Re: [PATCH V2 6/8] perf/x86/intel/uncore: Introduce PMU flags and broken state

[Chen, Zide]

On 6/2/2026 9:13 PM, Mi, Dapeng wrote:
> 
> On 6/2/2026 1:01 AM, Zide Chen wrote:
>> Replace the boolean 'registered' field in intel_uncore_pmu with an
>> unsigned long 'flags' field, and add a PMU_BROKEN flag to track box
>> setup failures. Broken flag is sticky, means it won't be cleared
>> unless module reload or system reboot.
>>
>> Broken PMUs are skipped in the CPU hotplug and box allocation paths.
>>
>> When any box fails to initialize, the PMU is marked broken.  Broken
>> PMUs reject new event assignments and skip future box setup attempts.
>> If the PMU was already registered, it remains so to avoid disrupting
>> in-flight events on other boxes.
>>
>> Signed-off-by: Zide Chen <zide.chen@xxxxxxxxx>
>> ---
>> v2:
>> - Make the broken flag sticky by clear_bit() in uncore_pmu_unregister()
>> other than zeroing out pmu->flag.
>> - In uncore_change_type_ctx(), don't stop CPU migration on broken PMU
>>   for in-flight events.  (Sashiko).
>> - Use box->cpu == -1 to identify inactive boxes that don't need
>>   migration, no need to check uncore_die_has_box(), which is incomplete.
>> ---
>>  arch/x86/events/intel/uncore.c     | 44 ++++++++++++++++++++++--------
>>  arch/x86/events/intel/uncore.h     | 13 ++++++++-
>>  arch/x86/events/intel/uncore_snb.c |  2 +-
>>  3 files changed, 46 insertions(+), 13 deletions(-)
>>
>> diff --git a/arch/x86/events/intel/uncore.c b/arch/x86/events/intel/uncore.c
>> index 283e41933ba7..f2cb3fde2dda 100644
>> --- a/arch/x86/events/intel/uncore.c
>> +++ b/arch/x86/events/intel/uncore.c
>> @@ -757,7 +757,7 @@ static int uncore_pmu_event_init(struct perf_event *event)
>>  
>>  	pmu = uncore_event_to_pmu(event);
>>  	/* no device found for this pmu */
>> -	if (!pmu->registered)
>> +	if (!uncore_pmu_available(pmu))
>>  		return -ENOENT;
>>  
>>  	/* Sampling not supported yet */
>> @@ -953,16 +953,18 @@ static int uncore_pmu_register(struct intel_uncore_pmu *pmu)
>>  
>>  	ret = perf_pmu_register(&pmu->pmu, pmu->name, -1);
>>  	if (!ret)
>> -		pmu->registered = true;
>> +		uncore_pmu_set_registered(pmu);
>>  	return ret;
>>  }
>>  
>>  static void uncore_pmu_unregister(struct intel_uncore_pmu *pmu)
>>  {
>> -	if (!pmu->registered)
>> +	if (!uncore_pmu_registered(pmu))
>>  		return;
>>  	perf_pmu_unregister(&pmu->pmu);
>> -	pmu->registered = false;
>> +
>> +	/* Keep PMU_BROKEN_BIT stick. */
>> +	uncore_pmu_clear_registered(pmu);
> 
> Why to set PMU_BROKEN_BIT to a stick bit and doesn't clear it when
> unregistering the PMU? Per my understanding, there is no active boxes when
> uncore_pmu_unregister() is called, seem all flags can be cleared. When the
> new CPU or PCI device are hot plug-in again, it may be a new device or the
> old issue which leads to the PMU can't be initialized has been fixed, the
> uncore PMU can work correctly, the PMU_BROKEN_BIT should be set any more.

Woops, a typo here: stick->sticky.

