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

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Message-ID: <5ec19557-6a62-4158-af82-c70bac75226f@amd.com>
Date: Wed, 28 Jan 2026 10:01:39 -0600
From: "Moger, Babu" <bmoger@....com>
To: "Luck, Tony" <tony.luck@...el.com>, Babu Moger <babu.moger@....com>
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Subject: Re: [RFC PATCH 13/19] x86/resctrl: Add PLZA state tracking and
 context switch handling

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