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

["Luck, Tony" <tony.luck@intel.com>]

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

-Tony

N.B. I don't have a PLZA enabled system. So I faked it with this
patch.

From 1655fea0049947218fa5400916d57109be8521ef Mon Sep 17 00:00:00 2001
From: Tony Luck <tony.luck@intel.com>
Date: Wed, 28 Jan 2026 13:02:51 -0800
Subject: [PATCH] fake PLZA

---
 arch/x86/include/asm/resctrl.h            | 10 ++++++----
 arch/x86/kernel/cpu/resctrl/core.c        |  4 ++--
 arch/x86/kernel/cpu/resctrl/ctrlmondata.c |  3 ++-
 3 files changed, 10 insertions(+), 7 deletions(-)

diff --git a/arch/x86/include/asm/resctrl.h b/arch/x86/include/asm/resctrl.h
index 2c11787c5253..7ee35bebb64c 100644
--- a/arch/x86/include/asm/resctrl.h
+++ b/arch/x86/include/asm/resctrl.h
@@ -90,14 +90,16 @@ static inline void resctrl_arch_disable_mon(void)
 
 static inline void resctrl_arch_enable_plza(void)
 {
-	static_branch_enable_cpuslocked(&rdt_plza_enable_key);
-	static_branch_inc_cpuslocked(&rdt_enable_key);
+	pr_info("resctrl_arch_enable_plza\n");
+	//static_branch_enable_cpuslocked(&rdt_plza_enable_key);
+	//static_branch_inc_cpuslocked(&rdt_enable_key);
 }
 
 static inline void resctrl_arch_disable_plza(void)
 {
-	static_branch_disable_cpuslocked(&rdt_plza_enable_key);
-	static_branch_dec_cpuslocked(&rdt_enable_key);
+	pr_info("resctrl_arch_disable_plza\n");
+	//static_branch_disable_cpuslocked(&rdt_plza_enable_key);
+	//static_branch_dec_cpuslocked(&rdt_enable_key);
 }
 
 /*
diff --git a/arch/x86/kernel/cpu/resctrl/core.c b/arch/x86/kernel/cpu/resctrl/core.c
index e41fe5fa3f30..780cdfb0e7cd 100644
--- a/arch/x86/kernel/cpu/resctrl/core.c
+++ b/arch/x86/kernel/cpu/resctrl/core.c
@@ -295,7 +295,7 @@ static __init bool __rdt_get_mem_config_amd(struct rdt_resource *r)
 
 	r->alloc_capable = true;
 
-	if (rdt_cpu_has(X86_FEATURE_PLZA))
+	if (1 || rdt_cpu_has(X86_FEATURE_PLZA))
 		r->plza_capable = true;
 
 	return true;
@@ -318,7 +318,7 @@ static void rdt_get_cache_alloc_cfg(int idx, struct rdt_resource *r)
 		r->cache.arch_has_sparse_bitmasks = ecx.split.noncont;
 	r->alloc_capable = true;
 
-	if (rdt_cpu_has(X86_FEATURE_PLZA))
+	if (1 || rdt_cpu_has(X86_FEATURE_PLZA))
 		r->plza_capable = true;
 }
 
diff --git a/arch/x86/kernel/cpu/resctrl/ctrlmondata.c b/arch/x86/kernel/cpu/resctrl/ctrlmondata.c
index 79ed41bde810..24a37ebed13a 100644
--- a/arch/x86/kernel/cpu/resctrl/ctrlmondata.c
+++ b/arch/x86/kernel/cpu/resctrl/ctrlmondata.c
@@ -136,7 +136,8 @@ static void resctrl_plza_set_one_amd(void *arg)
 {
 	union qos_pqr_plza_assoc *plza = arg;
 
-	wrmsrl(MSR_IA32_PQR_PLZA_ASSOC, plza->full);
+	pr_info("wrmsr(MSR_IA32_PQR_PLZA_ASSOC, 0x%lx)\n", plza->full);
+	//wrmsrl(MSR_IA32_PQR_PLZA_ASSOC, plza->full);
 }
 
 void resctrl_arch_plza_setup(struct rdt_resource *r, u32 closid, u32 rmid)
-- 
2.52.0