Re: [PATCH v2 00/16] fs,x86/resctrl: Add kernel-mode (e.g., PLZA) support to the resctrl subsystem

[Reinette Chatre]

Hi Tony,

On 4/21/26 8:11 AM, Luck, Tony wrote:
> On Mon, Apr 20, 2026 at 05:21:50PM -0700, Reinette Chatre wrote:
>> On 4/20/26 5:03 PM, Luck, Tony wrote:
...

>>>> # echo "global_assign_ctrl_inherit_mon_per_cpu" > info/kernel_mode
>>>
>>> This mode needs a CLOSID for PLZA, but doesn't need an RMID.
>>>
>>>> At this stage, only the kernel mode is being changed. However, there is no
>>>> way to know which control group the user intends to assign to kernel mode.
>>>> All we know here is the selected mode.
>>>>
>>>> After this operation, the info/kernel_mode_assignment interface should
>>>> become visible. But the question is: what should it contain or point to at
>>>> this moment?
>>>>
>>>> # cat info/kernel_mode_assignment
>>>> ??
>>>>
>>>> Next operation: Assign the group
>>>>
>>>> # echo "ctrl1//" > info/kernel_mode_assignment
>>>
>>> Now ring0 code is using the CLOSID from the ctrl1 group.
>>
>> ... and user space tasks also continue to use the CLOSID from the
>> ctrl1 group.
>> It is up to user space to decide if a group is dedicated to kernel
>> mode or not. resctrl does not enforce it.
>>
>>>
>>> But the RMID for this group isn't used.
>>
>> RMID is still used by user mode that maintains existing behavior concerning
>> this group when considering its tasks/cpus/cpus_list files. RMID assigned to this
>> group is just not used for kernel mode.
> 
> True, that the RMID is used if the user makes assignments using tasks/cpus/cpus_list
> for the ctrl1 group. But they might not do that.
> 
>>
>>>
>>> Are we OK with "wasting" an RMID in this way?
>>
>> How do you see this RMID as "wasted"?
> 
> Suppose the user doesn't assign tasks to the ctrl1 group?
> 
> Perhaps the resources they want to make available to the kernel do
> not exactly match with resources that they want to provide to any
> tasks. In this case the RMID is wasted.

Under these circumstances, yes, the RMID will not be used. 

A related scenario (when considering  what may happen if user does not assign tasks to
the ctrl1 group) is when user space disables PLZA on all CPUs in a domain then the CLOSID
(as well as RMID since this is irrespective of rmid_en mode) associated with kernel_mode
will be unused in that domain.

Reinette