Let's examine a theoretical flow for a particular PMU:

PCI dev on die0 online: activeboxes=1, PMU registered
PCI dev on die1 online: init_box failed, pmu->activeboxes=1, set broken
PCI dev on die0 offline: activeboxes=0, PMU unregistered

If the broken bit is cleared in uncore_pmu_unregister():
PCI dev on die0 or die2 online: activeboxes=1, PMU registered

This is wrong, because the box for die1 is not working.

In reality, box_allocate() or init_box() failures may not happen at all,
and when they do, the system may be experiencing severe out-of-memory or
other catastrophic conditions. Keeping the broken flag sticky should be
fine.

One alternative to avoid making it sticky is to use pmu->activeboxes as
pmu->die_refcnt to track online dies, not necessarily active boxes, as
v1 does.

The problem with this approach is that it could cause a pmu->die_refcnt
leak in the uncore_pci_remove() path: if init_box() fails but
pmu->die_refcnt has already been incremented, there is no chance to call
uncore_pci_pmu_unregister() because pci_get_drvdata() returns NULL.

>>  }
>>  
>>  static void uncore_free_boxes(struct intel_uncore_pmu *pmu)
>> @@ -1153,7 +1155,12 @@ static int uncore_box_setup(struct intel_uncore_pmu *pmu,
>>  {
>>  	int ret;
>>  
>> -	uncore_box_init(box);
>> +	if (uncore_pmu_broken(pmu))
>> +		return -ENODEV;
>> +
>> +	ret = uncore_box_init(box);
>> +	if (ret)
>> +		goto err;
>>  
>>  	/* First active box registers the pmu. */
>>  	if (atomic_inc_return(&pmu->activeboxes) > 1)
>> @@ -1167,6 +1174,16 @@ static int uncore_box_setup(struct intel_uncore_pmu *pmu,
>>  
>>  	return 0;
>>  err:
>> +	/*
>> +	 * On failure on any box, mark the per-package PMU as broken regardless
>> +	 * of whether it was registered or not.
>> +	 *
>> +	 * Don't decrement refcnt to avoid other in-die CPUs from trying to set
>> +	 * up the PMU box again.
>> +	 *
>> +	 * Don't kfree box; MSR and MMIO boxes are freed at module exit only.
>> +	 */
>> +	uncore_pmu_set_broken(pmu);
>>  	uncore_box_exit(box);
>>  	return ret;
>>  }
>> @@ -1190,8 +1207,10 @@ static int uncore_pci_pmu_register(struct pci_dev *pdev,
>>  		return -EINVAL;
>>  
>>  	box = uncore_alloc_box(type, NUMA_NO_NODE);
>> -	if (!box)
>> +	if (!box) {
>> +		uncore_pmu_set_broken(pmu);
>>  		return -ENOMEM;
>> +	}
>>  
>>  	box->dieid = die;
>>  	box->pci_dev = pdev;
>> @@ -1504,7 +1523,8 @@ static void uncore_change_type_ctx(struct intel_uncore_type *type, int old_cpu,
>>  
>>  		if (old_cpu < 0) {
>>  			WARN_ON_ONCE(box->cpu != -1);
>> -			if (uncore_die_has_box(type, die, pmu->pmu_idx)) {
>> +			if (uncore_die_has_box(type, die, pmu->pmu_idx) &&
>> +			    !uncore_pmu_broken(pmu)) {
>>  				box->cpu = new_cpu;
>>  				cpumask_set_cpu(new_cpu, &pmu->cpu_mask);
>>  			}
>> @@ -1512,12 +1532,14 @@ static void uncore_change_type_ctx(struct intel_uncore_type *type, int old_cpu,
>>  		}
>>  
>>  		WARN_ON_ONCE(box->cpu != -1 && box->cpu != old_cpu);
>> -		box->cpu = -1;
>>  		cpumask_clear_cpu(old_cpu, &pmu->cpu_mask);
>> -		if (new_cpu < 0)
>> +		if (new_cpu < 0) {
>> +			box->cpu = -1;
>>  			continue;
>> +		}
>>  
>> -		if (!uncore_die_has_box(type, die, pmu->pmu_idx))
>> +		/* non-active box doesn't need migration. */
>> +		if (box->cpu == -1)
>>  			continue;
>>  		uncore_pmu_cancel_hrtimer(box);
>>  		perf_pmu_migrate_context(&pmu->pmu, old_cpu, new_cpu);
>> @@ -1593,7 +1615,7 @@ static int allocate_boxes(struct intel_uncore_type **types,
>>  		type = *types;
>>  		pmu = type->pmus;
>>  		for (i = 0; i < type->num_boxes; i++, pmu++) {
> 
> IIUC, the "type->num_boxes" actually indicates the number of PMU  or the
> number of box classes. Since currently there could be multiple boxes for
> each kind of PMUs on the multiple dies device, the name "num_boxes" become
> inaccurate and much misleading. could we change it to a more accurate name,
> like "num_pmus" or "num_box_classes"?

