Re: [RFC PATCH 16/19] fs/resctrl: Implement rdtgroup_plza_write() to configure PLZA in a group

[Babu Moger]

Hi Tony,

On 1/28/26 16:03, Luck, Tony wrote:
> On Wed, Jan 21, 2026 at 03:12:54PM -0600, Babu Moger wrote:
> > Introduce rdtgroup_plza_write() group which enables per group control of
> > PLZA through the resctrl filesystem and ensure that enabling or disabling
> > PLZA is propagated consistently across all CPUs belonging to the group.
> >
> > Enforce the capability checks, exclude default, pseudo-locked and CTRL_MON
> > groups with sub monitors. Also, ensure that only one group can have PLZA
> > enabled at a time.
> ...
>
> > +static ssize_t rdtgroup_plza_write(struct kernfs_open_file *of, char *buf,
> > +				   size_t nbytes, loff_t off)
> > +{
> > +	struct rdt_resource *r = resctrl_arch_get_resource(RDT_RESOURCE_L3);
> > +	struct rdtgroup *rdtgrp, *prgrp;
> > +	int cpu, ret = 0;
> > +	bool enable;
> ...
>
> > +	/* Enable or disable PLZA state and update per CPU state if there is a change */
> > +	if (enable != rdtgrp->plza) {
> > +		resctrl_arch_plza_setup(r, rdtgrp->closid, rdtgrp->mon.rmid);
> What is this for? If I've just created a group with no tasks, and empty
> CPU mask ... it seems that this writes the MSR_IA32_PQR_PLZA_ASSOC on
> every CPU in every domain.
>
> > +		for_each_cpu(cpu, &rdtgrp->cpu_mask)
> > +			resctrl_arch_set_cpu_plza(cpu, rdtgrp->closid,
> > +						  rdtgrp->mon.rmid, enable);
> > +		rdtgrp->plza = enable;
> > +	}
> > +
> > +unlock:
> > +	rdtgroup_kn_unlock(of->kn);
> > +
> > +	return ret ?: nbytes;
> > +}
> It also appears that marking a task as PLZA is permanent. Moving it to
> another group doesn't unmark it. Is this intentional?
>
> # mkdir group1 group2 plza_group
> # echo 1 > plza_group/plza
> # echo $$ > group1/tasks
> # echo $$ > plza_group/tasks
>
> My shell is now in group1 and in the plza_group
> # grep $$ */tasks
> group1/tasks:4125
> plza_group/tasks:4125
>
> Move shell to group2
> # echo $$ > group2/tasks
> # grep $$ */tasks
> group2/tasks:4125
> plza_group/tasks:4125
>
> Succcess in moving to group2, but still in plza_group

You are moving the task from group1 to group2. This basically changes 
the association in

MSR_IA32_PQR_ASSOC register,  It does not change the PLZA association.

To change it:

a. You either remove task from plza group which triggers task update (tsk->plza = 0)
   
   echo >> /sys/fs/resctrl/plza_group/tasks

b. Or you can change the group as regular group.

  echo 0 > /sys/fs/resctrl/plza_group/plza


Thanks for the trying it out.

- Babu