Re: [PATCH V11 05/10] arm64/perf: Add branch stack support in ARMV8 PMU

[Anshuman Khandual]

On 6/5/23 17:35, Mark Rutland wrote:
> On Wed, May 31, 2023 at 09:34:23AM +0530, Anshuman Khandual wrote:
>> This enables support for branch stack sampling event in ARMV8 PMU, checking
>> has_branch_stack() on the event inside 'struct arm_pmu' callbacks. Although
>> these branch stack helpers armv8pmu_branch_XXXXX() are just dummy functions
>> for now. While here, this also defines arm_pmu's sched_task() callback with
>> armv8pmu_sched_task(), which resets the branch record buffer on a sched_in.
> 
> This generally looks good, but I have a few comments below.
> 
> [...]
> 
>> +static inline bool armv8pmu_branch_valid(struct perf_event *event)
>> +{
>> +	WARN_ON_ONCE(!has_branch_stack(event));
>> +	return false;
>> +}
> 
> IIUC this is for validating the attr, so could we please name this
> armv8pmu_branch_attr_valid() ?

Sure, will change the name and updated call sites.

> 
> [...]
> 
>> +static int branch_records_alloc(struct arm_pmu *armpmu)
>> +{
>> +	struct pmu_hw_events *events;
>> +	int cpu;
>> +
>> +	for_each_possible_cpu(cpu) {
>> +		events = per_cpu_ptr(armpmu->hw_events, cpu);
>> +		events->branches = kzalloc(sizeof(struct branch_records), GFP_KERNEL);
>> +		if (!events->branches)
>> +			return -ENOMEM;
>> +	}
>> +	return 0;
> 
> This leaks memory if any allocation fails, and the next patch replaces this
> code entirely.

Okay.

> 
> Please add this once in a working state. Either use the percpu allocation
> trick in the next patch from the start, or have this kzalloc() with a
> corresponding kfree() in an error path.

I will change branch_records_alloc() as suggested in the next patch's thread
and fold those changes here in this patch.

> 
>>  }
>>  
>>  static int armv8pmu_probe_pmu(struct arm_pmu *cpu_pmu)
>> @@ -1145,12 +1162,24 @@ static int armv8pmu_probe_pmu(struct arm_pmu *cpu_pmu)
>>  	};
>>  	int ret;
>>  
>> +	ret = armv8pmu_private_alloc(cpu_pmu);
>> +	if (ret)
>> +		return ret;
>> +
>>  	ret = smp_call_function_any(&cpu_pmu->supported_cpus,
>>  				    __armv8pmu_probe_pmu,
>>  				    &probe, 1);
>>  	if (ret)
>>  		return ret;
>>  
>> +	if (arm_pmu_branch_stack_supported(cpu_pmu)) {
>> +		ret = branch_records_alloc(cpu_pmu);
>> +		if (ret)
>> +			return ret;
>> +	} else {
>> +		armv8pmu_private_free(cpu_pmu);
>> +	}
> 
> I see from the next patch that "private" is four ints, so please just add that
> to struct arm_pmu under an ifdef CONFIG_ARM64_BRBE. That'll simplify this, and
> if we end up needing more space in future we can consider factoring it out.

struct arm_pmu {
	........................................
        /* Implementation specific attributes */
        void            *private;
}

private pointer here creates an abstraction for given pmu implementation
to hide attribute details without making it known to core arm pmu layer.
Although adding ifdef CONFIG_ARM64_BRBE solves the problem as mentioned
above, it does break that abstraction. Currently arm_pmu layer is aware
about 'branch records' but not about BRBE in particular which the driver
adds later on. I suggest we should not break that abstraction.

Instead a global 'static struct brbe_hw_attr' in drivers/perf/arm_brbe.c
can be initialized into arm_pmu->private during armv8pmu_branch_probe(),
which will also solve the allocation-free problem. Also similar helpers
armv8pmu_task_ctx_alloc()/free() could be defined to manage task context
cache i.e arm_pmu->pmu.task_ctx_cache independently.

But now armv8pmu_task_ctx_alloc() can be called after pmu probe confirms
to have arm_pmu->has_branch_stack.

> 
>> +
>>  	return probe.present ? 0 : -ENODEV;
>>  }
> 
> It also seems odd to ceck probe.present *after* checking
> arm_pmu_branch_stack_supported().

I will reorganize as suggested below.

> 
> With the allocation removed I think this can be written more clearly as:
> 
> | static int armv8pmu_probe_pmu(struct arm_pmu *cpu_pmu)
> | {
> |         struct armv8pmu_probe_info probe = {
> |                 .pmu = cpu_pmu,
> |                 .present = false,
> |         };   
> |         int ret; 
> | 
> |         ret = smp_call_function_any(&cpu_pmu->supported_cpus,
> |                                     __armv8pmu_probe_pmu,
> |                                     &probe, 1);
> |         if (ret)
> |                 return ret; > | 
> |         if (!probe.present)
> |                 return -ENODEV;
> | 
> |         if (arm_pmu_branch_stack_supported(cpu_pmu))
> |                 ret = branch_records_alloc(cpu_pmu);
> |              
> |         return ret; 
> | }