Re: [PATCH v3 04/12] x86,fs/resctrl: Program PLZA through kmode arch hooks

[Luck, Tony]

On Wed, May 20, 2026 at 12:49:25PM -0500, Babu Moger wrote:
> Hi Tony,
> 
> 
> On 5/19/26 15:59, Luck, Tony wrote:
> > On Thu, Apr 30, 2026 at 06:24:49PM -0500, Babu Moger wrote:
> > > +void resctrl_arch_configure_kmode(cpumask_var_t cpu_mask, u32 closid, u32 rmid, bool enable)
> > > +{
> > > +	union msr_pqr_plza_assoc plza = { 0 };
> > > +
> > > +	plza.split.rmid = rmid;
> > > +	plza.split.rmid_en = 1;
> > 
> > Shouldn't there be a parameter for the value of rmid_en?
> 
> 
> I realized that behavior is not required—it was actually due to a mistake in
> my v2 series implementation.
> 
> Below are the relevant definitions:
> 
> 
> GLOBAL_ASSIGN_CTRL_INHERIT_MON_PER_CPU:
> The CLOSID is applied to kernel work, while the RMID used for monitoring is
> inherited from the currently running user task.
> No separate monitoring group is assigned for kernel work, so kernel
> execution naturally inherits the user-space RMID.
> 
> 
> GLOBAL_ASSIGN_CTRL_ASSIGN_MON_PER_CPU:
> Both CLOSID and RMID are explicitly assigned to kernel work.
> This allows assigning a dedicated monitoring group for kernel execution and
> therefore requires a separate RMID.
> 
> Example: For GLOBAL_ASSIGN_CTRL_INHERIT_MON_PER_CPU:
> 
> # mount -t resctrl resctrl /sys/fs/resctrl
> 
> # cat /sys/fs/resctrl/info/kernel_mode
> [inherit_ctrl_and_mon:group=//]
> global_assign_ctrl_inherit_mon_per_cpu:group=none
> global_assign_ctrl_assign_mon_per_cpu:group=none
> 
> # mkdir /sys/fs/resctrl/ctrl1   (PQR_ASSOC closid=1 rmid=1)
> 
> This configures all the CPU threads to use closid=1 and rmid=1 for both
> allocation and monitoring across user and kernel modes.
> 
> 
> # echo "global_assign_ctrl_inherit_mon_per_cpu:group=ctrl1//" \
>   > /sys/fs/resctrl/info/kernel_mode
> 
> # cat /sys/fs/resctrl/info/kernel_mode
> inherit_ctrl_and_mon:group=none
> [global_assign_ctrl_inherit_mon_per_cpu:group=ctrl1//]
> global_assign_ctrl_assign_mon_per_cpu:group=none
> 
> This overrides the previous configuration, and PQR_PLZA_ASSOC is written.
> 
> Possible options:
> 
> 1. (closid=1, rmid_en=0, rmid=1)
> Here, hardware uses closid=1 for kernel work, but RMID tracking is disabled
> for kernel mode.
> 
> As a result, reading RMID 1 reports only user-mode activity
> This contradicts the definition of this mode, since kernel work is expected
> to inherit the user RMID for monitoring.
> 
> 2. (closid=1, rmid_en=1, rmid=1)
> In this case, RMID tracking is enabled for both user and kernel modes.
> 
> Reading RMID 1 reports combined user + kernel activity
> This aligns with the expected inherit_monitoring behavior
> 
> 
> The preferred approach is to separate kernel monitoring by assigning it a
> dedicated monitoring group and updating PQR_PLZA_ASSOC to use a different
> RMID (e.g., closid=1, rmid_en=1, rmid=2). This is exactly the behavior
> implemented by GLOBAL_ASSIGN_CTRL_ASSIGN_MON_PER_CPU.

So maybe I'm just confused by the name "global_assign_ctrl_inherit_mon_per_cpu"

That sounds like "Use the CLOSID from PLZA, but keep the RMID from
legacy PQR_ASSOC.

So:

# mkdir ctrl1 # maybe gets CLOSID=1, RMID=1
# echo global_assign_ctrl_inherit_mon_per_cpu:group=ctrl1//" > info/kernel_mode
# mkdir ctrl2 # maybe gets CLOSID=2, RMID=2
# echo $$ > ctrl2/tasks

My shell, and all children run with CLOSID=2 and RMID=2 from ctrl2. But
when they do system calls, take page faults or there is an interrupt I'd
expect the code in the kernel to run with the CLOSID=1, while inheriting
RMID=2.

To make that happen, I thing the PLZA MSR should have rmid_en = 0. But
the only code I see that sets this always sets rmid_en=1.

> 
> Thanks
> Babu

-Tony