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

["Moger, Babu" <bmoger@amd.com>]

Hi Reinette,

On 4/21/2026 5:44 PM, Reinette Chatre wrote:
> Hi Babu,
> 
> On 4/21/26 3:04 PM, Moger, Babu wrote:
>> My bad. My only motivation was to keep the mode listing display consistent.
> 
> The listing display is already inconsistent since the different modes have different
> global properties, no?
> 

Yes. That is true.

>>
>> 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.

> 
> 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@intel.com/
> 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.

> 	# 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.


> 
>> # 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=uninitialized
> This does not look right. After switching the kernel_mode to inherit_ctrl_and_mon
> I expect inherit_ctrl_and_mon to be the active mode?

Yes. inherit_ctrl_and_mon should be active here.

Thanks
Babu