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

["Moger, Babu" <bmoger@amd.com>]

On 1/28/2026 11:41 AM, Moger, Babu wrote:

> 

Outlook adds AMD  header.  Removed it now.

>> -----Original Message-----
>> From: Luck, Tony <tony.luck@intel.com>
>> Sent: Wednesday, January 28, 2026 11:12 AM
>> To: Moger, Babu <bmoger@amd.com>
>> Cc: Babu Moger <babu.moger@amd.com>; corbet@lwn.net;
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>> Subject: Re: [RFC PATCH 13/19] x86/resctrl: Add PLZA state tracking and context
>> switch handling
>>
>> On Wed, Jan 28, 2026 at 10:01:39AM -0600, Moger, Babu wrote:
>>> 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.
>>>
>>
>> Babu,
>>
>> I've read a bit more of the code now and I think I understand more.
>>
>> Some useful additions to your explanation.
>>
>> 1) Only one CTRL group can be marked as PLZA
> 
> Yes. Correct.
> 
>> 2) It can't be the root/default group
> 
> This is something I added to keep the default group in a un-disturbed,
> 
>> 3) It can't have sub monitor groups
>> 4) It can't be pseudo-locked
> 
> Yes.
> 
>>
>> Would a potential use case involve putting *all* tasks into the PLZA group? That
>> would avoid any additional context switch overhead as the PLZA MSR would never
>> need to change.
> 
> Yes. That can be one use case.
> 
>>
>> If that is the case, maybe for the PLZA group we should allow user to
>> do:
>>
>> # echo '*' > tasks
> 
> Yea. It can be done.
> 
> Thanks
> Babu
> 
>