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

[Moger, Babu]

Hi Reinette,

On 4/9/2026 10:41 PM, Reinette Chatre wrote:
> Hi Babu,
>
> On 4/9/26 4:42 PM, Moger, Babu wrote:
> > Hi Reinette,
> >
> > On 4/9/2026 3:50 PM, Reinette Chatre wrote:
> > > Hi Babu,
> > >
> > > On 4/9/26 11:05 AM, Moger, Babu wrote:
> > > > On 4/9/2026 12:26 PM, Reinette Chatre wrote:
> > > > > On 4/9/26 10:19 AM, Moger, Babu wrote:
> > > > > > On 4/8/2026 6:41 PM, Reinette Chatre wrote:
> > > > >
> > > > > > > When the user switches to either "global_assign_ctrl_inherit_mon_per_cpu" or
> > > > > > > 'global_assign_ctrl_assign_mon_per_cpu" then "info/kernel_mode_assignment" is created
> > > > > > > (or made visible to user space) and is expected to point to default group.
> > > > > > > User can change the group using "info/kernel_mode_assignment" at this point.
> > > > > > >
> > > > > > > If the current scenario is below ...
> > > > > > >        # cat info/kernel_mode
> > > > > > >        [global_assign_ctrl_inherit_mon_per_cpu]
> > > > > > >        inherit_ctrl_and_mon
> > > > > > >        global_assign_ctrl_assign_mon_per_cpu
> > > > > > >
> > > > > > > ... then "info/kernel_mode_assignment" will exist but what it should contain if
> > > > > > > user switches mode at this point may be up for discussion.
> > > > > > >
> > > > > > > option 1)
> > > > > > > When user switches mode to "global_assign_ctrl_assign_mon_per_cpu" then
> > > > > > > the resource group in "info/kernel_mode_assignment" is reset to the
> > > > > > > default group and all CPUs PLZA state reset to match. The kernel_mode_cpus
> > > > > > > and kernel_mode_cpuslist files become visible in default resource group
> > > > > > > and they contain "all online CPUs".
> > > > > > >
> > > > > > > option 2)
> > > > > > > When user switches mode to "global_assign_ctrl_assign_mon_per_cpu" then
> > > > > > > the resource group in "info/kernel_mode_assignment" is kept and all
> > > > > > > CPUs PLZA state set to match it while also keeping the current
> > > > > > > values of that resource group's kernel_mode_cpus and kernel_mode_cpuslist
> > > > > > > files.
> > > > > > >
> > > > > > > I am leaning towards "option 1" to keep it consistent with a switch from
> > > > > > > "inherit_ctrl_and_mon" and being deterministic about how a mode is started with
> > > > > >
> > > > > > Yes. The "option 1" seems appropriate.
> > > > > >
> > > > > > > a clean slate. What are your thoughts? What would be use case where a user would
> > > > > > > want to switch between "global_assign_ctrl_inherit_mon_per_cpu" and
> > > > > > > "global_assign_ctrl_assign_mon_per_cpu" to just switch rmid_en on and off?
> > > > > >
> > > > > >
> > > > > > This is a bit tricky.
> > > > > >
> > > > > > Currently, our requirement is to have a CTRL_MON group for
> > > > > > global_assign_ctrl_inherit_mon_per_cpu. In this scenario, we use the
> > > > > > group’s CLOSID for PLZA configuration, and RMID is not used (rmid_en
> > > > > > = 0) when setting up PLZA.
> > > > > >
> > > > > > Our requirement is also to have a CTRL_MON/MON group for
> > > > > > global_assign_ctrl_assign_mon_per_cpu. In this case as well, the
> > > > > > group’s CLOSID and RMID (rmid_en = 1)  both are used configure PLZA.
> > > > >
> > > > > ah, right. Good catch.
> > > > >
> > > > > > Actually, we should not allow these changes from
> > > > > > global_assign_ctrl_inherit_mon_per_cpu  to
> > > > > > global_assign_ctrl_assign_mon_per_cpu or visa versa.
> > > > >
> > > > > resctrl could allow it but as part of the switch it resets the "kernel mode group" to
> > > > > be the default group every time? This would be the "option 1" above.
> > > >
> > > > Other options.
> > > >
> > > > Allow global_assign_ctrl_inherit_mon_per_cpu -> global_assign_ctrl_assign_mon_per_cpu. As part of the switch, reset the "kernel mode group" to the default group.
> > > >
> > > > Allow global_assign_ctrl_assign_mon_per_cpu -> global_assign_ctrl_inherit_mon_per_cpu. In this case switch
> > > > to CTRL_MON/MON -> CTRL_MON.
> > >
> > > ok. Could you please return the courtesy of providing feedback on the
> > > suggestion you are responding to and also include the motivation why your
> > > suggestion is the better option?
> >
> > Yea. Sure.
> >
> > We need to allow the switch between the modes. Otherwise only way to reset is to remount the resctrl filesystem. That is not a good option.
> >
> > Allow global_assign_ctrl_inherit_mon_per_cpu -> global_assign_ctrl_assign_mon_per_cpu. As part of the switch, reset the "kernel mode group" to the default group.
> >
> > This option is same as you suggested.
> >
> > Allow global_assign_ctrl_assign_mon_per_cpu -> global_assign_ctrl_inherit_mon_per_cpu. In this case switch
> > to CTRL_MON/MON -> CTRL_MON. This option basically disables monitor (rmid_en=0). It is less disruptive. Move is between child group to parent group.
>
> ok. I am concerned that this creates an inconsistent interface. Specifically, sometimes
> when switching the mode the kernel group will reset and sometimes it won't. This inconsistency
> may be more apparent when writing the user documentation as part of this work. If you are
> able to clearly explain how this resctrl fs interface behaves (this cannot be about PLZA
> internals as above) then this could work.

Yes, certainly. I’ll begin work on v3, and we can continue refining it 
as we move forward.

Thanks
Babu