Yes, agree. Will do more clean up in the future.
> 
>> -			if (pmu->boxes[die])
>> +			if (pmu->boxes[die] || uncore_pmu_broken(pmu))
>>  				continue;
>>  			box = uncore_alloc_box(type, cpu_to_node(cpu));
>>  			if (!box)
>> diff --git a/arch/x86/events/intel/uncore.h b/arch/x86/events/intel/uncore.h
>> index d732b87be0a9..0adb477d9708 100644
>> --- a/arch/x86/events/intel/uncore.h
>> +++ b/arch/x86/events/intel/uncore.h
>> @@ -146,13 +146,24 @@ struct intel_uncore_pmu {
>>  	struct pmu			pmu;
>>  	char				name[UNCORE_PMU_NAME_LEN];
>>  	int				pmu_idx;
>> -	bool				registered;
>> +	unsigned long			flags;
>>  	atomic_t			activeboxes;
>>  	cpumask_t			cpu_mask;
>>  	struct intel_uncore_type	*type;
>>  	struct intel_uncore_box		**boxes;
>>  };
>>  
>> +#define PMU_REGISTERED_BIT	0
>> +#define PMU_BROKEN_BIT		1
>> +
>> +#define uncore_pmu_registered(pmu)	test_bit(PMU_REGISTERED_BIT, &(pmu)->flags)
>> +#define uncore_pmu_broken(pmu)		test_bit(PMU_BROKEN_BIT, &(pmu)->flags)
>> +#define uncore_pmu_available(pmu)	(uncore_pmu_registered(pmu) &&  \
>> +					 !uncore_pmu_broken(pmu))
>> +#define uncore_pmu_set_registered(pmu)	set_bit(PMU_REGISTERED_BIT, &(pmu)->flags)
>> +#define uncore_pmu_set_broken(pmu)	set_bit(PMU_BROKEN_BIT, &(pmu)->flags)
>> +#define uncore_pmu_clear_registered(pmu) clear_bit(PMU_REGISTERED_BIT, &(pmu)->flags)
>> +
>>  struct intel_uncore_extra_reg {
>>  	raw_spinlock_t lock;
>>  	u64 config, config1, config2;
>> diff --git a/arch/x86/events/intel/uncore_snb.c b/arch/x86/events/intel/uncore_snb.c
>> index c5347920541c..055131c508ff 100644
>> --- a/arch/x86/events/intel/uncore_snb.c
>> +++ b/arch/x86/events/intel/uncore_snb.c
>> @@ -940,7 +940,7 @@ static int snb_uncore_imc_event_init(struct perf_event *event)
>>  
>>  	pmu = uncore_event_to_pmu(event);
>>  	/* no device found for this pmu */
>> -	if (!pmu->registered)
>> +	if (!uncore_pmu_available(pmu))
>>  		return -ENOENT;
>>  
>>  	/* Sampling not supported yet */