Re: [RFC PATCH 13/19] x86/resctrl: Add PLZA state tracking and context switch handling

[Moger, Babu]

Hi Tony,

Thanks for the comment.

On 1/27/2026 4:30 PM, Luck, Tony wrote:
> On Wed, Jan 21, 2026 at 03:12:51PM -0600, Babu Moger wrote:
> > @@ -138,6 +143,20 @@ static inline void __resctrl_sched_in(struct task_struct *tsk)
> >   		state->cur_rmid = rmid;
> >   		wrmsr(MSR_IA32_PQR_ASSOC, rmid, closid);
> >   	}
> > +
> > +	if (static_branch_likely(&rdt_plza_enable_key)) {
> > +		tmp = READ_ONCE(tsk->plza);
> > +		if (tmp)
> > +			plza = tmp;
> > +
> > +		if (plza != state->cur_plza) {
> > +			state->cur_plza = plza;
> > +			wrmsr(MSR_IA32_PQR_PLZA_ASSOC,
> > +			      RMID_EN | state->plza_rmid,
> > +			      (plza ? PLZA_EN : 0) | CLOSID_EN | state->plza_closid);
> > +		}
> > +	}
> > +
>
> Babu,
>
> This addition to the context switch code surprised me. After your talk
> at LPC I had imagined that PLZA would be a single global setting so that
> every syscall/page-fault/interrupt would run with a different CLOSID
> (presumably one configured with more cache and memory bandwidth).
>
> But this patch series looks like things are more flexible with the
> ability to set different values (of RMID as well as CLOSID) per group.

Yes. this similar what we have with MSR_IA32_PQR_ASSOC. The association 
can be done either thru CPUs (just one MSR write) or task based 
association(more MSR write as task moves around).
> It looks like it is possible to have some resctrl group with very
> limited resources just bump up a bit when in ring0, while other
> groups may get some different amount.
>
> The additions for plza to the Documentation aren't helping me
> understand how users will apply this.
>
> Do you have some more examples?

Group creation is similar to what we have currently.

1. create a regular group and setup the limits.
   # mkdir /sys/fs/resctrl/group

2. Assign tasks or CPUs.
   # echo 1234 > /sys/fs/resctrl/group/tasks

   This is a regular group.

3. Now you figured that you need to change things in CPL0 for this task.

4. Now create a PLZA group now and tweek the limits,

   # mkdir /sys/fs/resctrl/group1

   # echo 1 > /sys/fs/resctrl/group1/plza

   # echo "MB:0=100" > /sys/fs/resctrl/group1/schemata

5. Assign the same task to the plza group.

   # echo 1234 > /sys/fs/resctrl/group1/tasks


Now the task 1234 will be using the limits from group1 when running in 
CPL0.

I will add few more details in my next revision.

Thanks
Babu