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

[Moger, Babu]

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@xxxxxxxxx/
>
> ...
>
> > 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