linux-kernel - Re: [RFC PATCH 16/19] fs/resctrl: Implement rdtgroup_plza

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Message-ID: <57d7a46a-c3d6-4c7c-bed1-ee61d6d690fb@amd.com>
Date: Thu, 29 Jan 2026 13:42:24 -0600
From: Babu Moger <bmoger@....com>
To: "Luck, Tony" <tony.luck@...el.com>, Babu Moger <babu.moger@....com>
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Subject: Re: [RFC PATCH 16/19] fs/resctrl: Implement rdtgroup_plza_write() to
 configure PLZA in a group

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