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 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? 

> 
> 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?

sidenote: Should the last line be "[global_assign_ctrl_assign_mon_per_cpu:group=ctrl1/mon1/]"?

> 
> # 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

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
	 */
	# 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?

> # 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?

Reinette