] x86/resctrl: resctrl_exit() teardown resctrl but leave the mount point

James Morse
2025-04-24T09:15:23Z

Hi Reinette,

On 16/04/2025 01:25, Reinette Chatre wrote:
> On 4/11/25 9:42 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.
>> In this scenario the MPAM driver will reset the hardware, but it needs
>> a way to tell resctrl that no further configuration should be attempted.
>>
>> In particular, moving tasks between control or monitor groups does not
>> interact with the architecture code, so there is no opportunity for the
>> arch code to indicate that the hardware is no-longer functioning.
>>
>> 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.

> The caveat here is that resctrl pretends to be mounted (resctrl_mounted == true)
> but there is nothing there. The undocumented part of this is that for this
> to work resctrl fs depends (a lot) on the architecture's callbacks to know
> if they are being called after a resctrl_exit() call so that they return data
> that will direct resctrl fs behavior to safest exit for those
> resctrl fs flows that are still possible after a resctrl_exit(). Not ideal
> layering.

It was the arch code that called resctrl_exit() - there is no other path into it.
I don't think its a problem for the arch code to also know to return an error.
I haven't found anything where which error is returned actually matter - so there
is no 'direction', only errors.

I agree the documentation can be improved.

> I understand from a previous comment [1] that one of the Arm "tricks" is to
> offline all domains. This seems to be a good "catch all" to ensure that at least
> current flows of concern are not running anymore.

Yup, that is necessary to stop the limbo and overflow workers for trying to read the
counters - which is a waste of time.

> Considering this,
> what if there is a new resctrl_error_exit() that does something like below?
>
> void resctrl_error_exit(void)
> {
> mutex_lock(&rdtgroup_mutex);
> WARN_ON_ONCE(resctrl_new_function_returns_true_if_any_resource_has_a_control_or_monitor_domain());
> resctrl_fs_teardown();
> mutex_unlock(&rdtgroup_mutex);
> resctrl_exit();
> }

Makes sense - the alternative would be to dig around to cancel the limbo/overflow
work, and a subsequent CPU-online might start them again.

> I do not see this as requiring anything new from architecture but instead
> making what Arm already does a requirement and keeping existing behavior?

I agree.

> This leaves proc_resctrl_show() that relies on resctrl_mounted but as I see
> the resctrl_fs_cleanup() will remove all resource groups that should result
> in the output being as it will be if resctrl is not mounted. No dependence
> on architecture callbacks returning resctrl_exit() aware data here.

Great - I'd missed that one,

>> diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>> index fdf2616c7ca0..3f9c37637d7e 100644
>> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c

>> @@ -4416,11 +4429,26 @@ int __init resctrl_init(void)
>> return ret;
>> }
>>
>> +/**
>> + * resctrl_exit() - Remove the resctrl filesystem and free resources.
>> + *
>> + * Called by the architecture code in response to a fatal error.
>> + * Resctrl files and structures are removed from kernfs to prevent further
>> + * configuration.
>
> Please write with imperative tone. For example, "Remove resctrl files and structures ..."
>
>> + */
>> void __exit resctrl_exit(void)
>> {
>> + mutex_lock(&rdtgroup_mutex);
>> + resctrl_fs_teardown();
>> + mutex_unlock(&rdtgroup_mutex);
>> +
>> debugfs_remove_recursive(debugfs_resctrl);
>
> Is it possible for the fatal error handling to trigger multiple calls here?
> To protect against multiple calls causing issues debugfs_resctrl can be set to NULL here.

It's not, the driver keeps track of whether resctrl_init() had been called, and only calls
resctrl_exit() once. But I agree it would be better to make it robust to this.

>> unregister_filesystem(&rdt_fs_type);
>
> unregister_filesystem() seems to handle an already-unregistered filesystem.
>
>> - sysfs_remove_mount_point(fs_kobj, "resctrl");
>> +
>> + /*
>> + * The sysfs mount point added by resctrl_init() is not removed so that
>> + * it can be used to umount resctrl.
>> + */
>
> (needs imperative)
>
>>
>> resctrl_mon_resource_exit();
>> }

Thanks,

James