Re: [PATCH V3 7/7] arm64/perf: Enable branch stack sampling

From: Suzuki K Poulose <suzuki.poulose@arm.com>
To: James Clark <james.clark@arm.com>,
	Anshuman Khandual <anshuman.khandual@arm.com>
Cc: linux-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org, peterz@infradead.org,
	acme@kernel.org, mark.rutland@arm.com, will@kernel.org,
	catalin.marinas@arm.com
Subject: Re: [PATCH V3 7/7] arm64/perf: Enable branch stack sampling
Date: Mon, 10 Oct 2022 16:48:32 +0100	[thread overview]
Message-ID: <6805165c-8aaf-086f-f5a6-181876cd7975@arm.com> (raw)
In-Reply-To: <0adcc3f7-bf60-516f-c8d7-4250d1f901cb@arm.com>

On 10/10/2022 14:55, James Clark wrote:
> 
> 
> On 29/09/2022 08:58, Anshuman Khandual wrote:
>> Now that all the required pieces are already in place, just enable the perf
>> branch stack sampling support on arm64 platform, by removing the gate which
>> blocks it in armpmu_event_init().
>>
>> Cc: Mark Rutland <mark.rutland@arm.com>
>> Cc: Will Deacon <will@kernel.org>
>> Cc: Catalin Marinas <catalin.marinas@arm.com>
>> Cc: linux-kernel@vger.kernel.org
>> Cc: linux-arm-kernel@lists.infradead.org
>> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
>> ---
>>   drivers/perf/arm_pmu.c | 32 +++++++++++++++++++++++++++++---
>>   1 file changed, 29 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/perf/arm_pmu.c b/drivers/perf/arm_pmu.c
>> index 93b36933124f..2a9b988b53c2 100644
>> --- a/drivers/perf/arm_pmu.c
>> +++ b/drivers/perf/arm_pmu.c
>> @@ -537,9 +537,35 @@ static int armpmu_event_init(struct perf_event *event)
>>   		!cpumask_test_cpu(event->cpu, &armpmu->supported_cpus))
>>   		return -ENOENT;
>>   
>> -	/* does not support taken branch sampling */
>> -	if (has_branch_stack(event))
>> -		return -EOPNOTSUPP;
>> +	if (has_branch_stack(event)) {
>> +		/*
>> +		 * BRBE support is absent. Select CONFIG_ARM_BRBE_PMU
>> +		 * in the config, before branch stack sampling events
>> +		 * can be requested.
>> +		 */
>> +		if (!IS_ENABLED(CONFIG_ARM_BRBE_PMU)) {
>> +			pr_warn_once("BRBE is disabled, select CONFIG_ARM_BRBE_PMU\n");
>> +			return -EOPNOTSUPP;
>> +		}
>> +
>> +		if (event->attr.branch_sample_type & PERF_SAMPLE_BRANCH_KERNEL) {
>> +			if (!perfmon_capable()) {
> 
> I'm still getting different behaviour compared to x86 when using
> perf_event_paranoid because of this perfmon_capable() call here.

Given the generic events framework already checks this for any
privileged branch samples (i.e., for both KERNEL and HV), the
individual drivers must not add additional restrictions.

> 
>> +				pr_warn_once("does not have permission for kernel branch filter\n");
> 
> Also I was under the impression that this should be more like a
> KERN_INFO loglevel rather than a KERN_WARNING. It's more like expected
> behavior rather than unexpected behavior and as far as I know anyone who
> sees something in dmesg might think something has gone wrong and try to
> follow it up. It is quite a useful message but I remember getting a
> review like this before and it made sense to me.

+1

Suzuki

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