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

[Reinette Chatre]

Hi Babu,

On 4/21/26 5:17 PM, Moger, Babu wrote:
> On 4/21/2026 5:44 PM, Reinette Chatre wrote:
>> On 4/21/26 3:04 PM, Moger, Babu wrote:

>>> That said, I agree we need to support this. Without it, we won’t be able to move the group from PLZA to non-PLZA.
>>>
>>> # cat info/kernel_mode
>>>      inherit_ctrl_and_mon:
>>>      global_assign_ctrl_assign_mon_per_cpu:group=uninitialized
>>>      [global_assign_ctrl_assign_mon_per_cpu]:group=ctrl1/mon1/
>>
>> Like above where the listing is inconsistent. Is this what you mean?
> 
> I meant the listing of "inherit_ctrl_and_mon" does not have groups while other modes have it.

I think this is ok since it does not need a group or any other (for now?) property.
What issues do you foresee here?

>>
>> sidenote: Should the last line be "[global_assign_ctrl_assign_mon_per_cpu:group=ctrl1/mon1/]"?
> 
> Yes.
> 
>>
>>>
>>> # echo "inherit_ctrl_and_mon:group=ctrl1/mon1/" > info/kernel_mode
>>
>> This does not look right. Why is a "group" property needed here? Can the mode not just
>> be set by itself? Specifically, why not just:
>>
>>     # echo "inherit_ctrl_and_mon" > info/kernel_mode
> 
> We can go with this based on your another comment below. While changing the mode use the defaults if properties are not provided.
> 
> 
>>
>> This reminds me that there is still an open remaining from
>> https://lore.kernel.org/lkml/71099958-1ddf-40dc-8a3c-aa13d0c56fee@xxxxxxxxx/
>> Specifically this from that message:
>>     The named fields could be made optional, if group is omitted then it will become the
>>     default resource group, and if cpus/cpus_list is omitted then it will default to all CPUs.
>>     This may not be intuitive since a user may expect that not mentioning a field means
>>     that the field is left untouched. Have you considered this scenario in your proposal?
>>
>> I think this needs some clear description of behavior wrt properties, for example:
>> - Is it required to provide all properties on each write? More specifically, can user expect there
>>    to be "default" values when a property is not provided or is user required to provide a value
>>    for each property? We need to be careful here because we do not want user scripts to fail when a new
>>    property is added in the future. What if resctrl specifies that if user space does not provide
>>    a property then resctrl will pick a default. For example, if user runs:
>>     # echo "global_assign_ctrl_assign_mon_per_cpu" > info/kernel_mode
>>    then resctrl will switch to "global_assign_ctrl_assign_mon_per_cpu" mode initialized to
>>    the default group.
>>    I am not sure if resctrl needs to support re-configuration of modes in the future where the
>>    mode stays the same but a property changes? Consider, for example,
>>
>>     # cat info/kernel_mode
>>     [inherit_ctrl_and_mon:]
>>     global_assign_ctrl_assign_mon_per_cpu:group=uninitialized
>>
>>     # echo "global_assign_ctrl_assign_mon_per_cpu" > info/kernel_mode
>>     /*
>>      * resctrl switches to "global_assign_ctrl_assign_mon_per_cpu" mode and sets
>>      * PLZA group to default group
>>      */
>>     # cat info/kernel_mode
>>     inherit_ctrl_and_mon:
>>     [global_assign_ctrl_assign_mon_per_cpu:group=//]
>>     # echo "global_assign_ctrl_assign_mon_per_cpu:group=ctrl1/mon1/" > info/kernel_mode
>>     /*
>>      * resctrl stays in "global_assign_ctrl_assign_mon_per_cpu" mode and sets
>>      * PLZA group to default group
>>      */
> 
> I think you meant "PLZA group to ctrl1/mon1/" here.

Indeed, yes. Thank you.

> 
>>     # cat info/kernel_mode
>>     inherit_ctrl_and_mon:
>>     [global_assign_ctrl_assign_mon_per_cpu:group=ctrl1/mon1/]
>>     # echo "global_assign_ctrl_assign_mon_per_cpu" > info/kernel_mode
>>     /*
>>      * TBD: should resctrl switch back to default group or just keep
>>      * group as ctrl1/mon1/ ?
>>      */
>>
>>    resctrl could thus specify different behavior for switching to a mode where all properties
>>    not specified obtains default values and re-configuring a mode where only specified
>>    properties are changed. That means, the "TBD" above would be that the group stays
>>    as ctrl1/mon1/. So,
>>     # cat info/kernel_mode
>>     inherit_ctrl_and_mon:
>>     [global_assign_ctrl_assign_mon_per_cpu:group=ctrl1/mon1/]
>>
>>    What do you think?
> 
> Yes. Sure. We can do that. We only have 2 properties now (mode and group). We should be able to handle that.

Thank you for considering.

Reinette