Re: [PATCH] drivers/perf: Change WARN_ON() to dev_err() on irq_set_affinity() failure

[Yicong Yang]

On 2022/8/15 17:39, Jonathan Cameron wrote:
> On Mon, 15 Aug 2022 17:28:15 +0800
> Yicong Yang <yangyicong@xxxxxxxxxx> wrote:
> 
>> From: Yicong Yang <yangyicong@xxxxxxxxxxxxx>
>>
>> The WARN_ON() on irq_set_affinity() failure is misused according to the [1]
>> and may crash people's box unintentionally. This may also be redundant since
>> in the failure case we may also trigger the WARN and dump the stack in the
>> perf core[2] for a second time.
>>
>> So change the WARN_ON() to dev_err() to just print the failure message.
>>
>> [1] https://github.com/torvalds/linux/blob/master/include/asm-generic/bug.h#L74
>> [2] https://github.com/torvalds/linux/blob/master/kernel/events/core.c#L313
>>
>> Suggested-by: Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx>
>> [https://lore.kernel.org/lkml/YuOi3i0XHV++z1YI@xxxxxxxxx/]
>> Signed-off-by: Yicong Yang <yangyicong@xxxxxxxxxxxxx>
> 
> Looks like progress in a sensible direction to me.
> 
> Reviewed-by: Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx>
> 
> Kind of unrelated question inline.
> 

Thanks for the quick review! replies below.

>>
> 
> 
>> diff --git a/drivers/perf/arm_smmuv3_pmu.c b/drivers/perf/arm_smmuv3_pmu.c
>> index 00d4c45a8017..05e1b3e274d7 100644
>> --- a/drivers/perf/arm_smmuv3_pmu.c
>> +++ b/drivers/perf/arm_smmuv3_pmu.c
>> @@ -646,7 +646,8 @@ static int smmu_pmu_offline_cpu(unsigned int cpu, struct hlist_node *node)
>>  
>>  	perf_pmu_migrate_context(&smmu_pmu->pmu, cpu, target);
>>  	smmu_pmu->on_cpu = target;
>> -	WARN_ON(irq_set_affinity(smmu_pmu->irq, cpumask_of(target)));
>> +	if (irq_set_affinity(smmu_pmu->irq, cpumask_of(target)))
>> +		dev_err(smmu_pmu->dev, "Failed to set interrupt affinity\n");
>>  
>>  	return 0;
>>  }
>> @@ -892,7 +893,8 @@ static int smmu_pmu_probe(struct platform_device *pdev)
>>  
>>  	/* Pick one CPU to be the preferred one to use */
>>  	smmu_pmu->on_cpu = raw_smp_processor_id();
>> -	WARN_ON(irq_set_affinity(smmu_pmu->irq, cpumask_of(smmu_pmu->on_cpu)));
>> +	if (irq_set_affinity(smmu_pmu->irq, cpumask_of(smmu_pmu->on_cpu)))
>> +		dev_err(dev, "Failed to set interrupt affinity\n");
> 
> In this case we have the option to fail probe.  Failing to set affinity means
> we are broken anyway, so perhaps that is cleaner than carrying on.
> 

This patch only switch the way on error notification with no functional change intended.
So if we need to change the behaviour here it should be in a separate patch. Indeed I'm
not sure it's necessary to fail probe here since we can use the pmu if it fails here.

> As a side note, I wonder if other drivers could benefit from what I think
> is a micro optimization to short cut calling the hp handlers when the
> decision of which CPU is easy...
> 

It seems sensible to me but may differ in differenct pmu drivers since they may need the
hp handlers called. Needs more check.

Thanks.

>>  
>>  	err = cpuhp_state_add_instance_nocalls(cpuhp_state_num,
>>  					       &smmu_pmu->node);
> .
>