Re: [PATCH v3 00/12] [PATCH v3 00/12] x86/resctrl: Add kernel-mode (e.g., PLZA) support to the resctrl subsystem

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

On 6/11/2026 4:53 PM, Reinette Chatre wrote:
> Hi Babu,
> 
> On 4/30/26 4:24 PM, Babu Moger wrote:
>> Design
>> ======
>>
>> A new sysfs file, info/kernel_mode, holds a single global policy that
>> selects what kernel work is steered and which rdtgroup it is steered
> 
> How should "selects *what* kernel work is steered" be interpreted? Do these
> modes not all apply to *all* kernel work?

How about?

A new sysfs file, info/kernel_mode, holds a single global policy for 
kernel contexts and the rdtgroup associated with the policy.

> 
>> to.  Reads describe the supported modes and the currently-active
>> binding; writes change the policy or rebind to a different group.
>> Look at the thread below for design discussion.
>> https://lore.kernel.org/lkml/14a8ad0a-e842-4268-871a-0762f1169e03@intel.com/
>>
> 
> ...
> 
>> Examples
>> ========
>>
>> (See Documentation/filesystems/resctrl.rst, "kernel_mode" and
>> "kmode_cpus" sections, for the full UAPI.)
>>
>>    # Mount resctrl
>>    # mount -t resctrl resctrl /sys/fs/resctrl
>>    # cd /sys/fs/resctrl
>>
>>    # Read the supported modes.  The active mode is bracketed and reports
>>    # the bound "<ctrl>/<mon>/" group; other supported modes report
>>    # ":group=none" because nothing is bound to them.
>>    # cat info/kernel_mode
>>    [inherit_ctrl_and_mon:group=//]
> 
> This is unexpected since associating a group to this mode implies that this
> group is used to manage allocations and monitoring of kernel work but this
> is not true, right? From what I understand there should be no group associated with
> this default "inherit_ctrl_and_mon" mode.

The default mode is "inherit_ctrl_and_mon", where both user mode and 
kernel mode share the same CLOSID and RMID. This is current mode 
(without this series).

I thought we are going to set the default mode with the default group 
when system boots up. No?


> 
>>    global_assign_ctrl_inherit_mon_per_cpu:group=none
>>    global_assign_ctrl_assign_mon_per_cpu:group=none
> 
> nit: "none" does not reflect state as clearly as "unset"/"uninitialized"/"NA"

Lets go with "uninitialized".

> 
>>
>>    # Create a CTRL_MON group plus a MON child and bind both the kernel
>>    # CLOSID and RMID to them.
>>    # mkdir ctrl1
>>    # mkdir ctrl1/mon_groups/mon1
>>    # echo "global_assign_ctrl_assign_mon_per_cpu:group=ctrl1/mon1/" \
>>            > info/kernel_mode
>>    # cat info/kernel_mode
>>    inherit_ctrl_and_mon:group=none
>>    global_assign_ctrl_inherit_mon_per_cpu:group=none
>>    [global_assign_ctrl_assign_mon_per_cpu:group=ctrl1/mon1/]
>>
>>    # kmode_cpus and kmode_cpus_list are visible only on the bound group.
>>    # ls ctrl1/kmode_cpus*
>>    ctrl1/kmode_cpus  ctrl1/kmode_cpus_list
> 
> Since it is ctrl1/mon1 that was bound, should these CPU files not appear
> in ctrl1/mon_groups/mon1 ?

Correct. Will fix it.


>>
>>    # Restrict the binding to a CPU subset; the write is incremental.
> 
> Does "incremental" mean that if the file contains CPUs 0-3 then writing
> "4" would set the CPUs to 0-4? This does not sound right since it is
> expected that user space can remove CPUs also?

Will remove incremental. Writing "4" will remove 0-3 and keep only 4.


> 
>>    # echo 0-3 > ctrl1/kmode_cpus_list
>>    # cat ctrl1/kmode_cpus
>>    f
>>    # cat ctrl1/kmode_cpus_list
>>    0-3
>>
>>    # Empty masks are rejected; use info/kernel_mode to reset to
>>    # "every online CPU".
>>    # echo "" > ctrl1/kmode_cpus_list
>>    bash: echo: write error: Invalid argument
>>    # cat info/last_cmd_status
>>    Empty mask not allowed; use info/kernel_mode to unbind
> 
> Why are empty masks rejected/not allowed?

No specific reason.

When the mode is switched, we discussed earlier to globally apply the 
mode to all the online CPUs.

At this point reading "kmode_cpus_list" will still report empty.

Users can change it to selectively apply the mode by writing to 
"kmode_cpus_list".

I was not sure what was the action when empty masks are written.

Should the empty mask apply the mode to all the online CPUs?


> 
>>
>>    # Disable kernel-mode steering (back to inherit, default group).
> 
> This sounds like kernel work is steered to default group which I
> do not think is accurate for the "inherit_ctrl_and_mon" mode.

How about ?

Drop the kernel-mode binding and restore inherit_ctrl_and_mon on the 
default group.

thanks
Babu